Analysis of Vacuum and Argon Gas Fill Data from
the MiniCLEAN Dark Matter Experiment
by
Stephen H. Jaditz
Submitted to the Department of Physics
in partial fulfillment of the requirements for the degree of
w
Doctor of Philosophy
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2015
Massachusetts Institute of Technology 2015. All rights reserved.
Author ..
Signature redacted
Department of Physics
January 16, 2015
Certified by.
Signature redacted
..
Joseph A. Formaggio
Associate Professor of Physics
Thesis Supervisor
Signature redacted
Accepted by
Krishna Rajagopal
Associate Department Head for Education
2
Analysis of Vacuum and Argon Gas Fill Data from the
MiniCLEAN Dark Matter Experiment
by
Stephen H. Jaditz
Submitted to the Department of Physics
on January 16, 2015, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Abstract
.
The existence of particle dark matter provides a consistent framework for understanding many astronomical observations. The rotation curves of galaxies and galaxyclusters, for example, indicate the majority of mass in these structures is unseen.
The existence of weakly-interacting massive particles (WIMPs) was proposed in the
early 1980s to account for the anomalous rotation curves and provide a mechanism
for producing the cold dark matter relic density, which along with dark energy is
thought to dominate the current energy density of our universe.
Efforts to observe the rare interaction of WIMPs with normal matter have continued since their proposal, and so far have set limits on the WIMP-nucleon interaction
cross-section extending to 1 x 10-9 pb. Contemporary experiments seek to observe
~ one WIMP-nucleus scatter per year per 100 kg of detector mass. These experiments
must be conducted deep underground with stringent cleanliness requirements.
The MiniCLEAN dark matter experiment is a single-phase liquid argon scintillation detector which uses the wavelength-shifting fluor tetraphenyl butadiene and
cryogenic photomultiplier tubes for light detection. The active spherical region of
the detector contains 500 kg of liquid argon at temperature 87 K. Background events
which could mimic a WIMP signal are mitigated through pulse-shape discrimination
and position reconstruction.
At an intermediate stage of ongoing detector assembly 2 km underground at SNOLAB in Ontario, the complete instrumented inner vessel was commissioned by collecting photomultiplier waveform data for periods when the vessel was evacuated and
when filled with warm argon gas. Alpha decay events from radon progeny on the
wavelength-shifting surface occur in this data at a measured rate of 19.0 t 0.4 /h/m 2
MiniCLEAN's projected sensitivity to spin-independent WIMP-nucleon scattering,
derived from simulation of this surface rate, is -si < 1.5 x 10-8 pb.
Thesis Supervisor: Joseph A. Formaggio
Title: Associate Professor of Physics
3
4
Contents
Introduction . . . . . . . . . . . . . . . . . .
. . . . . .
31
1.2
Dark Matter Halo Parameters . . . . . . . .
. . . . . .
35
1.3
Basics of Terrestrial Scattering . . . . . . . .
. . . . . .
36
1.4
Setting Scattering Cross-section Limits . . .
.
. . . . . .
38
1.5
Comparing Results of Different Experiments
.
. . . . . .
38
1.6
Detector Technologies . . . . . . . . . . . . .
. . . . . .
39
.
.
.
.
1.1
47
The MiniCLEAN Detector
2.1.1
48
Light detection: tetraphenyl butadiene
50
53
2.2.1
Assembly of inner vessel . . . . . . .
56
2.2.2
Gas processing system . . . . . . . .
57
2.2.3
Data acquisition . . . . . . . . . . . .
61
.
.
. . . . . . . . . . . . .
MiniCLEAN Design
.
2.2
. . . . . . . .
.
Scintillation in Noble Liquids
.
2.1
63
Radioactive Backgrounds
External Backgrounds
. . . . . . . . . . . .
. . . . . . . .
63
3.2
Internal Backgrounds . . . . . . . . . . . . .
. . . . . . . .
64
3.2.1
39 A r
. . . . . . . . . . . . . . . . . .
. . . . . . . .
64
3.2.2
Gammas . . . . . . . . . . . . . . . .
. . . . . . . .
66
3.2.3
.
.
.
3.1
Fast neutrons . . . . . . . . . . . . .
. . . . . . . .
67
3.2.4
Radon progeny on wavelength shifter
.
3
31
. . . . . . . .
69
.
2
Direct Dark Matter Detection
.
1
5
4
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
Simulation and Analysis Software
73
4.1
Likelihood Method for Particle Identification . . . . . . . . . . . . . .
75
4.2
Likelihood Method for Position Reconstruction . . . . . . . . . . . . .
76
The MiniCLEAN Veto System
79
5.1
O verview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.2
H ardw are
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.2.1
PMT mounts . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.2.2
PMT testing . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
5.2.3
Electrical connections and data acquisition . . . . . . . . . . .
85
Sim ulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.3.1
Cosmic muons . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.3.2
Cosmogenic neutrons . . . . . . . . . . . . . . . . . . . . . . .
98
5.3.3
Summary of Cosmogenic Backgrounds
5.3
6
Sum m ary
.
3.3
. . . . . . . . . . . . .
100
Analysis of MiniCLEAN Vacuum Data
103
6.1
PMT and DAQ configuration
103
6.2
Data quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6.3
22
6.4
Vacuum data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
. . . . . . . . . . . . . . . . . . . . . .
Na calibration source . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7 Analysis of MiniCLEAN Gas Data
119
7.1
Run Summaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.2
Triplet Quenching Overview . . . . . . . . . . . . . . . . . . . . . . . 122
7.3
22 Na
7.4
Instrument Effects
7.4.1
7.5
calibration source . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
125
128
Instrument Effects Summary . . . . . . . . . . . . . . . . . . . 134
Surface alphas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
135
7.5.1
Surface alpha rate . . . . . . . . . . . . . . . . . . . . . . . . .
138
7.5.2
Light yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149
6
8
Sensitivity Outlook and Conclusion
151
8.1
158
Conclusion .......
.................................
A Select Topics in Detector Design
161
A.1 Validation of photomultiplier and base . . . . . . . . . . . . . . . . . 161
A.2 Selection of light guide reflector . . . . . . . . . . . . . . . . . . . . .
171
A.3 Acrylic length and PMT neutron simulation . . . . . . . . . . . . . . 174
A.4 Inverted conflat validation and bolt selection . . . . . . . . . . . . . . 178
7
8
List of Figures
1-1
Mean velocities in the plane of the galaxy, as a function of linear distance from the nucleus for 21 Sc (spiral) galaxies, arranged according
to increasing linear radius. From [5]. None of the curves fall off at large
radii, contrary to the expectation for galaxies comprised of visible mass. 32
1-2
A composite optical, X-ray (red), and weak lensing (blue) image of the
Bullet Cluster (lE 0657-558) [10]. . . . . . . . . . . . . . . . . . . . .
1-3
33
From [24]: the rotation curve for the Milky Way for values of Ro =
7.1 kpc, v, = 185 km - s- , and RO = 8.5 kpc, vc = 220 km - s-I, where
RO is the distance to the galactic center and v, is the circular velocity
of our solar system about the galactic center.
The figure also shows
one of the ways in which the rotation curve can be decomposed into the
contributions from different mass components: the bulge (dotted line);
the stellar disc (filled circles); the H1 layer (crosses, where negative
values mean that the force is directed outwards); the H 2 layer (circles);
and the dark halo (dashed line). The best-fitting model, which is obtained by summing the individual components in quadrature, is shown
as a full line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
37
1-4
Solid lines: measured spin-independent WIMP-nucleon cross-section
limits. Non-solid (dashed and dotted) lines: projected limits from future measurements. Shaded regions with solid-line delimiting: signal
claims. Shaded regions with no delimiting: predictions for theoretical dark matter candidates. The dual-phase liquid argon experiment
DarkSide-50, for which a projected limit is shown in this figure, recently
obtained a limit of 6.1 x 10-" cm 2 for a WIMP mass of 100 GeV [51].
1-5
45
Limits for SI and SD WIMP-nucleon cross-section from the LHC experim ent CM S [20]. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
2-1
Photograph of the assembled MiniCLEAN inner vessel at SNOLAB.
47
2-2
Measured Leff for liquid xenon as a function of nuclear recoil energy,
.
from [63]. The open circles represent that work's measurement; other
markers are from previous measurements and cited in [63]. The solid
line is from a best-fit analysis of XENON10 AmBe source data and
Monte Carlo, and the dashed line in the theoretical prediction of Hitachi [68].
2-3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measurement of Leff for liquid argon as a function of nuclear recoil
energy, made with the micro-CLEAN detector [69].
The rapid rise
below 20 keVr is from trigger effects and not physical. . . . . . . . . .
2-4
50
51
Measurement of Leff for liquid neon as a function of nuclear recoil energy, from [70]. Solid lines represent fits to quenching models based on
different ion stopping-power models from SRIM [71] and the reference
[66] of M ei et al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
Normalized visible reemission spectrum from a TPB film illuminated
with various UV wavelengths. From [72]. . . . . . . . . . . . . . . . .
2-6
51
52
Photon emission time probability density functions for electronic and
nuclear recoils in liquid argon and alpha scintillation in TPB . . . . .
10
52
2-7
(Left) SolidWorks model of the MiniCLEAN detector by Jeff Griego
(LANL). The liquid argon or neon (LAr, LNe) is contained in the cold
inner vessel (radius 75 cm), which is housed by the evacuated outer
vessel. One of the 92 optical modules is depicted on the right. ....
2-8
53
Components of an optical module. (Left) An 8-inch Hamamatsu R591202MOD PMT attached to a tophat.
(Center) An irregular-hexagon
type light guide. The guide is shown before electropolishing and without the reflective inner surface, which is 3M ESR foil.
(Right) An
irregular-hexagon acrylic plug, illuminated on the far side with a UV
LED so that the blue fluoresence of the TPB can be seen . . . . . . .
2-9
54
Model of the central detector housed in the muon veto, a 7.9 m tall
by 2.8 m radius water tank instrumented with 42 8-inch Hamamatsu
R1408 PMTs. Figure by Jeff Griego.
. . . . . . . . . . . . . . . . . .
55
2-10 (Left) Photograph of the Cube Hall in SNOLAB, looking down onto
the water tanks for MiniCLEAN (right) and DEAP3600 (left). The
right photograph is taken from the bottom of the MiniCLEAN water
tank, looking up to the (empty) outer vessel and beyond to the Cube
Hall and yellow gantry crane.
. . . . . . . . . . . . . . . . . . . . . .
55
2-11 (Left) Photograph from the top of the Cryopit, looking down 50 feet
onto the temporary soft-wall cleanroom used for IV assembly. Detector
components are in bags stacked along the right rear wall.
(Right)
The IV in the soft-wall cleanroom with some spools and their clear
polycarbonate covers attached . . . . . . . . . . . . . . . . . . . . . .
11
56
2-12 (Left) Photograph looking into an IV port during the "dry-fit" of the
light guides, when each guide was inserted to check for fit and to fix
its orientation for final installation, before being removed again for assembly with reflective foil and the acrylic plug. Note the guides do
not abut along their edges, but have several millimeter gaps through
which scintillation light could penetrate the central detector from outside the wavelength-shifting radius. Minimization of these gap sizes
motivated tight tolerances on IV dimensions, which was a driving factor in fabrication cost. (Right) Photograph looking into the IV during
final cassette insertion. The TPB-coated acrylic faces are outlined by
reflective flaps which cover the gaps between guides. The flaps were
made by creasing the reflective foil lining the light guide walls. .....
58
2-13 Photograph of the assembled IV in the softwall cleanroom, with DAQ
racks and gas bottles. . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
2-14 Diagram of the MiniCLEAN gas system for purification of warm argon
gas.........
......................................
59
2-15 Photograph of the partial purification system. The SAES getter is the
grey box on the right, with the multi-meter sitting atop. To the right
of the getter is the charcoal trap submerged in the dewar of ethanol
slu rry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
2-16 SolidWorks model of the cold head attachments from James Nikkel
(RHUL). The cold head is housed in a nitrogen-filled canister attached
to the OV, with helium lines extending out to the compressor on deck.
The cold "finger" extends into the insulation vacuum space, where it
is bonded by copper braids to cold fingers which penetrate the IV.
. .
60
2-17 Diagram (from Boston University) and picture of the DAQ configuration for warm argon gas data. This configuration does not incorporate
the veto system, which will be present for liquid argon data taking.
12
.
62
3-1
(Top) Comparison of DEAP-1
22Na
data and the collaboration's ana-
lytic model in the region 120-240 photoelectrons (approximately 43-86
keVee). (Bottom) P1cak distribution for the same data, showing the an-
alytic model with and without PMT noise included. The lower curve
shows the expected backgrounds from high-Fp events, mostly cosmogenic neutrons. For 50% nuclear recoil acceptance, the statistical model
with noise projects pulse shape discrimination better than parts in 10
billion. Figures from [78].
3-2
. . . . . . . . . . . . . . . . . . . . . . . .
Intensity of prominent gamma lines in the 2 38 U and
2
32 Th
65
decay chains
(assuming equilibrium), derived from a listing given in a project proposal by the Majorana neutrinoless double-beta decay experiment [81].
3-3
(a,n) neutron energy spectrum, for 0.103 ppm
232 Th
U and 0.170 ppm
contamination in borosilicate glass. Generated using the online
tool http://neutronyield.usd.edu, based on [83].
3-4
2 38
67
(Left) Scenarios for the surface decay of
dot, to an alpha particle and
20 1Pb
2 1 0 Po,
. . . . . . . . . . . .
68
depicted as a black
nucleus, which could mimic a dark
matter signal. (Right) Example of the fiducial volume cut on surface
events. ........
3-5
69
...................................
From [84]: an example radon exposure requirement to achieve a desired
surface activity of 0.1 and 1.0 a decay per m2 per day due to
210 Po
on
acrylic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
70
Cutaway view of the inner MiniCLEAN detector as rendered by the
RAT simulation. The PMTs, outer vessel, and water tank are included
in the geometry but not shown. . . . . . . . . . . . . . . . . . . . . .
4-2
73
(Left) Simulated differential leakage of electron events obtained using
the discrimination variables L, and Fp. (Right) Integral leakage of 39 Ar
events as a function of energy threshold into a 150 kg fiducial volume.
From [87]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
75
4-3
(Left) Diagram showing 0.
(Right) Example results for E(ij), the
average probability for the generation of a photoelectron in a PMT
at location
i after a simulated energy deposition, indexed by
position ',. Plot from Stan Seibert.
4-4
j,
at
. . . . . . . . . . . . . . . . . .
76
Average reconstruction resolution along the X-axis for uniformly distributed 20 keVee events at radii ranging from the center of the detector to the TPB surface. The resolution is defined to be the sigma of a
Gaussian fit to the difference between true and reconstructed position.
From the November 2010 MiniCLEAN proposal to the DOE [88].
. .
77
5-1
PMT mount drawing. . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
5-2
PM T string drawing. . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
5-3 Veto PMT shown in mount.
5-4
. . . . . . . . . . . . . . . . . . . . . . .
81
A complete string of four veto PMTs, suspended from a crane at Bates
lab. Pictured from left to right are the author, Bates engineer Jim
Kelsey, and then MIT post-doc Kim Palladino . . . . . . . . . . . . .
5-5
Dark rate as a function of time at 2100 V for PMT PHSS. Dark pulses
are those which descend 3 mV below baseline.
5-6
. . . . . . . . . . . . .
Single photoelectron charge spectrum for PHSS at 2250 V.
83
The fit
model is given in equation 5.1. . . . . . . . . . . . . . . . . . . . . . .
5-7
82
84
Dark rate and gain for PHSS as a function of voltage. Pulses counted
as dark are those whose charges exceed 1.5 PE. The vertical line indicates the operating voltage for this PMT determined at SNO, which
produced gain 1.6 pC. The SNO-measured dark rate is higher because
a 1/4 PE threshold was used.
5-8
. . . . . . . . . . . . . . . . . . . . . .
85
Summary of the measured operating voltages for the veto PMTs. The
operating voltages are chosen to produce 1.6 pC gain. . . . . . . . . .
14
86
5-9
Ratio of measured gain to the gain measured by SNO, at SNO operating voltage. The gain has degraded over time. The two tubes with
ratio ~ 1.4 have blown back-termination resistors, which causes the
gain to double.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
5-10 TRIUMF connector assembly drawing. . . . . . . . . . . . . . . . . .
87
5-11 TRIUMF connector for mating cable to the R1408. From top to bottom is shown the coupling nut, the plug body into which the cable is
inserted, the teflon insulator which is also inserted into the plug body,
and the pin with a stripped cable. . . . . . . . . . . . . . . . . . . . .
88
5-12 Block diagram of veto electronics. . . . . . . . . . . . . . . . . . . . .
88
5-13 Muon flux as a function of azimuthal angle of incidence, from parameterization in [92] referenced from [77], and cross-sectional area of the
veto water tank. Integrating the product of the two curves gives the
result 9.8
0.3 muons per day incident on the veto. . . . . . . . . . .
90
5-14 dE/dx for muons in silicon dioxide, from RAT simulation and the PDG. 91
5-15 Number of photoelectrons created in veto PMTs vs length of water
traversed by muon. The black line is an estimate of the total number of
Cherenkov photons produced by the muon. The red line is an estimate
of the number PEs generated in the PMTs, made using a rule-of-thumb
for typical PMTs given in the PDG Review of Particle Physics [14], and
the approximate 1% photocathode coverage of the veto tank surface by
the PMTs. The red line does not take reflections into account, which
is why it lies below the bulk of the points. . . . . . . . . . . . . . . .
92
5-16 Number of hit veto PMTs vs length of water traversed by muon. In
the top plot, greater than 1/4 of the average single PE anode charge
was required in order for a PMT to be registered as hit. The bottom
plot shows the result for a threshold of 4 PE. . . . . . . . . . . . . . .
15
93
5-17 Number of photoelectrons per veto PMT per event and time distribution of photoelectron creation relative to the first photoelectron creation per PMT. The yellow line is a fit of a sum of two exponentials to
the data........
...................................
94
5-18 Efficiency for muon tagging vs veto PMT NHit requirement, for various
thresholds. As examples: requiring NHit > 2 and threshold 1/4 PE
results in efficiency greater than 0.999 and 3 missed muons per year; an
NHit > 5 and threshold 4 PE results in efficiency 0.97 and 107 missed
muons per year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
5-19 Location of closest approach for muons unvetoed with trigger requirements NHit < 10 and threshold
Q>
4 PE. The black lines outline the
veto tank wall. Unvetoed muons clip the tank corners.
. . . . . . . .
96
5-20 Distance of closest approach, or impact parameter, of unvetoed muons
for different trigger requirements.
The impact parameter decreases
with more stringent trigger requirements. . . . . . . . . . . . . . . . .
96
5-21 Radial coordinate of location of closest approach for an unvetoed muon,
with different trigger requirements. Although the impact parameter decreases with more stringent trigger requirement, the radial coordinate
remains close to the radius of the tank. . . . . . . . . . . . . . . . . .
97
5-22 Left: Simplified cavern geometry, showing cavern walls and the MiniCLEAN veto. Right: Neutron origins, looking at cavern from East to
West. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
....
......
..
99
5-23 cos(O) distributions for cosmogenic-neutron trajectories: (red) for all
neutrons (including those which do not hit the veto) with respect to
parent-muon trajectory; (blue) for all neutrons with respect to vertical;
and (green) for neutrons incident on the veto with respect to vertical.
0 = 0 points along the muon trajectory for (red), and downward for
(blue) and (green). (red) is drawn from [77] Eq. 16. Note the peak
at 0 = 7r/2 for (green). The origin of this peak is the sudden increase
in the volume of possible neutron origins in the cavern floor as the
azimuthal angle of incidence on the veto descends past horizontal. . .
100
5-24 Argon recoil energy distribution due to elastic scatters with cosmogenic
neutrons within fiducial radius (29.5 cm), for qucnching factor 0.25.
The number of scatters per year with 20-100 keVee is 0.080
0.002 per
year. The plot shows 76 years worth of simulated neutrons. . . . . . . 101
6-1
A typical event collected in vacuum data. The top trace is the sum of
the nine PMTs which fire during the event. The integrated charge is
105
61pC, or 12PE......................................
6-2
(a) Photoelectron, (b) Fp, and (c) charge centroid distributions for
vacuum runs 475, 483, and 488, normalized to counts per hour. . . . .
6-3
(a) Fp vs PE, (b) Fp vs R 3 /R3PB, and (c) R 3 /R3PB vs PE distributions
for vacuum runs 475, 485, and 488.
6-4
. . . . . . . . . . . . . . . . . . .
111
Reconstructed angular coordinates of vacuum data. Peaks are evident
at the coordinates of the cassettes. PMT 53, at cos(O) = -0.2,
#=
-1.5 was turned off due to problems with sparking. . . . . . . . . . .
6-5
110
112
Distribution of PMTs with maximum charge per event. PMTs 0, 1115, 61-65, and 91 are the pentagons, which have the smallest TPB
surface area of the three types of cassettes. . . . . . . . . . . . . . . .
6-6
112
PE and R 3 /R3PB distributions for simulated gammas overlaid on data
with arbitrary normalization.
. . . . . . . . . . . . . . . . . . . . . .
17
113
6-7
(a) R 3 /R}PB and (b) F, vs R 3 /4Rp
for vacuum events with greater
than 80 PE. The high energy events are strongly peaked at the TPB
radius and fall into two F, bands, one above and one below Fp = 0.6.
6-8
Reconstructed azimuthal and polar angles for vacuum runs with "Na
source present.
There is an excess of events at the position of the
source in the upper right corner. . . . . . . . . . . . . . . . . . . . . .
6-9
114
115
Gammas with origin at the PMTs simulated at 400 Hz and 100 kHz.
The 100 kHz source does not cause pileup significant enough to shift
the PE spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6-10 (a) Radial and (b) PE distributions for simulated bursts of 1000-1300
photons with origin on the surface of the reflective foil. The group
of events with ~ 90 PE originate along the edge of the acrylic block,
causing a lower light yield than events originating closer to the PMT,
away from the acrylic.
. . . . . . . . . . . . . . . . . . . . . . . . . .
116
6-11 The measured centroid distribution for vacuum data. The distribution
is fit with a third order polynomial between 0.85 and 0.92. The extent
of the polynomial beyond 0.92 defines the projected contributions of
gammas (in white) and ESR scintillation events (in blue). The blue
region contains 1.4 million events, corresponding to 720 events per hour
per square meter of ESR foil. The total area of the foil is 18.5 m2
7-1
117
A typical gas event, with 132 PE. The top panel shows waveform traces
for all PMTs with charge. The bottom panel is the event display, where
the area of a polygon corresponds to the charge in that PMT. ....
7-2
PE, Fp, R 3 /RTPB distributions for the first hour of gas run 281, com-
pared with vacuum data. . . . . . . . . . . . . . . . . . . . . . . . . .
7-3
120
121
Photoelectron and Fp distributions for the first five hours of gas runs
281 - 301. The average PE value decreases with time, while the average
F, value for scintillation events increases. . . . . . . . . . . . . . . . .
18
122
7-4
Mean total, late, and prompt photoelectron values for events with Fp <
0.6 during the first five hours of runs 281- 301. . . . . . . . . . . . . .
7-5
123
Sum of PMT waveforms with Fp < 0.6 collected during the first 11
minutes of run 281, normalized to unit integral. The triplet time constant is 1.58 ps, determined by fitting with equation 7.1 . . . . . . . . 124
7-6
Decay of the triplet time constant with time. . . . . . . . . . . . . . . 124
7-7
PE, Fp, and R 3 /R
B
distributions for tagged
22Na
events, overlaid
on events anti-coincident with the tag. The tagged and untagged distributions are very similar, indicating there was a problem with the
tag........
7-8
126
......................................
Reconstructed charge centroid coordinates for data taken when the
22 Na
source was present and not present. Distributions are normalized
to unity. The source is located in a divot in the inner vessel, drilled at
(cos(0)
7-9
=
0.85, #
2.1).
. . . . . . . . . . . . . . . . . . . . . . . . .
An afterpulsing event with 1000 PE, Fp = 0.08, R 3 /R}PB
=
127
0.979.
Top: all PMT waveforms. Left: the summed waveform on top, with
the brightest PMT (WFD 9, Ch.
5) below.
The afterpulse occurs
~1 us after the primary pulse. Right: event display with brightest
PMT in center foreground. . . . . . . . . . . . . . . . . . . . . . . . . 129
7-10 PMTs with greatest charge for high-energy, low-Fp events. PMT 2 was
prone to discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
130
7-11 A PMT discharge event with 735 PE, Fp = 0.12, R 3 /RPB = 0.996.
Top: all PMT waveforms. Left: the summed waveform on top, with
the brightest PMT (WFD 1, Channel 2) below. Both summed and individual waveforms have amplitude
-
-500 mV. Right: event display
with brightest PMT in center foreground . . . . . . . . . . . . . . . .
19
131
7-12 An baseline sag event with 5286 PE, Fp = 0.13, R 3 /RP
Top: all PMT waveforms.
=0.004.
The baseline sag occurs around 6000 ns,
with subsequent rebound after 8000 ns. Left: the summed waveform
on top, with the brightest PMT (WFD 7, Ch. 4) below. Right: event
. . . . . . . . . . .
display with brightest PMT in center foreground.
133
7-13 Fp vs R 3 /R3PB distribution for events with more than 500 PE. The
cluster of events with Fp < 0.3 and R 3 /4PB
= 1
occur outside the
TPB radius and induce one of the instrumental effects described in the
text. .........
134
....................................
7-14 Fp vs PE distributions for the first six hours of runs 281-301.
The
group of events circled in the top left panel migrate to higher Fp and
lower PE values with time, due to the increasing water vapor content
in the argon gas which quenches the late triplet scintillation light. . .
7-15 An alpha event with 2568 PE, F = 0.37, R 3 /R3B = 0.55.
136
Top:
all PMT waveforms. Notice the high occupancy. Left: the summed
waveform on top, with the brightest PMT (WFD 4, Channel 1) below. The summed waveform has amplitude -6000 mV, the individual
PMT -1200 mV. Right: event display with brightest PMT in center
foreground. PMTs with greater charge have larger area polygons.
. .
137
7-16 (Top) R 3 /4PB vs PE distribution for 2000 simulated alpha events
which deposit 5.3 MeV in the argon gas along the surface of the TPB.
The argon scintillation yield has been set to 1800 photons per MeV
in order to match the PE yield of 1500 observed during the first hour
of run 281 (see Fig. 7-19). All the simulated events reconstruct away
from the TPB surface with R 3 /RiPB < 0.7. Of the 2000 simulated
events, 1960 cause an event trigger, and 35 generate less than 800 PEs.
(Bottom) Projection onto the PE axis.
The cluster of events near
600 PE originate near or in front of the baffle, which absorbs the UV
scintillation light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
20
7-17 Distributions of Fp values for alpha events which satisfy the centroid
and PE cuts, for several hours of runs 281-301. The fit function is a
gaussian plus a constant. The instrumental effects described in the
text cause the events to fall outside the central Fp value. . . . . . . .
140
7-18 Summary of the fitted mean and variance values for the Fp distributions
shown in Figure 7-17. The mean variance, averaged over is 0.024.
. .
141
7-19 Photoelectron distributions for events with R 3 /R}rPB > 0.96 during the
first three hours of run 281. Each panel contains at least one hour of
live time, and is labeled by the time since the beginning of the gas fill.
The vertical lines represent the position of the "by-eye" cut described
in section 7.5.1. The solid circles are surface alpha events which induce
an instrument-effect, identified by their Fp value which falls outside a
range defined in Figure 7-18. . . . . . . . . . . . . . . . . . . . . . . . 142
7-20 Photoelectron distributions several hours after the gas fill. The hour
labels are the time since the fill. . . . . . . . . . . . . . . . . . . . . .
143
7-21 Average photoelectron yield for alpha events which do not induce an
instrument effect, fit with a sum of three exponentials.
. . . . . . . .
144
7-22 Photoelectron cut. The solid line is the alpha PE yield multiplied by
half and is used to determine the alpha rate. The dashed line was
used to select alpha events to accept for the PE yield determination.
The dashed line is a fit to the points, which were identified by-eye for
several PE spectra where there was an obvious gap between the falling
gamma spectrum and alpha accumulation, for example the first two
hours of Figures 7-19. . . . . . . . . . . . . . . . . . . . . . . . . . . .
145
7-23 The photoelectron distribution for vacuum events, and the integral of
this distribution above the PE cut of Figure 7-22. . . . . . . . . . . .
21
146
7-24 Surface alpha rates. (Top) Runs 106-115, started Jan. 29 2014, with
17.1 hours livetime.
(Middle) Runs 215-232, started Feb. 14, with
63.2 hours livetime. (Bottom) Runs 281-301, started Feb. 28, with
53.6 hours livetime.
runs is 42.9
The livetime-weighted mean rate for the three
0.7 events per hour. . . . . . . . . . . . . . . . . . . . .
147
7-25 Distribution of PMTs with greatest charge for alpha scintillation events.
The exposure time for each cassette during assembly is represented by
the blue line, with an overall normalization determined by the integral
of the measured rate. . . . . . . . . . . . . . . . . . . . . . . . . . . .
148
7-26 Photograph taken during assembly of the inner vessel, looking into the
vessel before cassette installation was complete.
. . . . . . . . . . . .
148
7-27 Argon gas light yield for alpha emission following neutron capture
on boron film, from [98].
The emitted alpha has energy of 2.79 or
2.31 MeV, with branching ratios of 6% and 94%, respectively. The measurement does not extend to MiniCLEAN's 180 kPa pressure. However,
the plateau beyond 60 kPa suggests the light yield remains constant at
5600 photons per decay above the limit of the measurement.
This
corresponds to 2400 photons per MeV of energy deposition. . . . . . .
8-1
Schematic of 21OPo decays to alpha particle and
20 6Pb
150
near the wavelength-
shifting surface, drawn to approximate scale. The measured alpha rate,
represented on the left, receives contributions from parent nuclei on the
TPB-argon interface, in the TPB, on the TPB-acrylic interface, and
up to
-
10 pm depth in the acrylic. Class-I and II events can mimic a
dark m atter signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8-2
Reconstructed radius distribution for simulated class-I surface events,
displaying the effects of several cuts.
8-3
151
. . . . . . . . . . . . . . . . . .
154
(Left) Event origins along the TPB surface for simulated type-I events.
(Right) Origins for events which reconstruct with R < 29.5 cm. . .. .
22
155
8-4
Reconstructed radius using the Shellfit algorithm vs the charge centroid
m ethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
8-5
PE threshold required to ensure zero-background fiducial mass, for
various class-I type surface rates.
8-6
. . . . . . . . . . . . . . . . . . . .
157
Number of class-II surface events per year which leak into the WIMP
ROI with 150 kg fiducial volume.
8-8
156
Number of class-I surface events per year which leak into the WIMP
ROI with 150 kg fiducial volume.
8-7
. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .
157
Projected 90% confidence SI WIMP-nucleon cross-section limits for
MiniCLEAN. All except the red curve assume 6 PE/keV. All except
the green curve assume 150 kg fiducial volume. The blue and red curves
assume a PE threshold of 120; the black-dashed curve assumes 95 PE
threshold and 15 background events.
. . . . . . . . . . . . . . . . . .
159
A-1 Signal response of PMTs vs temperature from [106]. The R1221 and
R649 (triangles) have multialkali photocathodes, while the rest have
bialkali photocathodes and exhibit steep response degradation below
about - 100C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162
A-2 Photograph of several 8-inch Hamamatsu photomultipliers. The two
tubes on the left have lost vacuum, which causes evaporation of the
brown photocathode and the tubes' clear appearance compared to the
SNO (R1408) tube. The increased opacity of the platinum-coated tube
can be seen in the duller appearance of the dead R5912-02MOD compared to the R5912. MiniCLEAN uses the frosted variety of R591202M O D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
163
A-3 MiniCLEAN R5912-02MOD base schematic.
Positive bias voltage
(- 1100 V at TLAr) is applied across the PMT lead to ground.
Sig-
nal appears as negative pulses ( 100 mV) on the bias voltage. Back
termination can be seen in the 50Q-4.7nF path from PMT lead to
ground. Not shown is the bias-tee circuit which baseline-subtracts the
signal for input to the digitizers. . . . . . . . . . . . . . . . . . . . . .
A-4 Single photoelectron pulses at ~
TLAr
164
for bases with (blue) and without
(red) back termination. Ringing seen in the red trace is damped by
the back term ination. . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
A-5 Diagram and picture of cold gas cryostat for PMT testing. The picture shows the closed setup, with only the outer vessel of the dewar
visible. The PMT is housed in the stainless inner vessel where it can
be covered by nitrogen or neon gas. Low-mass cable from Gore is used
for electrical connections inside the dewar, while RG58 provides the
rest of the connection to the DAQ (CAEN V1720 digitizer) and power
supply (HV). Light from a YAG laser is sent to the PMT with optical
fiber. Not depicted in the diagram is the piping for gas handling.
.
.
166
A-6 (Left) Single photoelectron charge spectrum for PMT 26 at 950 V (top)
and 1000 V (bottom), at room temperature. (Right) Gain and noise
measurements vs voltage for PMT 108 at 298 K (top) and 90 K (bottom). The bias voltages which produce 5 pC gain are 1043 V and 1031 V 167
A-7 Gain and noise results vs temperature for PMT. The top panel shows
the operating voltage required to obtain 5 pC gain. The middle panel
shows the rate of noise pulses with charge greater than 3/4 single photoelectron, at the operating voltage from the top panel. The bottom
panel shows the gain at 950 V operating voltage. . . . . . . . . . . . . 168
A-8 Time separation between dark pulses. The non-Poisson distribution,
with a peak near time zero, suggests a mechanism other than thermal
emission causes dark pulses at cryogenic temperatures.
24
. . . . . . . .
169
A-9 Examples of single photoelectron pulses at liquid argon temperature.
The left (right) panels are classified as DLN (TLN). An ADC count
corresponds to 2V/2 12 = 4.88mV. . . . . . . . . . . . . . . . . . . . . 169
A-10 Reduced y2 distributions for Eqn. A.1 fit to PMT calibration data
taken at liquid argon temperature.
Each pulse is classified as DLN,
TLN, or multiple PE (upper right). . . . . . . . . . . . . . . . . . . .
170
A-11 Pulse time distributions for the different catagories of pulses, weighted
by their relative probability. . . . . . . . . . . . . . . . . . . . . . . . 170
A-12 Charge distributions for the different categories of pulses, weighted by
their relative probability. . . . . . . . . . . . . . . . . . . . . . . . . . 170
A-13 Simulated light collection vs reflectivity of light guide walls, based on
10 eV betas at detector center. . . . . . . . . . . . . . . . . . . . . . .
171
A-14 Measured reflectivities of coatings from two companies, JDecker Industries (red) and Silvex (blue). These are calibrated measurements
performed by Angstrom Sun Technologies (Acton, MA) using a Varian
Cary-500 spectrometer and, for total reflectivity, an integrating sphere.
The silver-based coating from JDecker is proprietary; the Silvex sample
is plated with silver per ASTM B700 Type 2 Grade C Class S. . .. .
172
A-15 Measured reflectivities at several angles of JDecker coating, 3M ESR
film, and an Al/SiO front-face mirror. These uncalibrated measurements were made with a Cary-500 and VASRA attachment for angular
manipulation. The result is that the JDecker coating performs better
than the ESR and Al/SiO at all angles. The lack of calibrated reference
mirror prevents a quantitative statement from being made. . . . . . .
25
173
A-16 Example determination of o for thin acrylic and energy 100-105 photoelectrons. (Left) F, for simulated electron and WIMP events, using
100 ns.
(Right) Contamination and difference between median
values of FP6 and Fpx, as a function of . The left (right) arrow indicates the value of
which minimizes electronic recoil contamination
(maximizes the difference between median Fp values).
o is chosen to
minimize contamination, and for this energy window takes the value
140 ns. ........
...................................
175
A-17 o for thin acrylic as a function of energy expressed in photoelectrons.
The average value 135 ns minimizes contamination.
. . . . . . . . . .
176
A-18 PMT neutron background as a function of reconstructed photoelectron
number, before cuts, then with FI > 0.7 and R < 29.5 cm cuts. After
cuts, 60.0 and 26.5 neutrons per year reconstruct in the energy region
of interest 75-150 PEs for thin (5mm) and 10 cm acrylic. The results
are normalized to 40000 neutrons per year generated in the PMT glass. 176
A-19 (Red) Number of neutrons per year which reconstruct in the energy
region of interest 75-150 PEs after F, and R cuts, as a function of
acrylic length. (Blue) Percentage light loss relative to thin acrylic as a
function of acrylic length. The collaboration chose to use 10 cm length
acrylic, coincident by chance with the cross on this plot.
26
. . . . . . .
177
A-20 (Top) Jeff Griego's (LANL) mechanical drawing of the spool from two
perspectives, showing a standard conflat on the left and inverted conflat
on the right. (Middle) Photograph of an IV port's inverted conflat,
without a spool attached. The inside of the IV can be seen through
the port. (Bottom) Close-up photograph of spool attached to IV. In
the foreground (toward the outside edges of the photo) is shown a
standard conflat, with bolt-holes outside the radius of the knife-edge,
where the tophat is to be attached. The black heads of bolts fastening
the spool to the IV via the inverted conflat are in the photo's middle
ground. The pictures were taken at Winchester during test-assembly
for pressure vessel certification.
. . . . . . . . . . . . . . . . . . . . .
A-21 Cassette test stand schematic (top).
180
This stand was conceived to
benchmark the optical properties of a full cassette in liquid argon, with
light guide, reflector, PMT, and TPB-coated acrylic. The schematic
shows the configuration used to purify gaseous argon and condense it
with liquid nitrogen providing cooling via a small condensing canister
attached to the main vessel. Fine temperature control of the condenser
is accomplished by pressurizing with nitrogen gas. During the inverted
seal test, assembled as depicted on the lower right, helium was substituted for argon, and a helium leak sniffer was attached to the vacuum
shroud.................
... .....
....
A-22 (Left) Alistair Butcher (RHUL) cleaning bolt holes.
.....
. . ..
.
181
The blue and
white wipes were used to keep track of which holes had been marginally
cleaned. (Right) Comparison of bolts before and after electropolishing. 182
A-23 X-ray fluorescence measurement of tape lifts from an Inconel bolt with
scaling and from an electropolished Inconel bolt. The listed metals are
all present in Inconel 718 alloy. The scaling is likely high-temperature
oxidation which occurred during annealing. . . . . . . . . . . . . . . .
27
182
28
List of Tables
1.1
Spin values for some relevant nuclides.
2.1
Scintillation parameters for three noble liquids. Results for the prompt
. . . . . . . . . . . . . . . . .
40
and late time constants, Leff, and F. are from the cited references. The
other parameters are assembled in [65]. Below the given lower electronequivalent energy bounds, the measurements for electronic and nuclear
Fp tend toward equal central values. The light yields for Fp measurements in neon and argon were 3.5 and 4.85 photoelectrons per keVee,
respectively. (*) Measurement from [64] was made with fission fragments. 49
3.1
Gamma fluxes from norite, measured during the installation of SNO
with a NaI(Tl) detector and various thicknesses of lead. "The calculations are based on neutron capture in the elements of norite with
neutron flux predicted from the measured Th and U concentrations in
the rock." From the SNOLAB User's Handbook, [76]. . . . . . . . . .
23
23 2 Th,
40 K
3.2
Summary of
3.3
Gammas generated per year by the PMT glass and steel of the inner
U,
and
gamma properties.
. . . . . . . . .
and outer vessels. Assays reported in mBq/kg, from [82]. . . . . . . .
3.4
64
66
68
Summary of background sources for MiniCLEAN and their reduction
via energy, fiducial volume, Fp, and F, cuts, derived from simulation
and tabulated in the internal document [85]. . . . . . . . . . . . . . .
5.1
71
Values for the parameters in equation 5.4 derived from measurements
of muon flux at several underground sites.
29
. . . . . . . . . . . . . . .
89
30
Chapter 1
Direct Dark Matter Detection
1.1
Introduction
We hold that the movement of celestial bodies is governed by the law of gravitation,
developed first by Isaac Newton in the 17th century and modified by Albert Einstein
in the early 20th. The power of physical laws derives from their utility in making
predictions. A dramatic success story of Newtonian gravity, among many, was the
discovery in 1846 of the planet Neptune [1]. Urbain Joseph Le Verrier inferred Neptune's position from its pull on the planet Uranus, whose orbit about the Sun was
known to deviate slightly from the Newtonian expectation. Le Verrier sent his prediction for Neptune's location to Johann Gottfried Galle at the Berlin Observatory,
who immediately observed the planet. Encouraged by his success, Le Verrier later
proposed the existence of another planet, Vulcan, to explain the anomalous precession
of Mercury's orbit. Vulcan, however, was never observed. Mercury's orbit was incorporated into our understanding by Einstein, who derived its anomalous precession as
a consequence of his modified law of gravitation, the general theory of relativity, in
1916 [2].
In the 1930s Fritz Zwicky uncovered another tension between theory and observation, this time beyond our solar system: the galaxies in the Coma Cluster orbited each
other at velocities too high to be gravitationally bound
[3].
This effect was confirmed
later for individual galaxies, notably by Vera Rubin and her collaborators in the
31
1970s and 80s [4, 5]. Figure 1-1, from Rubin, Ford, and Thonnard's influential paper
of 1980 [5], depicts the rotation curves for 21 galaxies with large spirals (Sc galaxies).
Rotational velocity is determined by measuring the redshift of Hot-lines, which have
extreme red wavelength. The measured line-of-sight velocity is converted to rotational velocity by projection onto the galactic plane, using the assumption that the
emission regions are moving in planar circular orbits about the center of their galaxy.
Remarkably, none of rotation curves fall-off with distance from the galactic center,
even at the faint outer extent of the optical images. The expectation for galaxies
comprised of visible mass is that the rotation curve falls off at large distances.
0
5
10
15
10
200
20
30
200
100
-
NOC 1421
NGC 4062
NGC 4321
NGC 2742
NGC 701
C-
100 km s-
NGC 3672
II
NGC 1035
E
kpc
21 Sc GALAXIES
NGC 4605
0
40
NGC 2715
_,-I
IC 467
NGC 2608
NGC 7541
NGC 3495
0
LIJ
NGC 7664
q
z
NGC 1087
-J
UGC 3691
z
NGC 482
NGC 2998
NGC 753
-I
0
NGC 801
LIi
200
UGC 2885
-
100
0
1.A
0
DISTANCE
10
FROM
20
30
NUCLEUS
40
(kpc)
50
0 60100 120
EH- 50 km s' Mpc9
Figure 1-1: Mean velocities in the plane of the galaxy, as a function of linear distance
from the nucleus for 21 Sc (spiral) galaxies, arranged according to increasing linear
radius. From [5]. None of the curves fall off at large radii, contrary to the expectation
for galaxies comprised of visible mass.
32
Following Le Verrier in his prediction of a massive, unseen planet, the implication
taken by Rubin et al.
in [5] was that "Sc galaxies of all luminosities must have
significant mass located beyond the optical image."
However, modified theories of
gravity were proposed, with modified force-laws [6, 7] or dynamics [8], to account
for the anomaly. The notion that most of the mass in a galaxy is invisible can be
disquieting, and Rubin herself came to express aesthetic preference for a modified
theory of gravity in 2005 [9].
Figure 1-2: A composite optical, X-ray (red), and weak lensing (blue) image of the
Bullet Cluster (lE 0657-558) [10].
Compelling evidence for the existence of non-luminous, weakly-interacting matter
came in 2006 with gravitational lensing observations of the Bullet Cluster [11]. Figure
1-2 shows the now-famous composite image of the two galaxy clusters which collided
100 million years ago. The image shows that the massive parts of the two clusters
continued along their original trajectories after the collision, while their x-ray emitting
gas clouds were violently deformed. The decoupling of the majority of the mass (in
blue) from the baryonic matter (in red) implies that the bulk of the matter in the
clusters interacts weakly with itself and the known particle constituents of the gas
33
clouds. Here is direct evidence for the existence of unidentified "dark matter", which
makes up most of the mass of galaxies.
The presence of stable, particle dark matter in large quantities fits well with our
current understanding of the development of the universe from the Big Bang. A cosmological model must account for the structure of the cosmic microwave background;
the abundance of the light elements hydrogen, helium, deuterium, and lithium; the
distribution of galaxies on large scales; and the accelerating expansion of the universe.
The six-parameter ACDM model is the simplest successful model. It is based upon a
spatially-flat, expanding universe with dynamics governed by general relativity and
constituents dominated by cold dark matter (CDM) and a cosmological constant (A)
at the current time. The parameters of the model, including the dark energy, baryonic matter, and dark matter density of the universe, can be extracted from analysis
of the cosmic microwave background (CMB) temperature, which has been measured
by three generations of space missions since the early 90s. Most recently, the Planck
Collaboration [12] has found that matter constitutes 31.5-1.% of the energy density
of the universe, with (84.5)
2.4% of this matter required to be non-relativistic (cold),
pressureless, and non-interacting (dark).
There is an overwhelming body of evidence that the dark matter required by
cosmology cannot be a particle of the Standard Model
[13, 14].
Theoretical candidates
for dark matter include primordial black holes or massive compact halo objects (for
example, [15]), axions [16], and weakly-interacting massive particles (WIMPs) [17].
Of these, the WIMP has been most popular, since a cosmologically stable particle
with mass 10 GeV-1 TeV and weak-nuclear-force-scale interactions would freeze-out
during universe expansion with about the right relic density which we observe today in
the CMB data. Supersymmetric extensions of the Standard Model provide a variety
of WIMP candidates like the sneutrino and the neutralino [17].
Despite the success of the dark matter model in providing a consistent framework
for understanding diverse astrophysical observations, all the widely-accepted evidence
for its existence is gravitational, and its particle nature remains unknown. Efforts
to characterize dark matter particles fall into three categories: indirect detection
34
experiments which aim to detect the products of dark matter annihilation or decay;
dark matter production at accelerator facilities such as the Large Hadron Collider;
and direct detection of dark matter scattering in dedicated, underground detectors.
Dark matter annihilation or decay to high-energy photons at the center of our galaxy
is a promising route to indirect detection, and recent analysis of data from the Fermi
telescope indicates there may indeed an excess of such photons coming from the
galactic center [18]. Another method of indirect detection is to observe the effect of
dark matter annihilation early in the history of the universe on the CMB temperature
and polarization anisotropies [19].
Efforts in this direction have not produced a
positive signal [19].
Searches at the LHC, where the signature is large missing transverse momentum,
have to date been negative [20], and are setting interaction cross-section limits comparable to contemporary direct detection experiments (see Figure 1-5). Direct searches
are the topic of the following sections, which survey direct detection principles and
techniques. Then in Chapter 2 comes description of the MiniCLEAN dark matter
detector that is the subject of this paper.
1.2
Dark Matter Halo Parameters
Dark matter preserved the primordial fluctuations in cosmological density on galactic
scales that were wiped out in baryonic matter by viscosity, when radiation decoupled
from baryons hundreds of thousands of years after the big bang. Gravitational collapse
produces dark matter halos, which provide most of the gravitation for formation of
stable structures in the universe. Unlike the baryonic components of galaxies, which
are supported from radial collapse by angular momentum, dark halos are supported
by random velocity which serves as collisionless pressure. The simplest halo model is
spherical and isothermal, with a Maxwellian velocity distribution for the constituent
WIMPs.
For consistent interpretation of experimental results, most direct detection experiments assume the simplest halo model for our galaxy, following the standard reference
35
of Lewin and Smith [21]. The halo parameters of interest are those that determine the
dark matter energy distribution and flux onto a terrestrial detector: the local dark
matter density PD, the dispersion velocity of dark matter vo, and the escape velocity
vesc above which the dark matter velocity distribution is truncated. A halo density
which falls off as r- 2 (where r is the radial coordinate with origin at the galaxy center) generates the flat rotation curve observed in many spiral galaxies. Estimates for
PD based on this spherical density profile and the Milky Way rotation curve (Figure
1-3) have been in the range 0.2 GeV < PD < 0.4 GeV, leading to adoption by many
dark matter experiments of the central value PD
0.3 GeV. Use of more realistic
halo models which include flattening, or structures like rings in the galactic plane,
can cause PD to change by as much as a factor of four (for example, [22]).
For an isothermal, isotropic halo in hydrostatic equilibrium and with the Maxwellian
dark matter velocity distribution f(v) eM/v, Drukier
0 et al. [23] use the equation
of hydrostatic balance to show that vo = vc, the circular velocity of the solar system about the galaxy center. The calculation assumes vo and v, are independent of
galactic radius. Lewin and Smith [21] use vo = 230 km -s-'
20 km -s-1.
Drukier et al. [23] note that v.c is bounded below by the highest observed stellar
velocity, 583 km - s-1, and estimate a local upper bound of 625 km - s-1. The standard
value chosen by Lewin and Smith [21] is v... = 600 km -s-1.
1.3
Basics of Terrestrial Scattering
A WIMP of mass M. and kinetic energy E. which scatters elastically from a nucleus
with mass MA deposits energy
Er = 2E
MAMX
S(MA + MX) 2
(1 - cos(6))
(1.1)
where E, and 0 are the energy and scattering angle of the struck nucleus in the
center-of-momentum frame. Neglecting the motion of our solar system through the
dark matter halo, the recoil spectrum of the stuck nucleus is obtained by folding the
36
300
200
200
0
0 R,=8 5
X
pc
200
E 4
10
e g os1
2
Galactaocentric
Radius
[
kpc]
7.1 kpc,
Figure 1-3: From [24]: the rotation curve for the Milky Way for values of R0
1
22Okm -s- , where R0 is the distance
8.5 kpc, ve
and R0
oc=185 kmn - s,
to the galactic center and 'vc is the circular velocity of our solar system about the
galactic center. The figure also shows one of the ways in which the rotation curve
can be decomposed into the contributions from different mass components: the bulge
(dotted line); the stellar disc (filled circles); the H 1 layer (crosses, where negative
values mean that the force is directed outwards); the H 2 layer (circles); and the
dark halo (dashed line). The best-fitting model, which is obtained by summing the
individual components in quadrature, is shown as a full line.
Maxwellian kinetic energy distribution with equation 1.1. This gives an exponentially
falling recoil spectrum with the form dR/dE oc e-Er/Eor, where the average recoil
energy E0 =
M0vo and
(ile
(dtedln);te tlards
r
=4M
IIlyr
icls;th
MA / (My +i MA)2
cosewhr
egtv
(1.2)
As typical targets have atomic masses of several tens of GeV, detectors with keV
energy threshold are required. Another difficulty is the low rate of interaction, which
may be cast conveniently normalized:
_
(1.3)
NOP-D
A M,
37
405 (GeV
A
k Mx )
PD
(_____
0.3 GeV - cm-
oo
(230 km- s-1
counts
pb
kg - day
where A is the atomic number of the target, No the Avogadro number, and go the
'zero-momentum transfer' dark matter interaction cross-section per nucleus.1
1.4
Setting Scattering Cross-section Limits
Experimental efforts to detect WIMPs center on reducing background events in a
detector large enough to register the rare low-energy nuclear scattering predicted by
equation 1.4. A WIMP of mass 1 GeV and weak-scale cross-section ao=1 pb would
interact several times per day per kg of detector mass. When detectors fail to register
any WIMP scattering, an upper limit on ao may be set as a function of Mx. The
first dark matter experiments (of the late 1980s) set upper limits for co near 10-' pb,
while current limits extend to 10-9 pb (see section 1.6). In other words, contemporary
experiments seek to register about one dark matter scattering event per 100 kg per
year of exposure. Sources for such low-energy scattering events abound, and must be
carefully eliminated by reducing radioactive content within the detector and shielding
of external radiation incident on the detector. The characteristic nuclear scattering
of WIMPs must also typically be discriminated from more common electronic recoils.
1.5
Comparing Results of Different Experiments
In order to compare the limits obtained with different detector materials, a model
for WIMP-nucleus scattering is needed from extensions of the Standard Model. In
these extensions, the standard coupling of a WIMP proceeds via a scalar current
to the mass of a nucleus (known as the spin-independent interaction, SI), or via an
axial vector current to the spin of a nucleus (spin-dependent interaction, SD). The
omits suppression of the interaction rate due to the form-factor of the target nucleus. The
expression for R omits corrections accounting for detector response, including energy detection
efficiency, resolution, and threshold. These corrections are detailed in the standard reference of
1oo
Lewin and Smith[21].
38
distinction generates a natural division between SI detectors using high-A nuclei and
SD detectors using nuclei with unpaired nucleon spins. Groups conducting SI searches
typically report an upper limit on a WIMP-nucleon cross-section o-ule, which, for a
target nucleus with atomic number A, amounts to [17, 25]:
2
2
where PA and pnre are the reduced masses of the WIMP-nucleus and WIMP-nucleon
systems. For experiments with more than one type of nucleus as target, the total
cross-section a-i is obtained by averaging across all targets, obtaining 1/o-e
The normalization procedure for SD limits is more complicated. An SD experiment reports limits on WIMP-proton or WIMP-neutron cross-sections oSD. This
requires input of experimentally-determined values for the proton and/or neutron
spin expectation values (Sp,n) for the target nuclide(s). The theoretical models for
SD scattering are more varied than for SI, leading to further complexity. One method
by Giuliani [26] that has come into use 2 is to calculate, for each sensitive nuclide in
the target molecule, one of the following cross-sections:
/ J+1(S,,n)
(1.6)
os(A)
where
u,,n
3
are the reduced masses of the WIMP-proton and WIMP-nucleon systems,
and J is the total nuclear spin. The total cross-section may then be reported as in the
SI case: 1/o,2 = ZA [1/ SD(A)].
Reference spin values from
[26] for some relevant
nuclides are given in Table 1.1.
1.6
Detector Technologies
Dark matter searches have utilized three types of signatures to detect nuclear scattering: ionization, scintillation, and heat. A number of experiments use a combination
2
For example, by PICASSO [27], XENON100 [28], and CDMS [29].
39
Nucleus
19F
Z
9
J
1/2
(Sp)
0.441
(Sn)
-0.109
23
Na
11
3/2
0.248
0.020
27
Al
13
17
17
53
54
5/2
3/2
3/2
5/2
1/2
0.343
-0.059
-0.178
0.309
.0.028
0.030
0.011
0
0.075
0.359
3 5 C1
37
CI
127I
129
Xe
Table 1.1: Spin values for some relevant nuclides.
of two of these, to discriminate electronic from nuclear recoils.
Ionization: The first dark matter detectors used germanium diodes, carefully fabricated for low radioactivity, at liquid nitrogen temperature.
electronic vs nuclear recoils can be done with Ge diodes.
old is set by microphonic and electronic noise.
No discrimination of
The low-energy thresh-
Crystal masses for these detectors
ranged from about 250 g to 3 kg. Typical thresholds were a few keV. The event rates
at threshold were a few counts/keV/kg/day.
Notable experiments using Ge diodes
were the collaborations at Homestake (1987[30]) and Oroville (1988[31]), the Hei-
delberg/Moscow group (1994[32], 1998[33]), and the IGEX collaboration (2000[34]).
These groups obtained upper limits on oa
in the range of 10-4 to 10-
pb for
Mx = 60 GeV. More recently, the CoGeNT collaboration deployed a 440 g P-type
point-contact Ge detector at the Soudan Underground Laboratory.
CoGeNT had
a 2keV threshold and used pulse shape for discrimination against events occurring
near the edges of the crystal.
CoGeNT reported an excess of events over expected
backgrounds near threshold, and an annual modulation of the excess consistent with
ic ~1 x 10-4 pb for Mx
=
10 GeV
[35].
The modulation was subsequently found
to be an order of magnitude too high for realistic halo models, and inconsistent with
the lack of modulation seen in CDMS-II [36].
In another effort to exploit the ionization signature, several groups are pursuing
low-pressure gas time-projection chamber (TPC) technology as a means to measure
the energy and direction of nuclear recoils. This is motivated by the observation that
WIMP-induced nuclear recoils should be preferentially opposed to the direction of the
40
Sun's motion about the center of the galaxy, which changes considerably for points on
the surface of the Earth over a 24-hour period [37]. One practical difficulty with this
approach is the large volume of low-pressure gas required to obtain a significant target
mass. These experiments therefore use nuclei with large expected SD, rather than
SI, coupling. CF 4 gas is usually used, with
19F
providing the WIMP target. Energy
thresholds in these detectors are several tens of keV, below which a nuclear recoil
track is too short to be reconstructed with directional ("head-tail") sense. Groups
which have recently set dark matter limits using TPCs underground include DRIFTIld (2012[38]) and NEWAGE (2010[39]) for CF 4 . Other collaborations developing
TPCs are DMTPC [40] and MIMAC [41].
Scintillation: Ionizing radiation in crystals like Nal, CsI, and CaF, or the noble
liquids and gases Xe, Ar, and Ne, induce scintillation photons which may be detected
with photomultiplier tubes or more efficient semiconductor photodiodes.
Nuclear
recoils typically develop light pulses with shorter decay time constants than electron
recoils, allowing discrimination. Light collection efficiencies range from 2-8 photons
per keV. For example, the NAIAD experiment in Boulby mine (2003) used thalliumdoped Nal crystals to set an upper limit on osi of about 10-6 pb for a WIMP of
mass Mx = 60 GeV [42]. This result excluded the positive annual modulation signal
reported by DAMA in 2000 [43], which was using the same type of scintillator crystal
in the underground Gran Sasso National Lab. The "annual modulation signal" is
based on the observation that the WIMP flux onto Earth should be greater when the
Earth's velocity around the Sun is in the same direction as the Sun's rotation around
the galaxy. At the time, DAMA's result favored the existence of a WIMP with M. =
52 GeV and ouc = 7 x 10-6 pb. The modulation signal has persisted since then, but
is in extreme tension with contemporary cross-section limits.
The scintillation light from noble liquids and gases has also been used for dark
matter detection. The first collaboration to use a single-phase noble liquid was ZeplinI, operated in the UK Boulby Mine in 2001 and 2002 [44]. This detector held liquid
Xenon with 3.1 kg fiducial volume, viewed by three photomultipliers through silica
windows. The light yield was 2.5 photoelectrons per keV, and the low-energy analysis
41
threshold was 2 keV. With 293 kg - d of exposure the collaboration set the limit Us
<
1 x 10-6 pb for Mx ~ 80 GeV. This is, to date, the only dark matter limit set with a
single-phase liquid noble detector. Two collaborations are currently installing singlephase liquid argon detectors at SNOLAB, including the MiniCLEAN detector which
is the subject of this paper and described in Chapter 2.
The more common type of liquid noble experiment is the dual-phase detector,
which operates on the principles of a time-projection chamber. The target is a noble liquid, which produces scintillation upon energy deposition. An electric field is
applied across the target, causing ionization electrons to drift though the liquid toward the anode where the noble is in gaseous state. An avalanche of electrons is
produced as the ionization electrons cross the liquid surface into the gas, generating a secondary scintillation signal as electroluminescence. Discrimination between
electronic and nuclear recoils is provided by the ratio of prompt to late scintillation
signals, and accurate position reconstruction can be accomplished along the direction of the electric field. A variety of liquid Xenon experiments have obtained dark
matter limits over the years, including Zeplin-I in 2007 [45], Zeplin-III (2009[46]),
LUX (2014[47]), XENON-10 (2008[48]), and XENON-100 (2012[49]) which currently
holds the strongest SI cross-section limit for higher mass WIMPs. Dual-phase liquid
argon detectors have been operated by the WARP [50] and DarkSide [51] collaborations, with DarkSide recently obtaining a WIMP-nucleon SI cross-section limit of
6.1 x 10-4 cm2 for a WIMP mass of 100 GeV.
Cryogenic bolometers are sensitive thermometers, including the associated heat
absorber, operated at temperatures of several mK.
Scattering events in crystals
like sapphire (A1 2 0 3 ), Ge, LiF, TeO 2 , and Si, produce phonons which are absorbed
with near 100% efficiency by a superconducting film evaporated onto the surface of
the crystal.
The film serves as a thermometer.
The detector is operated within
the superconducting-to-normal transition of the film, so that a small temperature
rise AT produces a relatively large rise AR of its resistance. The resistance of the
film is determined by passing a reference current through the film and a SQUID
in parallel. A positive AR produces an increased current in the SQUID, which is
42
measured. For example, the CRESST experiment [52] used 262 g of saphire crystals
with superconducting tungsten film, installed in Gran Sasso Underground Laboratory.
The energy threshold was 580 eV, and energy resolution was about 300 eV at 1.5 keV.
In 2000, the group published both SI and SD limits, with o-
<
1
x
10-3
pb for
Mx = 60 GeV.
Other bolometer experiments have used heat measurement in combination with
scintillation or ionization to discriminate electron vs nuclear scatters. CRESST-II
[53] used the scintillating crystal CaWO 4 . The setup was as in CRESST, with a large
crystal serving as the phonon channel, but with an additional small crystal serving as
the light detection channel, all encased in reflective housing. Since the light yield for
nuclear recoils is smaller than for electronic recoils, the ratio of light to total energy
provides discrimination. The use of scintillation light, however, pushes the low-energy
threshold higher (to a few keV), since the conversion of energy to scintillation is less
efficient than conversion to phonons. In 2012 CRESST-II reported an exponentially
rising nuclear recoil spectrum at low energies which they interpreted to be consistent
with a 10 to 30 GeV WIMP with o-SI in the range of 1 x 10-5 pb [53].
As with
CoGeNT, this result is inconsistent with XENON-100 and CDMS-II limits.
CDMS [54] and EDELWEISS [55] are bolometer experiments which collected ionization charge via electrodes electrolithically patterned onto their crystals. The ionization yield is lower for nuclear recoils than electronic, so that the ratio of ionization
yield to phonon yield again can be used for discrimination. In 2011 the EDELWEISS
collaboration reported results obtained with 4kg of Ge installed at the Laboratoire
Souterrain de Modane. With 384 kg -d of exposure they set an upper-limit
4.4 x 10-8pb for M. = 85 GeV [55]. CDMS used 19 Ge (~
-SI <
230 g each) and 11 Si (~
105 g each) crystals installed at the Soudan Underground Laboratory with 398 kg - d
of exposure to set the limit o-i
< 6.6 x 10-8 pb for M. = 60 GeV, reported in 2008
[54].
Superheated liquid detectors are based on the technique of the classic bubble chambers. Energy deposition in a superheated liquid nucleates a gas bubble, provided that
the energy deposit and energy loss per unit path distance exceed thermodynamically43
defined minima.
These conditions allow tuning of the chamber, via pressure and
temperature adjustment, such that the detector is sensitive to nuclear recoils but
largely blind to electronic recoils. Bubble nucleation is accompanied by an acoustic sound wave which can be recorded along with optical data. Alpha recoils have
been found to produce louder acoustic emissions than nuclear recoils [56, 57]. Eventby-event energy measurement is not possible (although the threshold is adjustable).
There have been two types of technical realizations: the COUPP bubble chambers,
and the PICASSO and SIMPLE superheated droplet detectors. The COUPP experiment operated with 3.5kg of CF 3 I in a shallow underground site for four months
in 2009 [57], and is now running a 60 kg vessel at SNOLAB. As with the old bubble
chambers, the vessel must be compressed after every event to recondense the vapor.
The process took a full minute for the smaller vessel, allowing 28.1 kg -d of effective
exposure in the four month period. At the shallow site, the collaboration set an SD
limit better than 0.01 pb for a 60 GeV WIMP incident on the high-spin proton of
fluorine.
The superheated droplet detectors (SDDs) consist of liquid droplets (diameter ~
10-300pm) suspended in a viscous polymer or aqueous gel [58]. Apart from occasional
recompression periods, SDDs can operate continuously. Acoustic data collection is
done with piezoelectric transducers. There are two experiments employing SDDs:
the PICASSO experiment at SNOLAB, which uses C 4 F10 , and SIMPLE at LSSB (in
France) which uses C 2 CIF5 . PICASSO has set SD limits for a 24 GeV WIMP on
protons oSD < 0.16pb and neutrons
SD
< 2.60 pb, with 13.75 kg -d (active mass
134g) of exposure in 2009 [27]. SIMPLE set somewhat stronger limits in 2010 with
similar exposure [59].
Figures 1-4 and 1-5 show current competive WIMP-nucleon cross-section limits
from direct detection experiments, projections for future experiments, and limits from
the CMS collaboration at the Large Hadron Collider. Also displayed in Figure 1-4 is
the neutrino-nucleus coherent scattering limit, which is the lowest spin independent
WIMP-nucleon limit that can be set before neutrino scattering becomes an irreducible
background to a WIMP signal.
44
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46
Chapter 2
The MiniCLEAN Detector
Figure 2-1: Photograph of the assembled MiniCLEAN inner vessel at SNOLAB.
The Cryogenic Low Energy Astrophysics with Noble Liquids (CLEAN) detector
concept uses scintillation light from liquid argon or liquid neon to register nuclear
recoils in a spherical target mass with 47r photomultiplier tube (PMT) coverage. The
MiniCLEAN detector, pictured in Figure 2-1, holds 500 kg of liquid argon in the active
detector region, viewed by 92 PMTs. Assembly of the detector is currently (Winter
2014-15) ongoing at SNOLAB, an underground science facility near Sudbury, Ontario
47
which provides a 2 km rock overburden (6000 m water-equivalent) for reduction of
cosmic rays. Radioactive backgrounds in MiniCLEAN are mitigated through pulse
shape discrimination and event-position reconstruction. The detector design allows
exchange of liquid neon for the argon. This target exchange can be used to verify a
putative dark matter signal by confirming the correct A 2 dependence of the signal
rate, while maintaining very similar sources of background. This chapter reviews the
physics of scintillation in noble liquids before describing the MiniCLEAN detector and
its assembly. The next chapter summarizes the sources of radioactive backgrounds
for the dark matter search and their reduction through analysis techniques.
2.1
Scintillation in Noble Liquids
Ionizing radiation in noble liquids produces excited diatomic molecules (excimers)
which emit scintillation upon decay. The wavelength of scintillation is too low to cause
atomic excitation, making the liquid largely transparent to the light. The singlet and
triplet excimers have different lifetimes, and are excited in ratios dependent on the
type of ionizing energy deposition. Thus the relative amplitudes of fast singlet and
slow triplet components in a scintillation pulse can be used to determine the type of
scatter that produced the pulse. This is called pulse shape discrimination (PSD).
Excimers may be produced in two ways [66].
Free excitons generated by the
scattering event can combine with ground states, X* + X
-4
X*, where X is any
noble liquid and X2* is the excimer. Alternatively, free ions may undergo collision
(X+ + X
-
Xf), recombination (X2 + e ->
(X** a X* + heat, X* + X
-a
X** + X), and deexcitation processes
X2). The excimers then decay radiatively from the
lowest-excited molecular states 'E+ and 'E+ to the repulsive ground state 1 E+. The
triplet
3E+
must undergo a forbidden spin flip to decay, extending its lifetime beyond
that of the singlet 1E2.
Nuclear recoils produce less light than electronic recoils of the same energy. This
effect is known as quenching, and is characterized by the parameter Lepf which is the
ratio of light yields for nuclear to electronic recoils of a given energy. Quenching is
48
Parameter
Light yield (photons/keV)
Prompt time constant (ns)
Late time constant
Cef f (above keVr)
Electronic Fp (above keVee)
Nuclear Fp (above keVee)
Peak wavelength (nm)
Raleigh scattering length (cm)
Density (g - cm- 3 )
Boiling point (K)
Ne[60]
25
18.2
14.9 ps
0.24 (50)
0.15 (50)
0.3-0.4 (50)
77
60
Ar[61, 62]
40
7
1.5 ps
0.25 (20)
0.28 (32)
0.7 (32)
128
90
Xe[63, 64]
42
2.2
21 ns
0.19 (10)
0.23
0.61(*)
174
30
1.20
1.40
2.95
27.1
87.3
165.1
Table 2.1: Scintillation parameters for three noble liquids. Results for the prompt and
late time constants, Cef , and Fp are from the cited references. The other parameters
are assembled in [65]. Below the given lower electron-equivalent energy bounds, the
measurements for electronic and nuclear Fp tend toward equal central values. The
light yields for Fp measurements in neon and argon were 3.5 and 4.85 photoelectrons
per keVee, respectively. (*) Measurement from [64] was made with fission fragments.
caused by several mechanisms. Nuclear recoil energy generates atomic motion, reducing the contribution to atomic excitation or ionization. The quantitative description
of this mechanism, proposed by Lindhard [67], poorly describes data from xenon scattering experiments [63]; therefore an additional mechanism, described by Hitachi [68],
is thought to contribute. Here, collisions between excitons result in production of an
ion and a ground state atom. The rate increases for higher exciton densities, leading
to higher quenching for higher energy density deposition. Several measurements of
Leff vs energy for nuclear scattering in xenon are shown in Figure 2-2, from [63],
along with the parameterization of [68]. Average values for Leff in xenon, argon,
and neon are listed in Table 2.1, for recoils above certain energies. The rapid rise
in stopping power for recoils below a few tens of keV leads to sensitive dependence
of Leff on energy, and measurement is complicated by trigger effects and decreased
energy resolution. Recent measurements of Leff for argon and neon as a function of
recoil energy are displayed in Figures 2-3 and 2-4.
The slow triplet scintillation light is further suppressed relative to the singlet by
destructive triplet molecular interactions, primarily the Penning process [66]. This
49
0. 5
0.4
0.3
0.2--
0.1
01
10
102
Nuclear Recoil Energy [keV]
Figure 2-2: Measured Leff for liquid xenon as a function of nuclear recoil energy, from
[63]. The open circles represent that work's measurement; other markers are from
previous measurements and cited in [63]. The solid line is from a best-fit analysis of
XENON10 AmBe source data and Monte Carlo, and the dashed line in the theoretical
prediction of Hitachi [68].
occurs when two excimers collide to form one excimer and one ground state. Again,
the effect increases with excimer density, resulting in less triplet light for nuclear
recoils than electronic recoils. This allows discrimination through a simple parameter
Fp, the ratio of prompt to total light in a pulse. Nuclear recoils have higher Fp than
electronic recoils. Table 2.1 lists prompt fractions for electronic and nuclear recoils
in neon, argon, and xenon.
To express scattering energy without reference to the type of scatter, dark matter
experimenters use the unit keVee where the subscript means "electron-equivalent".
This is distinguished from the unit keVr which refers specifically to the recoil energy.
Thus, for nuclear recoils, E[keVee] = E[keVr] x Leff. In liquid argon, for example,
where the quenching factor Leff
=
0.25, a nuclear recoil event which deposits 200 keV,
of energy produces 50 keVee of scintillation light.
2.1.1
Light detection: tetraphenyl butadiene
Noble liquids scintillate in the ultraviolet. Traditional borosilicate glass PMTs cannot
detect UV light, since the photons are rapidly attenuated in the bulb.
In liquid
xenon, synthetic silica bulbs may be used. However, the scintillation wavelengths of
50
*
O0.5
micro-CLEAN
Mean O.25
0.4
0.01 .0.01
0.3
0.2
0.1
50
100
150
200
250
Energy (keVr)
Figure 2-3: Measurement of Lejf for liquid argon as a function of nuclear recoil
energy, made with the micro-CLEAN detector [691. The rapid rise below 20 keV, is
from trigger effects and not physical.
.
0.5
+ MicroCLEAN result
+ PicoCLEAN result
S0.45
0.4
- Lindhard+Birks model (SRIM)
-Lindhard+Birks model (DM)
0.35
0.3
0.25
0.2
-1
0.15
0.1
0.05
C
C
50
100
150 200
250 300
350 400 450
Energy (keVr)
Figure 2-4: Measurement of Leff for liquid neon as a function of nuclear recoil energy, from [701. Solid lines represent fits to quenching models based on different ion
stopping-power models from SRIM [71] and the reference [66] of Mei et al.
argon and neon are too short even for synthetic silica, necessitating the use either
of avalanche photo-diodes, or of a wavelength-shifting fluor which absorbs the UV
light and reemits visible wavelengths which are detected with traditional PMTs.'
The commonly-used fluor is an organic molecule, tetraphenyl butadiene (TPB). TPB
may be applied to acrylic or glass using vacuum vapor deposition, or by mixing
with polystyrene and a solvent like toluene and dipping or painting. Light yield and
durability of the coating are the primary concerns. Reference [73] of Gehman et al.
found no difference in conversion efficiency between the two application methods.
'More exotic PMT bulbs may also be used like MgF 2 , but these are difficult to fabricate.
51
.lumination
E 0.014
Waveleith:
128 M
-
160 n
0.01
So.oos0.004
---
0.002
350
500
450
400
550
W00
Wavelength [nm]
Figure 2-5: Normalized visible reemission spectrum from a TPB film illuminated with
various UV wavelengths. From [72].
For 130 nm light (close to liquid argon's peak at 128 nm), this efficiency is 1.2. The
greater than unity efficiency is possible since the reemitted photons are lower energy
than those absorbed. The fluorescence spectrum is independent of UV illumination
wavelength, as shown in Figure 2-5. The spatial flux distribution of TPB fluorescence
is an outstanding question, and is hypothesized in [72] to be Lambertian. The time
constant for TPB fluorescence is thought to be
1 ns. 2
electrons
nuclear recoils
10-2----
Alpha in TPB
10.4
10-5
1
102
10
3
10
1e
photon tlm.(nsj
Figure 2-6: Photon emission time probability density functions for electronic and
nuclear recoils in liquid argon and alpha scintillation in TPB.
TPB also scintillates when ionized. MiniCLEAN's sister collaboration DEAP3600
has studied alpha-induced scintillation of TPB to improve understanding of surface
backgrounds, discussed in section 3.2.4. Reference
2
[75]
of Pollmann et al. found
Although the reference [74] of November 2014 finds an intermediate time component.
52
that vacuum-deposited TPB films emit 882
210 photons per MeV of alpha energy
deposition. The decay profile is characterized by a double exponential with time components 11
5 ns and 275
10 ns. Figure 2-6 displays the scintillation time profiles
for electrons and neutrons in liquid argon and alphas in TPB. The unique time profile of TPB motivates the introduction of the discrimination variable F", which is the
fraction of the waveform's total charge in the intermediate time region 12.9 - 775.4 ns.
The mean value of F, for an alpha scintillating in TPB is 0.6, while nuclear recoils in
liquid argon have (F,) = 0.2. This quantity is useful for identifying the alpha decay
of radon progeny on TPB-coated surfaces.
2.2
MiniCLEAN Design
Calibration
Portsx
Top Hat
SOptical
PMT
Outer--Vessel
Spool
Target
Volume
Light
Guide
Inner
Acrylic
Vessel
plug
Figure 2-7: (Left) SolidWorks model of the MiniCLEAN detector by Jeff Griego
(LANL). The liquid argon or neon (LAr, LNe) is contained in the cold inner vessel
(radius 75 cm), which is housed by the evacuated outer vessel. One of the 92 optical
modules is depicted on the right.
Figure 2-7 shows models of the primary detector components. The liquid argon
or neon is contained in the spherical inner vessel. The inner vessel has 92 circular
ports, to which the optical modules are attached. An optical module consists of a
cryogenic photomultiplier tube, and a faceted, reflective light guide which supports
a 10 cm acrylic plug with TPB applied on the inner surface (Fig. 2-8). The TPB53
coated acrylic faces form the "wavelength-shifting sphere" (WLS) at radius 43.5 cm
from detector center. The WLS is a 92-sided truncated icosahedron assembled with
12 pentagon-shaped acrylic faces, 20 regular hexagons, and 60 irregular hexagons.
The WLS defines the active central region of the detector, where argon scintillation
is able to be absorbed by the TPB and re-emitted at optical wavelengths. The inner
vessel is contained by the outer vessel which is put under vacuum to provide thermal
insulation between the two components. The whole detector is submerged in a tank
of water (Figures 2-9, 2-10), which provides shielding from external radiation. The
water shield is instrumented with 48 PMTs to detect the Cherenkov light generated
by through-going muons. Argon scintillation events coincident with muon detection
are vetoed, since muons interacting with rock or water generate cascades of neutrons
and other spallation products which might mimic a WIMP signal.
Figure 2-8: Components of an optical module. (Left) An 8-inch Hamamatsu R591202MOD PMT attached to a tophat. (Center) An irregular-hexagon type light guide.
The guide is shown before electropolishing and without the reflective inner surface,
which is 3M ESR foil. (Right) An irregular-hexagon acrylic plug, illuminated on the
far side with a UV LED so that the blue fluoresence of the TPB can be seen.
54
SNOLAB Deck
Outer Vessel
&
Inner Vessel
Optical Cassettes
Muon Veto PMTs
Support Stand
Figure 2-9: Model of the central detector housed in the muon veto, a 7.9 m tall
by 2.8 m radius water tank instrumented with 42 8-inch Hamamatsu R1408 PMTs.
Figure by Jeff Griego.
Figure 2-10: (Left) Photograph of the Cube Hall in SNOLAB, looking down onto the
water tanks for MiniCLEAN (right) and DEAP3600 (left). The right photograph is
taken from the bottom of the MiniCLEAN water tank, looking up to the (empty)
outer vessel and beyond to the Cube Hall and yellow gantry crane.
55
2.2.1
Assembly of inner vessel
Preparation of the IV for optical module insertion, including cleaning and attachment
of the spools (see Fig. 2-7), was done in the Cryopit cavern at SNOLAB in a class 500
soft-walled cleanroom (Figure 2-11). Particle counts in the cleanroom were typically
below 100 0.5 prm per m3 . All detector components were ultra-sonically cleaned and/or
wiped with methanol before being sealed in bags and stored on the Cryopit floor to
await assembly onto the vessel. After spool attachment, the IV ports were sealed
with temporary polycarbonate covers, and the vessel was tested for leaks using a
helium leak-sniffer. Then commenced insertion of the optical modules, also called
Figure 2-11: (Left) Photograph from the top of the Cryopit, looking down 50 feet onto
the temporary soft-wall cleanroom used for IV assembly. Detector components are
in bags stacked along the right rear wall. (Right) The IV in the soft-wall cleanroom
with some spools and their clear polycarbonate covers attached.
cassettes. Protection of the TPB-coated acrylic from physical abrasion and exposure
to radon was the primary consideration during cassette assembly, to reduce the surface
backgrounds discussed in section 3.2.4. To mitigate radon contamination into the
vessel, the cleanroom was modified to accept compressed air from surface as input
rather than the ambient air of the lab. Surface air has radon content 1-2 pCi/l, a
factor of 2-4 less than the air in the lab. To prevent diffusion of air into the IV while
a port was opened, the IV was overpressured with liquid nitrogen boil-off. Since radon
has a lower boiling point than nitrogen, it tends to remain in the liquid. The boil-off
radon concentration is estimated to be 1 x 10- pCi/l. During periods of inactivity
(for example, overnight), the IV was pumped down to further mitigate radon exposure
56
of the inner surfaces. The acrylic plug remained in its nylon bag while the PMT was
attached to the tophat and the reflective foil was arranged in the light guide, which
had been previously assigned a position and orientation in the IV (Figure 2-12). Then
the plug was removed from its bag and inserted into the light guide using suction cups
on the surfaces without TPB. The polycarbonate cover was detached from the port,
allowing the complete light guide to be inserted and attached to the inner flange of
the spool. Then the tophat with its PMT was sealed to the outer flange of the spool.
.
The radon concentration in the cleanroom was continuously monitored with a RAD73
This allowed identification of time periods where radon levels were higher than usual,
for instance during a lab shutdown when mining operations disrupted the air supply.
Figure 2-13 is a photograph of the complete assembled IV in the cleanroom in March
2014, with gas processing and data acquisition systems configured for vacuum and
argon gas data collection. Analysis of this data is the subject of Chapters 6 and 7.
2.2.2
Gas processing system
The purification system shown in Figures 2-14 and 2-15 was assembled to allow data
collection for warm argon gas. The SAES getter, which uses a heated zirconium
alloy to capture impurity molecules, reduces 02, H 2 0, CO, C0 2 , N 2 , H2 , and CH4
to less than part-per-billion levels. These molecules degrade the scintillation yield by
quenching argon excimers or absorbing the UV photons. The cold activated charcoal
trap removes radon. Activated charcoal is carbon which is processed to have high
surface area, onto which solid radon adsorbs. The trap is immersed in a temporary
liquid-nitrogen-cooled ethanol slurry to maintain the temperature near 160 K, below
the freezing point of radon (202K) and above the freezing point of argon (84K).
Use of the slurry was a work-around employed while a more permanent refrigerator
was unavailable. Function of the purification system was verified with a residual gas
analyzer.
The purification system will remain essentially unchanged during the liquid argon
3
Made by Durridge (Billerica, MA), the RAD7 is a sniffer which collects alpha-emitters electrostatically and uses a silicon detector to identify the alpha decays of 2 18Po and 2 4 po.
57
Figure 2-12: (Left) Photograph looking into an IV port during the "dry-fit" of the
light guides, when each guide was inserted to check for fit and to fix its orientation
for final installation, before being removed again for assembly with reflective foil and
the acrylic plug. Note the guides do not abut along their edges, but have several
millimeter gaps through which scintillation light could penetrate the central detector
from outside the wavelength-shifting radius. Minimization of these gap sizes motivated tight tolerances on IV dimensions, which was a driving factor in fabrication
cost. (Right) Photograph looking into the IV during final cassette insertion. The
TPB-coated acrylic faces are outlined by reflective flaps which cover the gaps between guides. The flaps were made by creasing the reflective foil lining the light
guide walls.
dark matter search. Cooling will be accomplished via two cold heads bonded to the
IV with the heat sink provided by a Gifford-McMahon type single-stage refrigerator,
a Cryomech Model AL325. A SolidWorks model of the cold head connection is shown
in Figure 2-16.
two months.
Condensation of the full argon mass is expected to take almost
A critical question to be addressed during the cold run is whether
recirculation of the argon is necessary to maintain purity requirements. This would
necessitate addition of a recirculation pump to the process system.
58
Figure 2-13: Photograph of the assembled IV in the softwall cleanroom, with DAQ
racks and gas bottles.
Burst
To
dl.c
Currently
vent Onm
W"
plastic
tubing
GV360
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o
s
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nlv
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--------------
n :
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te: ANl
og s eV H
gas
vources arm
located Outsude SWCR
Rev:
Draw |
ig: MCLACryops
RGvsH
no, Thomas Caldwell, Natalia Guerrero
H
Date: 1/31/2014
Flowsheet
Figure 2-14: Diagram of the MiniCLEAN gas system for purification of warm argon
gas.
59
Figure 2-15: Photograph of the partial purification system. The SAES getter is the
grey box on the right, with the multi-meter sitting atop. To the right of the getter is
the charcoal trap submerged in the dewar of ethanol slurry.
Figure 2-16: SolidWorks model of the cold head attachments from James Nikkel
(RHUL). The cold head is housed in a nitrogen-filled canister attached to the OV,
with helium lines extending out to the compressor on deck. The cold "finger" extends
into the insulation vacuum space, where it is bonded by copper braids to cold fingers
which penetrate the IV.
60
2.2.3
Data acquisition
The purpose of the MiniCLEAN DAQ is to digitize the waveforms from the central
detector's 92 PMTs. CAEN V1720 waveform digitizers (WFDs) were chosen for this
task. The V1720 is an 8 channel module with 12-bit resolution over 2 V peak-to-peak
input and 250 MHz sampling rate. The adjustable length of the digitized waveform is
set to 16 ps, ten times the slow scintillation time constant for argon. Twelve WFDs
are used to collect data from the central 92 PMTs, while one WFD is allocated for the
48 PMTs of the veto (see section 5.2.2 for description of veto electronics). The rate
of data written to disk may be reduced through hardware zero-suppression, where
the CAEN digitizers omit empty sections of the waveform, and a software reduction,
where a decision is made in software to save only summary data for events outside
the dark matter region of interest.
Figure 2-17 shows a diagram of the DAQ system. Bias voltage for the PMTs is
supplied via a custom HV block, which consists of the 92 bias-tee circuits required
to pick-off the PMT signal from the single cable supplying bias. The HV block also
supplies pulse injection signals for timing-calibration. PMT signals are sent to the
12 WFDs, which are controlled in groups of three by four front-end PCs (FEPCs).
Each WFD outputs an internal hit-sum, which are passed to NIM analogue summing
and discrimination electronics to generate an event trigger based on the number of hit
PMTs (NHIT trigger). The trigger is passed to the VENATOR module (V1720 Event
Number And Trigger ORganizer), an FPGA-based module which controls triggering
of the digitizers and sets the trigger pattern, event time, and event number. Partial
events are read out over optical link by the four FEPCs, and pulse charge and time
information is sent over network to a data reduction PC (DRPC). The level of data
reduction is decided by the DRPC, which sends the decision back to the FEPCs for
processing. The FEPCs send the data to the event builder PC (EBPC) for writing
to disk. The target NHIT threshold is five, and the baseline rate is 600 Hz.
61
kVM
"
(D"im
Naot
Figure 2-17: Diagram (from Boston University) and picture of the DAQ configuration
for warm argon gas data. This configuration does riot incorporate the veto system,
which will be present for liquid argon data taking.
62
Chapter 3
Radioactive Backgrounds
Reduction of scattering events which would mimic a WIMP signal is the primary
challenge of any dark matter experiment. Background events may be separated into
two classes based on the origin of the scattering particle: external, for sources outside
MiniCLEAN's water tank; and internal, for sources within the detector.
3.1
External Backgrounds
External sources of background for MiniCLEAN are gammas from radioactivity in
the cavern rock and fast neutrons induced by muon spallation in the rock. During
the experiment's data taking, these will be shielded by the active water-tank veto,
which provides a minimum 150 cm water barrier. The gamma flux from the rock was
measured during the installation of SNO and is tabulated in Table 3.1. Gammas with
.
energy greater than 4.5 MeV are emitted from the cavern at a rate of ~ 1000 m- 2 -d- 1
,
Since the absorption length for few-MeV gammas on hydrogen is about log. cm- 2
the minimum 150 cm of water shielding in the radial coordinate reduces the gamma
flux by a factor of e
15
to ~ 3 /m 2 /year. The actual rate will be lower since the path
length for a rock gamma through the water to the outer vessel is greater than 150 cm
for most angles of incidence.
The fast neutron flux at SNOLAB, determined by FLUKA simulation in [77], is
1.7 /m 2 /year. Detailed simulation described in Section 5.3.2 shows this flux is reduced
63
Measured Flux
y m- 2 - d-'
510 + 200
360 220
180 + 90
< 20
(MeV)
4.5-5
5-7
> 7
> 8
Calculated Flux
m-2 - d-1
320
250
15
Table 3.1: Gamma fluxes from norite, measured during the installation of SNO with
a NaI(Tl) detector and various thicknesses of lead. "The calculations are based on
neutron capture in the elements of norite with neutron flux predicted from the measured Th and U concentrations in the rock." From the SNOLAB User's Handbook,
[761.
to much less than unity by the water shielding.
3.2
Internal Backgrounds
The dominant backgrounds in MiniCLEAN originate inside the detector:
" beta decays of the naturally occurring isotope
39 Ar,
" gammas from 238U/ 23 2Th contamination in the borosilicate glass bulbs of the
PMTs and IV/OV steel,
" fast neutrons from (a,n) processes in the PMT bulbs and IV/OV steel,
" alpha decays from radon daughters deposited on the wavelength-shifting surface
and
238 U/ 23 2 Th
contamination of the TPB and acrylic.
The collaboration aims to eliminate these backgrounds in a fiducial argon mass of
150 kg and an energy region of interest corresponding to 75-150 photoelectrons.
3.2.1
3 9Ar
The radioactive isotope
39 Ar
is a beta-emitter with half-life 269 years and cndpoint
energy 565 keV. It is produced in the atmosphere through cosmic ray activity on 40Ar,
and in the ground through neutron capture on
64
39 K
or alpha emission by calcium.
39Ar
is present in natural argon at a level of 8 parts in
zero
39
1016,
or 1 Bq . kg-
1
. Thus to obtain
Ar events in 150 kg fiducial argon volume over one year requires discrimination
Analytic model
1W5
Data
1W
.1-
10
C
0
-
better than parts in a billion.The DEAP-1 experiment indicated this discrimination
102
I
I0
0
0-2
0.8
0.6
O.A
FPMpr projection at 120 to 240 pc
I
101
10
103
-- DEAP-1 data, 2Nay's
101
Statist"ca
10,5
...... ...
....... ....
model
--Model with no pe noise
S10-
108
10
101
10-1
01
.
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fpmmpt
Figure 3-1: (Top) Comparison of DEAP-1 2 2 Na data and the collaboration's analytic
model in the region 120-240 photoelectrons (approximately 43-86 keVee). (Bottom)
Pleak distribution for the same data, showing the analytic model with and without
PMT noise included. The lower curve shows the expected backgrounds from high-F,
events, mostly cosmogenic neutrons. For 50% nuclear recoil acceptance, the statistical
model with noise projects pulse shape discrimination better than parts in 10 billion.
Figures from [78].
level may be possible [78]. The detector consisted of two PMTs viewing a 7 kg mass
of liquid argon, and was run until 2009 on surface at Queen's University. Using a
tagged
22 Na
source to identify scattering of a 511 keV photon in the liquid argon,
the collaboration found the fraction of mis-identified electron recoil events less than
6 - 10-
for scattering energies between 43 and 86 keV. This result was statistically
65
238
U
2 32
Th
40 K
# -y
El.w
Ehigh
5.3
3.8
0.11
46.5 keV
40 keV
2.20 MeV
2.62 MeV
Table 3.2: Summary of
2 38U, 23 2 Th,
and
Most intense
795 keV (99%)
2.62 MeV (99%)
1.46MeV (11%)
40 K
gamma properties.
limited by nuclear recoils induced by cosmic-ray generated neutrons, but may be
extrapolated to greater discrimination levels using a simple analytical model. The
result is shown in Figure 3-1. The variable Pleak is defined as
Pleak (F')
= fF(x)dx(3.1)
fo' Fp (x) dx
for a given Fp distribution, and represents the pulse shape discrimination.
Lower energy electronic recoil events are harder to discriminate against, since random fluctuations in scintillation photon production time convolved with low efficiency
for conversion to photoelectrons wipe out the power of Fp as a discrimination variable.
The energy threshold for the MiniCLEAN dark matter search will be set as
low as possible while still maintaining the effectiveness of Fp to eliminate the billions
of
39
Ar decays per year. To this end, the collaboration has investigated a likelihood
method of electronic recoil discrimination, which uses individual photoelectron ar-
rival times rather than the simple integrated charge of F>. This likelihood method
has performed well in application to calibration data from the DEAP-1 detector [79].
Simulation indicates an energy threshold as low as 75 photoelectrons may be possible
for MiniCLEAN [80]. It is described in Chapter 4.
3.2.2
Gammas
High energy photons are produced during the relaxation of alpha-emitters in the
primordial
23
8U and 23 2Th decay chains, and also by decay of an excited state of 4 0 Ar
formed by electron capture on 40K. These gammas may Compton scatter in the liquid
argon and must be discriminated against. Table 3.2 gives some properties of these
66
gammas, and Figures 3-2 show the line intensities for
238U
and 2 32 Th. Assay results for
the PMT glass and steel are listed in Table 3.3 along with the corresponding gamma
rates, which total 29 billion per year. These must be reduced at a level comparable
to
39
Ar.
F05
01
=10-1
10-2
10-
3
10-4 k5
10-
1.5
0.5
2
2.5
MeV
2
2.5
MeV
|aTh
2:1 V_
_c10-1-
1 02
V~H
10-31111
0
0.5
1]
1.5
1
1
Figure 3-2: Intensity of prominent gamma lines in the 2 38U and 232 Th decay chains
(assuming equilibrium), derived from a listing given in a project proposal by the
Majorana neutrinoless double-beta decay experiment [81].
3.2.3
Fast neutrons
The dominant source of neutrons in the inner detector is (a,n) reactions in the borosilicate glass of the PMTs.
238U
and
232 Th
67
are present in the PMTs at 0.103 and
#
PMT glass
IV steel
OV steel
Mass (kg)
66
4600
2800
23 8 U
1274
2.72
0.92
2 32
Th
691
7.46
3.52
"K
3523
8.57
< 3.65
-y per year
20 x 109
6.3 x 109
2.7 x 109
Table 3.3: Gammas generated per year by the PMT glass and steel of the inner and
outer vessels. Assays reported in mBq/kg, from [82].
0.170 ppm, determined by assay of a broken tube. In reference [83], Mei et al. calculate neutron production yields for various elements as a function of
2
38U
and
232 Th
contamination. These calculations predict 42000 neutrons per year originate in the
66kg of PMT glass. Alpha reactions with B 2 0 3 (SiO 2 ) account for 90% (10%) of the
total. A similar calculation for the steel IV and OV predicts a neutron yield of 1800
per year. The energy spectrum for PMT neutrons is shown in Figure 3-3.
I
10
C
z
10,
101
I
1
2
3
4
5
6
7
8
Neutron Energy (MeV)
Figure 3-3: (oz,n) neutron energy spectrum, for 0.103 ppm 238U and 0.170 ppm
23 2Th contamination in borosilicate glass.
Generated using the online tool
http://neutronyield.usd.edu, based on [83].
Neutrons incident on the target volume must be reduced by shielding, which is
provided by the 20 cm of liquid argon and 10 cm of acrylic in each light guide. The
hydrogen-rich acrylic is a better moderator than liquid argon, but attenuates light
more strongly.
The 10 cm acrylic length was chosen to balance these competing
effects, quantified in simulations described in Section A.3. These simulations indicate
shielding and fiducialization may reduce the number of neutron events in the dark
68
matter region of interest to order unity.
3.2.4
Radon progeny on wavelength shifter
TPB
-LAr
Fiducial Volume Cut
Rf = 29.5 cm
Mf = 150 kg
Rate x 1/1000
101
01
0
50
100
150
200
250
300
350
400
reconstructed radius (mm)
Figure 3-4: (Left) Scenarios for the surface decay of 210Po, depicted as a black dot, to
an alpha particle and 20 Pb nucleus, which could mimic a dark matter signal. (Right)
Example of the fiducial volume cut on surface events.
The alpha decays of radon daughters, which have plated out onto the TPB surface
during TPB-deposition or detector assembly, constitute another background to the
dark matter search. Primordial
2 38
U and
2 32
Th contamination in the bulk of the
acrylic and TPB may also contribute. This is represented in Figure 3-4, where the
decay of
210Po
produces a 5.3 MeV alpha particle and 100 keV
20
Pb nucleus. These
decay products deposit energy in some combination of the acrylic, TPB, and liquid
argon.
Surface events which fall in the WIMP energy region-of-interest, projected to be
75 - 150 PE, are identified using the discrimination variable F, and position reconstruction. The scintillation time profile of TPB in response to ionization is distinct
from liquid argon's (see section 2.1.1), motivating the definition of F, as the fraction
of the PMT waveform's total charge in the intermediate time region 13 - 775 ns. The
mean value of F, for alpha-induced TPB scintillation is 0.6, while nuclear recoils in
liquid argon have (F0 ) = 0.2. F, is an effective discrimination variable for surface
events which deposit energy exclusively in the TPB.
The scintillation yield of TPB (~
900 optical photons/MeVr) is much lower than
69
liquid argon's (- 48000 optical photons/MeVee).
For surface events which eject a
daughter nucleus into the argon and an alpha through the TPB into the acrylic (the
bottom case in Figure 3-4), F, is a less effective discrimination variable and position
reconstruction becomes more important. The right panel of Figure 3-4 displays a
simulated reconstructed radial distribution for surface events. MiniCLEAN uses a
likelihood method for position reconstruction, described in the next chapter, with
-
a projected resolution of 6 cm. Simulation indicates that surface events with 75
150 PE and origin on the TPB-argon interface are reduced by a factor of
-
1000 by
requiring reconstructed radius less than 29.5cm.
Deposition tests described in [84] indicate that several hours exposure of acrylic to
air with 10 Bq -m- 2 -d-
1
radon activity, a typical value for ambient air in SNOLAB,
results in surface activity of ~ la per m2 per day (Figure 3-5).
Desire to limit
exposure of the TPB surface to lab air motivated the assembly procedure described
in section 2.2.1: the assembly cleanroom was kept overpressured with low-radon air
from surface, and the IV was kept overpressured with liquid nitrogen boiloff during
200
E
E
a-0
X
w
0
1 a m~ 2
0.1 a m~
day
day
"
cassette insertion and pumped out during periods of inactivity.
150
100
50
0.01
0.1
1
10
Radon Concentration [Bq m-]
100
Figure 3-5: From [84]: an example radon exposure requirement to achieve a desired
surface activity of 0.1 and 1.0 a decay per m2 per day due to 210 Po on acrylic.
70
Background Source
Intrinsic
39 Ar
a in Acrylic
a at Acrylic-TPB interface
a in TPB
a at TPB-Ar interface
PMT (o,n)
Steel (a,n)
y from PMTs
y from steel
y-e Cherenkov
Cosmogenic and wall n
Raw Rate
IBq/kg
24 000 /year
10 000 /year
1000 /year
10 000 /year
42 000 /year
1840 /year
20 x 109 /year
9 x 109 /year
29 x 109 /year
3650 /year
12.5-25 keVee
R < 29.5 cm
4.2 x 108
1.2 x 108
284 4
1.0 0.5
75 t 3
3000 5
352.2 t 2.1
6 x 106
2 x 106
3500
0.08 0.01
0.02
<<
0.007
0.82 t
91.6
0.01
1
0.003
0.09
1.1
3 x 105
1 x 105
< 0.1
Fp, Fa, Lr
<1
< 1 x 10-4
< 1 x 10-4
< 1 x 10-4
0.24 t 0.05
3.8 0.02
< 0.2
< 0.08
< 0.02
< 0.1
Table 3.4: Summary of background sources for MiniCLEAN and their reduction via
energy, fiducial volume, Fp, and F, cuts, derived from simulation and tabulated in
the internal document [85].
3.3
Summary
Table 3.4 compiles the background sources for MiniCLEAN and their reduction
through energy, radius, Fp, and F cuts. The energy region of interest corresponds
to 75 - 150 PE/keV with a projected light yield of 6 PE/keV. The fiducial volume is
150 kg, corresponding to R = 29.5cm. The nuclear-recoil acceptance efficiencies for
the cuts Fp > 0.7 and F, > 0.55 are 50% and > 99%, respectively. The effect of
the likelihood method for discrimination against electronic recoils is included, represented by the variable L, ("likelihood-recoil")'. The PMT neutron background may
be further reduced with a multiple-scattering tag.
Discrimination variables are defined in detail in Chapter 4.
71
72
Chapter 4
Simulation and Analysis Software
Figure 4-1: Cutaway view of the inner MiniCLEAN detector as rendered by the RAT
simulation. The PMTs, outer vessel, and water tank are included in the geometry
but not shown.
The MiniCLEAN collaboration uses a GEANT4 and ROOT-based simulation
package called RAT, developed originally by the Braidwood collaboration [86] as a
general-use tool for photomultiplier-based detectors with scintillating targets. GEANT4
provides the detector geometry implementation (Figure 4-1) and simulation of electromagnetic and hadronic physics processes relevant for particle propagation. Scintillation and fluorescence are handled with custom code, which generates photons with
time-dependent intensity appropriate for the type and amount of energy deposition
73
in the active material. The ROOT framework is used for data processing and storage.
RAT simulates the following detector effects:
" Propagation of primary particles such as electrons, gammas, nuclear recoils,
alphas, and neutrons through the detector materials, using the physics processes
implemented by GEANT4 including electromagnetic and nuclear scattering,
ionization, Cherenkov emission, and nuclear capture.
" Generation of UV scintillation in the liquid argon in response to ionization, with
the correct yield and time-dependence for the ionizing-particle type.
" Propagation of UV and optical photons through the detector with GEANT4,
which implements transmission, reflection, refraction, and absorption at material interfaces, and Rayleigh scattering and absorption in material bulk.
" Absorption of UV photons and reemission of optical photons by the wavelength
shifting surface.
" Detection of photons by the PMTs. Optical processes for the incident photon
are handled in detail, including transmission, reflection, and absorption in the
PMT glass, photocathode, and dynode stack. A realistic pulse shape is produced, according to the model described in section A.1 which includes time,
charge, and shape variation, as well as pre-, after-, late-, and double-pulsing.
" Detector triggering and data acquisition response, including waveform digitization, zero-suppression, and data-packing. Zero-suppression refers to the functionality of the digitizer which allows storage only of those waveform-segments
which descend below a settable threshold.
Assembly of simulated events into ROOT files with physical measured quantities
then proceeds as for real data. This involves: converting the digitizer's ADCs to
voltages, determining the trigger time and aligning the time offset across channels,
baseline determination and subtraction, and waveform integration to determine event
charge and the discrimination variables Fp and F,.
74
4.1
Likelihood Method for Particle Identification
The collaboration has developed a Bayesian method for identifying the times of single
Compared to simpler methods
photoelectrons in a sampled PMT waveform [79].
like charge integration or peak counting, the method improves energy resolution and
particle identification of low-energy events in DEAP-1 calibration data and simulation
of MiniCLEAN. The application of this method's results to particle identification is
described here.
2.0-
1.5-
0
1.0
1010
o
F-rmp
Lrecolw
S
6
0.5-
0.0-
so
100
150
200
250
30
10
15
0 photodectrons (pe)
20
25
so
3
5
40
45
50
Energy Threshold (keVe
Figure 4-2: (Left) Simulated differential leakage of electron events obtained using
the discrimination variables L, and Fp. (Right) Integral leakage of 39 Ar events as a
function of energy threshold into a 150 kg fiducial volume. From [87].
With a set of detected photoelectron times for an event T, a discrimination variable L,. can be defined as a normalized log-likelihood difference, comparing the nuclear
recoil hypothesis with the electronic recoil hypothesis:
L,
1
-
(log P,(t E) - log Pe(t E)) ,
(4.1)
where m is the number of photoelectrons in the event (the size of the list T), P is
the time probability density function (PDF) for the nuclear recoil hypothesis given
an event energy E, and Pe is the time PDF for the electronic recoil hypothesis given
an event energy E. Positive values for L, are more nuclear recoil-like, and negative
values are more electronic recoil-type. The performance of L, compared to Fp for
75
identifying electronic recoils is shown in Figure 4-2.
Likelihood Method for Position Reconstruc-
4.2
tion
The ShellFit algorithm, developed by collaboration member Stan Seibert, is a likelihood method for reconstructing the event position. For an event at position rev
generating Nuv scintillation photons and an average number of photoelectrons C,
the probability density of observing charge qj in a PMT at position Fi is
M
H P (qi
L (Njv, rev)
(4.2)
C(ii,Nuv, Fev))
i=1
The value of C is found with Monte Carlo. N events are generated to determine the
average photoelectron probability E(Oig) in the expression
C (Fi, Nuv,
e
1
)=
N
1
(4.3)
E (Oi )
j=1
where
0
2j
is the angle between the emitted UV photon and the hit PMT, as shown
in Figure 4-3 along with an example of results for E (Oj). Finally, the charge PDF is
.
Visible
00
W
UV-
TPB
0
0.5
1
1.5
.2
2.5
A
0,, (radians)
Figure 4-3: (Left) Diagram showing 0. (Right) Example results for E(ij), the average
probability for the generation of a photoelectron in a PMT at location Fi after a
simulated energy deposition, indexed by j, at position re,. Plot from Stan Seibert.
76
calculated using
Poisson(0, C)
P (qj, C) =
E'
if qj = 0, i.e. PMT not hit
(4.4)
Poisson (k,C) x Pk(qi) otherwise
where Pk(qi) is the charge PDF for k photoelectrons. For events with 120 PEs, ShellFit
has a resolution in the X-direction of 6 cm in the center of the detector and 3 cm at
the edge, as shown in Figure 4-4.
100
80
60
--
40
F1
20
00
0.1
0.2
0.3
0.4
0.5
0.6
0.9
1
0.7
0.8
(True radius/439 mm)
Figure 4-4: Average reconstruction resolution along the X-axis for uniformly distributed 20 keVee events at radii ranging from the center of the detector to the TPB
surface. The resolution is defined to be the sigma of a Gaussian fit to the difference
between true and reconstructed position. From the November 2010 MiniCLEAN
proposal to the DOE [88].
77
78
Chapter 5
The MiniCLEAN Veto System
5.1
Overview
MiniCLEAN's active muon veto serves to detect through-going muons and to shield
the inner detector from neutrons and gamma rays originating in the surrounding
cavern rock. The water tank for the MiniCLEAN veto is a bottomless silo, with radius
2.8 m and height 7.9 m. These dimensions provide 1.5 m or greater water thickness
between the cavern air and outer vessel.
The veto is lined by a highly-reflective
waterproof bladder, with four holes in the bottom through which the supports for
the main detector pass into the cavern floor. Four 8-inch R1408 PMTs from SNO
are attached to each of 12 equally spaced poles which hang in the water along the
inside wall of the tank. The 48 total PMTs provide approximately 1% photocathode
coverage of the tank.
5.2
5.2.1
Hardware
PMT mounts
The PMT support structures, which attach the PMTs to poles or "strings" which
attach to the veto tank lid, were built at Bates Linear Accelerator Center. Drawings
of the 316L stainless steel mount and string are shown in Figures 5-1 and 5-2, with
79
pictures in Figure 5-3. The Buna-N rubber pads contacting the base enclosure and
bulb accommodate the variations in dimensions across the set of PMTs. The base
enclosures' and bulbs' diameters were measured to vary between 7.62 and 8.48cm
and between 20.09 and 20.37 cm. The mount has been tested successfully with one of
the larger and one of the smaller diameter PMTs. During these tests the pads were
wet with water and force was applied by hand to try to dislodge the PMT from the
mount. Nevertheless the mounts were fitted by hand to each PMT at Bates before
shipment to SNOLAB. A complete string of four PMTs was also suspended from a
crane at Bates for testing (Fig. 5-4). No test of a full string in water was performed.
0
~DESCR
0
4 PL
MiniCleo
Receptacle
AlSI Type 316L stainless steel
I
Type 316L stainless steel
4
ASI Type 316L stainless steel
2
MiniClear Veto PMT
4
AISI Type 316L stainless steel
Itainless
tee
ReceptacteS belt lop SM
2
5
3
Round Frame
MinClear Veta PMT
Receptack Frame Leg
2
4 PL
QTY.
RIP1ON
ReCle
o
Receptacle
ub Comp SM
As
AlSI
Stainless St set Pop Rivet
5
8
Stainless Steel
MASSACHUSETTS INSTTITE OF TECHNLOGY
111
WEIH N POUNDS.
3.9UNESOHRIESEIID
F
r
-ATES LINEAR ACCELERATOR CENTER
MiniClean Veto PMT
Receptacle SM
Nn
N GL E P RO JECTIO
OA
5R(
I
Figure 5-1: PMT mount drawing.
80
11 OF
11
plete Assemblies
Cm
R2
Req Wed
iMwo-V..
Figure 5-2: PMT string drawing.
Figure 5-3: Veto PMT shown in mount.
81
Figure 5-4: A complete string of four veto PMTs, suspended from a crane at Bates
lab. Pictured from left to right are the author, Bates engineer Jim Kelsey, and then
MIT post-doc Kim Palladino.
82
5.2.2
PMT testing
Sixty-six R1408 PMTs have been passed down from SNO for use in the veto. Sixtythree of these have undergone gain and dark rate measurement as a function of bias
voltage at the 12-PMT test stand at Los Alamos National Lab, which used a CAEN
V1720 and optical card with DCDAQ for data acquisition. PMTs were conditioned at
2100 V for 12 hours. After this, single photoelectron gain measurement as a function
of bias voltage was made using a blue LED flashed at low intensity, so that PMT
occupancy was several percent. Dark rates at various thresholds can be determined
by counting pulses outside a prompt window in the single PE data. Data were also
taken with LED intensity set to produce a number of PE per PMT comparable to
the average number produced during a muon event (determined from simulation to
be 27), to characterize a PMT pulse expected to be typical in the veto. Figures 5-5
through 5-7 show results for one PMT.
Channel #3
1000
'800-+
600-
400-
200-
2
4
6
8
10
12
14
Hours
Figure 5-5: Dark rate as a function of time at 2100 V for PMT PHSS. Dark pulses
are those which descend 3 mV below baseline.
The gain as a function of bias voltage is determined for each PMT by fitting with
a model that consists of three components: a Gaussian for the pedestal, a Poisson for
the charge response at the anode for an SPE generated at the photocathode which
strikes the first dynode, and an exponential for the other contributions to the charge
83
pmt3 Single PE Rate, no threshold
0.0022-
j
0.002
0.0018-
X!/NDF=221 /117
Prob. = 2.2.-08
Ag =0.076
P 0 = 0.026
-
0.0016
OW =0.17
A, = 0.002
8=4.9
I
0.0014
gain 1.3
A 2 = 0.0015
0.0012 -
0.96
01 Popt
-t Empty
Ilj tj
0.0006
0.0006
X
~~IIII
if
0.001-
Fit to prompt
1
-
0.0004
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Charge (pC)
Figure 5-6: Single photoelectron charge spectrum for PHSS at 2250 V. The fit model
is given in equation 5.1.
integral. The model has seven parameters:
- G2
-(x-PBG) 2
f(x; ABG,
PBG, 'BG, A 1 , 6, A 2 , A) = ABGe
G
u
+
A1 e
!
X
A
2
ei
(5.1)
where x is the charge produced at the anode in pC, u is a dimensionless number that
can be understood as the number of electrons produced at the first dynode
6
(5.2)
(Ndynode-1)
1.6- 10-7
and the gain in PC is given by
(scale) - (gain) = 1.6where
Ndynodc =
10~76NVnode pC
(5.3)
9 for our R1408s. Scale is a factor used to correct for an impedance
mismatch present during the LANL calibration.
The 75 Q cables attached to the
bases of the PMTs were mated at the feedthrough panel of the dark box to 50 Q
cables, which were subsequently attached to a 50 Q, digitizer. The signal loss due to
reflection at the 75-50 Q junction is 20%, corresponding to a value 0.8 for (scale).
The operating voltages for the veto PMTs are chosen so that the single photo-
84
Mllillipilliilimrilizl
gi
|
PHSS
-2.2
.
0.81
v
SNOOp. VofSgo0
Noise: 3.81 kHz
0
X
-0.7 z
21.8
x0.6
0
0
1.6
0.5
1.40
0.4
0
1.2-
x
O-0.3
0
0.8
0
0
0
-X
xNoise
x Gain
0
-
1800
1900
2000
2100
2200
2300
0
.2
2400 2500
Voltage (V)
Figure 5-7: Dark rate and gain for PHSS as a function of voltage. Pulses counted as
dark are those whose charges exceed 1.5 PE. The vertical line indicates the operating
voltage for this PMT determined at SNO, which produced gain 1.6 pC. The SNOmeasured dark rate is higher because a 1/4 PE threshold was used.
electron gain is 1.6 pC. Figure 5-8 summarizes the measured operating voltages in a
histogram. The voltages required to produce 1.6 pC gain are generally greater than
what was measured 20 years ago during SNO testing. The low gain of the tubes is
seen in Figure 5-9 which shows the ratio of the recently measured gain to the SNO
gain at the SNO operating voltage.
The lower gain of the tubes may be caused
by degradation with time of the photocathode and dynodes, which are depleted of
electrons by exposure to light.
5.2.3
Electrical connections and data acquisition
Cable used underwater at SNOLAB must fulfill the unusual requirement to be both
waterproof and fireproof. Since off-the-shelf RG59 does not exist with these properties, cable had to be custom built by Belden according to the original SNO specifications, which were passed to us through Super-K. This cable has epoxy bonding the
insulator, ground-braid shield, and outer jacket, to prevent water entering at pinholes
in the jacket from flowing along the length of the cable.
85
Operating Voltage
-
------
SNO Operating Voltage
54
3-
1A00
1700
1800
1900
2000
2100
2200
2300
2400
2500
V
Figure 5-8: Summary of the measured operating voltages for the veto PMTs. The
operating voltages are chosen to produce 1.6 pC gain.
Custom connectors which mate the 75 Q RG59 cable to the PMTs were built
by the MIT machine shop. This was necessary since the SNO R1408s have a nonstandard TNC-type jack. The connectors are a modified version of the original offthe-shelf TNC-type connector used at SNO, designed by Rich Helmer at TRIUMF to
eliminate electric discharges known as "sharkfins" due to the appearance of effected
waveforms[891. An assembly drawing of the connector is shown in Figure 5-10 along
with a picture in Figure 5-11. The essential part of the design is the lack of direct
path from central conductor to ground shield, effected by nesting the insulators of
the male and female components. The connector was never implemented at SNO,
since the sharkfin problem was resolved by refraining from degassing the water. Thus
MiniCLEAN is the first to use this connector design.
Data acquisition for the veto proceeds through a multiplexing subsystem designed
by electrical engineer Ben Buck at Bates. The system time-multiplexes the 48 PMT
signals into the veto's six allotted digitizer channels and provides an instantaneous
hit sum (NHIT) for the veto trigger. The NHIT amplitude is proportional to the
number of PMT signals descending below a settable threshold at a given time. A
discriminator applied to the NHIT signal is used to generate a veto trigger. A block
diagram of the electronics is shown in Fig. 5-12.
86
Our gain / SNO gain at SNO voltage
5l
4n
3
2--
00
0.2
0.6
0.4
1.4
1.2
1
0.8
Ratio
Figure 5-9: Ratio of measured gain to the gain measured by SNO, at SNO operating
voltage. The gain has degraded over time. The two tubes with ratio ~ 1.4 have blown
back-termination resistors, which causes the gain to double.
lo
11
UO
1A.
T6IN
lo1C
ULTOR
-1--
A1'
3
DIMS iN INCHES
"T
I
0y
.
R
R.H
3
TRIUMF
PLUG ASSEMBLY
SNO HIGH VOLTAGE CONNECTOR
SNO DETECTOR
XDE0376B
c
b
&4D PLW)
B
d1
Figure 5-10: TRIUMF connector assembly drawing.
87
Figure 5-11: TRIUMF connector for mating cable to the R1408. From top to bottom
is shown the coupling nut, the plug body into which the cable is inserted, the teflon
insulator which is also inserted into the plug body, and the pin with a stripped cable.
HV Bias
.
Delay Lines
JL.-
-ru'i
To PMT
To Digitizer
nSummer Card
Amplifier
Discriminator
Card
N-Hit
erSummer Card
Figure 5-12: Block diagram of veto electronics.
88
5.3
Simulation
The veto has been simulated in RAT versions r821 and later, using GEANT4.9.2.pOl.
A muon and cosmogenic neutron flux model based on reference [77] of Mei and Hime
was used to characterize the response of the veto to cosmic muons and to estimate
the cosmogenic neutron contribution to background for the dark matter search.
5.3.1
Cosmic muons
The SNO Collaboration has recently measured the total muon flux at SNO to be
(3.31
0.01(stat.) 0.09(sys.)) x 10-10 muons/s/cm 2 [90].
the value (3.77
This agrees at la- with
0.41) x 10-10 quoted in [77] from the earlier reference [91]. The
SNO reference [90] presents the flux as a function of slant-depth, determined from
the measurement of the muon's azimuthal angle and the flat overburden at Sudbury.
Simulations presented here use the SNO measurement for the total flux and the
angular parameterization of [77] referenced from [92] (rather than the SNO angular
distribution). This parameterization for the through-going muon flux Ith as a function
of vertical depth ho and azimuthal angle of incidence 0 is a sum of two exponentials:
Ith
(h, 0)
=
(Iie(-ho
sec(O)/Ai) -
1 2 e(-ho sec(O)/A2 )) sec(0)
(5.4)
where the fit parameters I1, I2, A 1 , and A2 are derived from the measured muon
flux at several underground sites and take the values listed in Table 5.1. Equation
Parameter
Value
1
(8.60 0.53) x 10-6 s- 1 cm- 2 sr- 1
(0.44 t 0.06) x 10-6 s- 1 cm- 2 sr-1
12
A,
0.45
0.01 kmwe
A2
0.87
0.02 kmwe
Table 5.1: Values for the parameters in equation 5.4 derived from measurements of
muon flux at several underground sites.
5.4 is plotted in Figure 5-13 for ho = 6011 mwe, the depth of SNOLAB in meters
89
water equivalent units. The cross-sectional area of the water tank as a function of
azimuth is also shown for comparison. The expected total rate of muons traversing
the MiniCLEAN water tank is 9.8
50
-
c
0.3 per day.
45-0.67
40
- -
0.4
Cro.s-.ctonal tank area
Muon flux
35
0.3
0.2
.
-
.
,
I
I
.
30
I
0.1
25
"I
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0
Figure 5-13: Muon flux as a function of azimuthal angle of incidence, from parameterization in [92] referenced from [77], and cross-sectional area of the veto water tank.
Integrating the product of the two curves gives the result 9.8 t 0.3 muons per day
incident on the veto.
Simulation of the physics of muon energy loss and Cherenkov radiation in water
is handled entirely by GEANT4. The muon losses energy primarily via ionization,
which is well-handled by GEANT4 as can be seen in the validation study for SiO 2 in
Figure 5-14. Propagation of optical photons in water is also handled by GEANT4.
Reflection of optical photons is treated in a simplified way, where the water tank
liner and outer vessel surfaces are assigned real reflectivities which are the probability
for photon reflection rather than absorption. (Snell's law is not implemented.) The
reflection coefficients for the stainless steel vessel and Tyvek tank liner are 55% and
95%, respectively. Reflection from these surfaces is diffuse, ie the reflection angle is
chosen from uniform density in the hemisphere normal to the reflection surface. The
interaction of optical photons with the PMT bulb and generation of electrons at the
photocathode and dynodes is handled by custom code described in chapter 4.
Figures 5-15 through 5-21 show results of simulation implementing the energy and
angular distributions for muons from [77]. Figure 5-15 shows agreement between the
90
Energy loss for muon In S102 I
,
PDG table
-.
--
Simulation
1
-
>10-
105
104
104
1
10
guon momentum (GeV/c)
Figure 5-14: dE/dx for muons in silicon dioxide, from RAT simulation and the PDG.
number of photoelectrons generated in the PMTs to an estimate from the PDG [14]
made using the 1% photocathode coverage of the veto tank surface. The number of
photoelectrons generated in the simulation exceeds the PDG estimate since the PDG
estimate does not take into account reflections of Cherenkov photons from the veto
liner. Figure 5-16 shows the number of hit veto PMTs as a function of the length of
water that a muon traverses for different hit thresholds. Figure 5-17 characterizes the
expected signals in individual PMTs: the mean number of photoelectrons per PMT
per event is 27, while the average creation time of PEs relative to the first PE for an
individual PMT is 14 ns. Figure 5-18 shows the muon tagging efficiency as a function
of the required veto trigger NHit and charge threshold for a PMT to register a hit.
As examples: requiring NHit > 2 and threshold 1/4 PE results in efficiency greater
than 0.999 and 3 missed muons per year; an NHit > 5 and threshold 4 PE results in
efficiency 0.97 and 107 missed muons per year.
91
go
5000 16
Estimate of number Cherenkov photons produced
0. 4500-
PDG estimate for typical PMTs (1% coverage)
14
4000
>
12
10
3500
3000
12500
.-
8
-J
z2000
*m
4
NN
0
100
200
300
400
500
600
700
80
L (cm)
Figure 5-15: Number of photoelectrons created in veto PMTs vs length of water
traversed by muon. The black line is an estimate of the total number of Cherenkov
photons produced by the muon. The red line is an estimate of the number Ps
generated in the PMTs, made using a rule-of-thumb for typical PMTs given in the
PDG Review of Particle Physics [14], and the approximate 1% photocathode coverage
of the veto tank surface by the PMTs. The red line does not take reflections into
account, which is why it lies below the bulk of the points.
92
NHit vs Length of traversed veto water
ENHitVsL
C45
to
C 40_
499.1
45.820
243.6
6.228
Mean x
y
RMS x
RMS y_
-.%
-mean
A
140
35
z30
-
L
120
25
100
208
15 _
60
10
40
5
20
100
0
200
300
400
500
600
700
800
L (cm)
NI lit vs Length of traversed veto water
hNHitVsL_4PE
Mean x
y
RMS x
RMS y
-Mean
45
A
~_0
z
499.1 0
39.32
243.6
12.58.
80
400
25
35
30 -60
25-
040
15
- i
10
a2
50
100
200
300
400
500
600
700
800
L (cm)
Figure 5-16: Number of hit veto PMTs vs length of water traversed by muon. In
the top plot, greater than 1/4 of the average single PE anode charge was required
in order for a PMT to be registered as hit. The bottom plot shows the result for a
threshold of 4 PE.
93
Number of PE per PMT per event
hNPE
37776
27.23
Entries
Mean
1800
RMS
28.37
1600
1400
1200
1000.
800-
.~
-
600
400:
200
00
I
20
80
60
40
100
Time distribution of PE creation rel. to first PE
140
120
Number of PEs
hTime
I
135617
Entrie
1368
Meani
RMS
2
x / ndf
Prob
10 5
p0
p1
p2
p3
104
23.04
3.279 e+04 198
0
2.535 +04 t 68
25 .81 0.05
2.9876 +05 428
1.84 7 0.000
103
102
1I0I
I I I I50l I
i
.
.
l
.
A
100
. .
150
. .
.
10
200
Time (ns)
Figure 5-17: Number of photoelectrons per veto PMT per event and time distribution
of photoelectron creation relative to the first photoelectron creation per PMT. The
yellow line is a fit of a sum of two exponentials to the data.
94
r099
-
0.98
0.97
0.96
0.95
IL
0.94 -
Q> 0.25 PE
> 1 PE
0.93
0
Q>4PE
09
2
.
I92
4
. . . .
6
.
8
10
Required NHit
Figure 5-18: Efficiency for muon tagging vs veto PMT NHit requirement, for various
thresholds. As examples: requiring NHit > 2 and threshold 1/4 PE results in efficiency greater than 0.999 and 3 missed muons per year; an NHit > 5 and threshold
4 PE results in efficiency 0.97 and 107 missed muons per year.
Figures 5-19 through 5-21 characterize the event shape of unvetoed muons. Figure
5-19 shows the location of closest approach to detector center of muons which go
undetected with a veto trigger requirenent of NHit < 4. Unvetoed muons intersect
the water along the edge of the vertical tank wall. Figure 5-20 shows the impact
parameter, or distance of closest approach to detector center, as a function of trigger
requirements, and Figure 5-21 shows the radial distance of closest approach. Although
the impact parameter decreases with more stringent trigger requirements, the radial
distance of closest approach remains pegged at the radius of the veto. In other words,
as the trigger requirements are made more stringent, the water intersection points
of unvetoed muons extend from the bottom (top) tank corners up (down) along the
vertical tank wall. This is an encouraging result: neutrons and photons produced
as secondaries in the water by unvetoed muons, which are a background to the dark
matter search, must still traverse a thickness of water at least as great as the minimum
water thickness, 1.5 m, which occurs at points along the veto wall at height equal to
that of the detector center.
95
Location of closest approach of unvetoed muon
16
N 3
2
E
2
1-1
L
C5
O
1C 0
-1
8
.
a.
-2
6
-3
4
-4-
2
-5-
,11.,,,.n
, ,1
_0
0.5
1
1.5
3
2
2.5
Impact parameter p (m)
Figure 5-19: Location of closest approach for muons unvetoed with trigger requirements NHit < 10 and threshold Q > 4 PE. The black lines outline the veto tank wall.
Unvetoed muons clip the tank corners.
I
Distance of closest approach of unvetoed muon In water to detector center
05.6
5.4
5.2
-I
-
4.8
0 Q > 0.25 PE
-
-
4.6
4.4
-
Q>1PE
I
0
2
6
4
10
8
Required NHIt
Figure 5-20: Distance of closest approach, or impact parameter, of unvetoed muons for
different trigger requirements. The impact parameter decreases with more stringent
trigger requirements.
96
Radius of closest approach of unvetoed muon In water to detector center
2.9C
02.85-
2.8
-
__
__
____
2.75-
2.7 -
-
Q>0.25
e
Q>1 PE
0
Q>4PE
PE
I
2.65- 1
0
2
6
4
10
8
Required NHit
Figure 5-21: Radial coordinate of location of closest approach for an unvetoed muon,
with different trigger requirements. Although the impact parameter decreases with
more stringent trigger requirement, the radial coordinate remains close to the radius
of the tank.
97
Cosmogenic neutrons
5.3.2
Muon-induced neutron events incident on the MiniCLEAN veto tank may be divided
into three classes using the two criteria that the muon traverses the veto or not and
that the neutron is created inside the veto or not. The three classes are: i) muon
traverses veto, neutron produced in veto; ii) muon traverses veto, neutron produced
outside veto; iii) muon does not traverse veto, neutron produced outside veto. Type
i) and ii) events may be reduced with a high muon-tagging efficiency, while events
of type iii) must be reduced with sufficient shielding. A simulation has been done of
cosmogenic neutron events where the neutron is created outside the veto, ie types ii)
and iii) [93]. The simulation does not take into account the potential tagging of muons
for type ii) events. The results are conservative when viewed with this perspective.
Neutron events in this simulation were generated according to the following logic:
" A simplified geometry, which includes only the cavern dimensions and detector
location, is created by the event generator, following the dimensions given in a
concept drawing of the cube hall with deck[94].
" The starting position of a neutron within the cavern wall is drawn from a distribution uniform in the dimensions tangent to the rock/wall surface, and with
normal distance drawn from the lateral-distance-from-parent-muon distribution
in
[77] Fig.
20. The simple cavern geometry and a projection of neutron starting
positions are shown in Figure 5-22.
" A muon energy and azimuthal angle are drawn from [77] Figs. 6 and 7. These
are used to draw a neutron angle-with-respect-to-muon-trajectory from [77]
Eq. 16. A polar angle for the neutron with respect to the muon trajectory
is drawn from a uniform distribution. Note no muon is passed to RAT here; the
information about the muon is used only to determine the neutrons trajectory.
* Assuming straight-line flight through the cavern air, and through the cavern
wall (an assumption justified by the claim in [77] that presented distributions
are given for neutron and muon fluxes in equilibrium), it is asked whether the
98
neutron trajectory intersects with the veto. If not, the generator returns to the
beginning and draws a new neutron origin. If yes, the generator proceeds to
the next step.
" A neutron energy is drawn from [77] Eq. 14. Note that no correlation is given
in [77] between neutron energy and angle of emergence with respect to parentmuon.
" The neutron energy, position, and momentum direction is reported to RAT,
where the simulation continues.
Simulation of 31887 neutron events (corresponding to 76 years) in RAT v877 and
use of the total neutron flux at SNOLAB derived from simulation in [77], 0.054 x
10-9m-2 s-1 , give the result that 0.2 neutrons/year are incident on the fiducial
volume (within r = 29.5cm). These create 0.080
0.002 neutron-Ar elastic scattering
events per year with recoil energy in the region 20-100 keVee, with a quenching factor
of 0.25. The angular distribution of neutrons incident on the veto and the argon recoil
energy spectrum are shown in Figures 5-23 and 5-24.
Starting position of neutron
hNwuronStrtPOSZ
Entries 58252
Men x -0.0903
Mean y -0.8259
RMS x
6.65
7.063
y
-
110 -RMS
0
i
-0
-5
i. A
-10
-5
0
5
10
Figure 5-22: Left: Simplified cavern geometry, showing cavern walls and the MiniCLEAN veto. Right: Neutron origins, looking at cavern from East to West.
99
C
Cos(s) wrt muon
Cos) wit vedtical
02
NO.024 -
CoS wit vetical kncident on tank
-
E0.022 0
0.02
C
Z0.018
-
p0.016
17
C0.014
C0012
0.01-
-
0.008
0.006
-1
-0.5
0
0.5
Cos(8)
Figure 5-23: cos(O) distributions for cosmogenic-neutron trajectories: (red) for all
neutrons (including those which do not hit the veto) with respect to parent-muon
trajectory; (blue) for all neutrons with respect to vertical; and (green) for neutrons
incident on the veto with respect to vertical. 0 = 0 points along the muon trajectory
for (red), and downward for (blue) and (green). (red) is drawn from [77] Eq. 16.
Note the peak at 0 = 7r/2 for (green). The origin of this peak is the sudden increase
in the volume of possible neutron origins in the cavern floor as the azimuthal angle
of incidence on the veto descends past horizontal.
5.3.3
Summary of Cosmogenic Backgrounds
The cosmogenic neutron simulation of the previous section did not take into account
the possible detection of the spallating muon by the active water veto. From this
perspective, the projected neutron background in the WIMP region of interest, 0.080
0.002 per day, is conservative, since events contributing to this rate may be vetoed
when a through-going muon is registered in the water tank.
Since the cosmogenic neutron flux is projected to be so low with a passive water
tank, one may ask whether the effort to instrument the veto is well-spent. The danger
of an uninstrumented veto is that neutrons may be generated close to the central
detector by muons traversing the steel of the outer and inner vessels. Reference [77]
of Mei and Hime, which has guided the presentation here, provides an estimate for the
neutron flux at SNOLAB generated by various shielding substances including lead,
polyethylene, copper, and germanium.
The values range from 1 x 10-12 n/cm 3 /s
for polyethylene to 2 x 10-11 n/cm 3 /s for lead. The rate of neutron production in
materials generally increases with increasing Z 2 /A. Using the total volume of steel in
100
Energy deposited by elastic scatter in fiducial radius (29.5cm)
90.06
hNEDep
Entries
59
Mean
10.67
RMS
14.83
Underflow
0
Overflow
0
integral
0.4264
0.05.
00.04
-
ZO. 03
0.02
0.01
0
20
40
60
80
100
Deposited energy of scatter (keVee)
Figure 5-24: Argon recoil energy distribution due to elastic scatters with cosmogenic
neutrons within fiducial radius (29.5 cm), for quenching factor 0.25. The number of
scatters per year with 20-100 keVee is 0.080t0.002 per year. The plot shows 76 years
worth of simulated neutrons.
MiniCLEAN's inner and outer vessels 9.26 x 105 cm3 and an estimate value for the
neutron production rate 1 x 10-11 n/cm 3 /s, the projected neutron flux from muons
interacting in the detector steel is 300 per year.
This number is less than 1% of
the (a, n) neutron flux from the inner detector's photomultipliers, 40000 per year.
Therefore the advantage of instrumenting the water veto is small.
101
102
Chapter 6
Analysis of MiniCLEAN Vacuum
Data
With the MiniCLEAN inner vessel (IV) assembled in the temporary cleanroom in the
Cryopit, the IV was commissioned and tested under vacuum and with purified argon
gas. In this configuration without the water shield, gammas from the cavern rock wall
contribute to the event rate. Data were taken in Spring 2014 for many days under
vacuum, and during three separate gas runs. This chapter investigates the vacuum
data.
6.1
PMT and DAQ configuration
PMT voltages were set to produce 5 pC single photoelectron gain, using calibration
data taken at Boston University. Noise was seen on some channels with a maximum
amplitude ~ 1/2 PE and a frequency of 100 MHz, likely generated by the DC high
voltage power supply. The trigger threshold for each channel was set just above this
noise. The DAQ was then configured to collect zero-suppressed waveforms whenever
five individual channels registered a trigger within a 16 ns window. These settings
resulted in approximate trigger rates of 350 Hz in vacuum and 450 Hz with fresh argon
gas, corresponding to disk write rates of 150 kB/s and 1.3 MB/s for vacuum and gas,
respectively. Gas events have larger file size than vacuum events due to the typically
103
higher PMT occupancy. The digitizers are configured to record only those portions
of waveforms which descend below the zero-suppression threshold 5mV.
6.2
Data quality
In the data presented in the following sections, events are considered for analysis only
if the maximum of the summed waveform occurs within 50 samples (200 ns) of the
trigger. This ensures that the slow triplet scintillation light is fully collected in the
16 ps waveform, and has the effect of removing many events which exhibit detector
effects like PMT discharge and afterpulsing.
Events which saturate the 2V input range of a digitizer channel are marked in
the trigger word in the data file. These are corrected by fitting lines to the voltage
trace on either side of the saturated segment. The intersection point of the two lines
is identified as the amplitude of the pulse, and the area of the triangle is added to the
integral of the digitized portion of the pulse. The contribution to the charge integral
introduced by this correction is always small (< 1%).
Voltage baselines are calculated separately for each pulse in the zero-suppressed
waveforms using the five samples (20 ns) collected before the voltage descends below
the zero-suppression level. The baseline is the mean of these five presamples.
6.3
2 2 Na
calibration source
During a subset of the vacuum runs and during one gas run, a 30 kBq
was used for calibration.
22 Na
source
The source was placed abutting the IV in a divot cut
to accommodate the source. Upon decay, radioactive
22 Na
emits a positron, which
annihilates to two back-to-back 511keV gammas. The source is encased in a Nal
scintillator crystal, which is optically coupled to a PMT. When one gamma scintillates
in the crystal, the PMT fires, and we are assured that the other gamma is cmitted in
the direction of the IV. Thus the PMT on the calibration source can be used as a tag
which identifies events coincident with a 511 keV gamma entering the IV. The
104
22 Na
nucleus produces an additional, isotropic 1.275 MeV gamma upon prompt relaxation.
6.4
Vacuum data
Vacuum data were taken over many days, primarily for the purpose of DAQ debugging. In this section, data taken over 97 hours of livetime during May 2014 (runs 475,
483, 488) are considered. The mean trigger rate during these runs was 332 Hz, and
0.1% of recorded events were eliminated from consideration by the data quality cuts.
Events recorded for vacuum data are generated primarily by gammas which CompThe recoiling
ton scatter with electrons in the acrylic blocks of the light guides.
electrons produce Cherenkov light which causes the event trigger. PMT traces for a
typical event are shown in Figure 6-1. Figures 6-2 through 6-3 show distributions for
File: skim 130724_0_1408638765 MCL_000475000300_000303.root;
4000
Run ID: 475;
0
1000
2000
3000
Voltage (mV)
I
1111111
II 111
1
II 11
111111 1
5000
I iii
6000
II
ii
7000
i
I
I
8000
Iii
1
1111111
10000
9000
1I
ii
1
1
11000
11111 I
I
Summed
WFD 12,In 1
WFD 11,In 6
WFD 9,In 2
WFD 4,In 0
WFD 3,in 4
WFD 3,ln 3
WFD 2,In 4
WFD
V
1,In 7
WFD 1,In 5
0
1000
2000
3000
4000
5000
Event ID: 58527076; Trigger Pattern: W,2
Trigger time: 175125986211595071; WFD time: 387253000; Reduction Level: 2; ZLE: 1;
6000
7000
8000
9000
10000
11000
fls
Figure 6-1: A typical event collected in vacuum data. The top trace is the sum of the
nine PMTs which fire during the event. The integrated charge is 61 pC, or 12 PE.
number of photoelectrons, Fp, and charge centroid.
F-f;
Vf(t)dt
f ' V(t)dt
105
Fp is defined here as
(6.1)
where the trigger time to is obtained when the summed waveform is maximum, T =
to - 20 ns, Tf = to + 16 ps, and o = 80 ns. The distribution for Fp peaks near one,
as expected for the very prompt Cherenkov light which triggers most of the events.
Figure 6-4 shows the angular distribution of the reconstructed charge centroid R,
defined as
R -EiQz
(6.2)
where 6i is the position of the i'th PMT and Qj is the charge in that PMT.1 The
angular distribution shows peaks at the positions of the optical cassettes, as expected
for events generated by light coming from the acrylic blocks. Figure 6-5 shows the
distribution of PMTs with greatest charge, indicating that there are no anomalously
hot PMTs during the vacuum runs.
Gammas from the
238
U and
232
Th chains have energies ranging from 40 keV to
2.6 MeV. Within the IV, the PMTs are the hottest source of these gammas. Assay
results for the PMTs indicate that the gamma emission rate is several hundred Hz
(see Section 3). The cavern rock wall also generates a large flux of gammas in this
energy range. The flux of higher energy gammas (> 4.5 MeV) from the rock wall is
1000 m- 2 - d- 1 , which is negligible.
Figures 6-6 show the PE and centroid distributions for simulated gammas with
a flat energy spectrum between 0.4-4.5 MeV overlaid on vacuum data. There is an
excess of high energy events in the data. Events generating more than 80 PE occur
at a rate of 0.38 Hz, or about 0.1% of the total rate. Figures 6-7 show the Fp and
centroid distributions for events with greater than 80 PE. The centroid is strongly
peaked near the TPB radius.
One may speculate that the high-energy excess is caused by pileup events, where,
for example, multiple gammas from the rock wall scatter in the acrylic during a 16 Ps
data collection window. This possibility is disfavored by the Fp distribution which is
peaked near one, since scattering events occuring outside the prompt window would
'This method of position reconstruction is biased inward.
106
push Fp down. Additional evidense against this possibility may be seen in the 8 hours
of data taken with the
22 Na
source, shown in Figure 6-8. The reconstructed charge
centroid shows a clear peak near the position of the source in angular space. Since
the untagged calibration source can be seen over background, the cavern rock cannot
be emitting gammas at a rate much higher than the 30 kHz source activity. A gamma
rate of this frequency cannot produce significant pileup since the event digitization
length is only 16 us. Figure 6-9 confirms this by comparing the PE distributions
for 1-2.6 MeV gammas generated at 100 kHz to those generated at a baseline rate of
400 Hz.
The high-energy events may be caused by scintillation in the reflective foil lining
the light guides. CRESST-2 found that 3m ESR foil emits 250 photons per MeV of
energy deposition [95]. When the foil arrives from the manufacturer, it is covered
with a protective plastic film. Removal of this film causes static electricity buildup
on the foil, perhaps attracting the 5.3 MeV alpha-emitter 2' 0Po. Figures 6-10 show
simulated bursts of 1000-1300 optical-wavelength photons originating at points on the
foil surfaces. The charge centroid for ESR scintillation events is peaked near the TPB
radius, but the PE distribution is bi-modal. The lower PE events occur when the
scintillation occurs along the acrylic plug, while the higher PE events originate closer
to the PMT. More detailed simulation of the ESR scintillation is needed, and the
optics along the plug need to be verified. The point here is simply that ESR scintillation events induced by
-
5 MeV alphas generate more than 80 PE and reconstruct
along the TPB radius.
The radial distribution of vacuum data in Figure 6-6 attains a shallow local maximum for 0.7 < R 3 /R3PB < 0.8, before sinking to a local minimum around 0.95 and
then peaking at the TPB radius. This distinctive behavior cannot be reproduced by
a sum of the simulated gamma PDFs. It is likely the gamma contribution to the
radial distribution descends beyond R3 /R3B = 0.8, as in Figure 6-11. If this is
correct, the rate of foil-scintillation events can be determined by counting the number of events above the falling gamma radial spectrum of Figure 6-11. The result
is 720 events/h/M 2 of ESR foil. This may be considered an upper limit on the ESR
107
scintillation rate. A lower limit is simply the rate of events with PE greater than 80,
which is 0.38 Hz or 70 events/h/m2 of foil.
This rate is very large compared to the expectation from radon deposition models.
For example, Borexino
[96] modeled
the expected
210Pb
surface contamination for a
sample using the equation:
O-(CPb) = Covot
where o- is the surface concentration of
21OPb
(6.3)
[atoms/M 2], Co the ambient radon con-
centration in the air [ 222 Rn atoms/M 3], vo the effective deposition velocity, and t
the exposure time of the sample. Borexino measured vo = 3 x 10-' m/s in a class
100 cleanroom. Radon concentration in the class 500 assembly cleanroom for MiniCLEAN, where particle counts were typically below 100 particles larger than 0.5 pm
per m 3 , was 1 Bq/m 3 . The exposure time of the foil was about two hours. The
expected
210Pb
activity per M 2 ,
A21oPb,
is then
A21Pblog(2) 2 b( 210 Pb) = 3.7 x 10- decays/rM 2
tOPb = 22 year
where the radon concentration Co is obtained by multiplying the activity 1 Bq/m 3 by
the lifetime of
222
Rn, 3.8 d/log(2). This predicted rate is 5 to 6 orders of magnitude
below the apparent rate.
It is likely the ESR foil attracted
21OPb
more efficiently than expected due to the
static electricity build-up generated by removal of the protective films. Advantage of
this property was taken during assembly, when the foil was temporarily secured to
the inside of the cassette by static cling. The charged foil was also likely an efficient
attractor of radon progeny.
Besides serving as the reflector in the light guides, the ESR foil was also creased
outward from the TPB surfaces to form baffles or "wipers" which close the gaps
between light guides. (See Figure 2-12.) This means that a portion of the foil is in
contact with the active region of argon, and alpha decays originating on those surfaces
108
-111-.1,1111-1-111 -.
1.
of foil will generate events. The total area of the baffles is about 0.1 m 2 , generating
between 7 and 70 events per hour. In the next chapter, the alpha scintillation rate in
argon is found to be 44.6
0.7 events per hour. It is possible the entire surface alpha
rate for MiniCLEAN is generated by the ESR baffles.
109
LulI
CO
0
02
10
01
10
1
0-1
Uc
0
200
400
600
800
1000
1200
1400
PE
(a)
05M CC0
0
-
T0CC
00
0
20C
1C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.6
0.7
0.8
0.9
1
R /RTPB
-
O
-
(b)
26000
24000
.)22000
20000
18000
16000
14000
12000
10000
0
0""" 0.2
0.1
0.3
0.4
0.5
(c)
Figure 6-2: (a) Photoelectron, (b) Fp, and (c) charge centroid distributions for
vacuum runs 475, 483, and 488, normalized to counts per hour.
110
LL
106
105
r.
*1
..
103
~-p
I.
102
10
| ,I,,i,,,I , ,i,,,I
,
,
, ~,
F ,
00
600
400
200
800
1000
1200
1
1400
PE
(a)
C.
1
104
0.9
0.8
1 03
0.7
0.6
102
10
0.2
0.1
0.4
0.3
0.5
0.6
0.7
0.8
0.9
1
1
R3 3
R /RTPB
(b)
106
0.9
105
0 .7
104
0.8
16
0.C
103
0.4
102
0.
0.11
10
0.1
0
400
600
800
1000
1200
1400
PE
1
(c)
Figure 6-3:
(a) Fp vs PE, (b) Fp vs R3/R'PB, and (c) R 3 /R3 PB vs PE distributions
for vacuum runs 475, 485,
and 488.
111
0 0.8
0.6
0.4
105
0.2
0
-0.2
-0.4
104
-0.6
-0.8
-1
Figure 6-4: Reconstructed angular coordinates of vacuum data. Peaks are evident at
the coordinates of the cassettes. PMT 53, at cos(O) = -0.2, <
-1.5 was turned off
due to problems with sparking.
1- 3000
-)
2500
0
2000
1500
1000
500
0
10
20
30
40
50
60
70
80
90
PMT ID
Figure 6-5: Distribution of PMTs with maximum charge per event. PMTs 0, 11-15,
61-65, and 91 are the pentagons, which have the smallest TPB surface area of the
three types of cassettes.
112
1__
10
.M102
0.4 - 1 MeV
1 - 2.6 MeV
-
- 4.5 MeV
__2.6
-
Data
10
10-1
10-2
04N0
00350
-
300
250
250
PE
0.4 - 1 MeV
1 - 2.6 MeV
~
200
150
100
50
0
-
2.6 - 4.5 MeV
Data, Scaled
200150100
50
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.93
1
R /R T 6
Figure 6-6: PE and R 3 /RrPB distributions for simulated gammas overlaid on data
with arbitrary normalization.
113
-
0
-- _ PE>80
---
210-1
No PE Cut, Scaled
10-2
--
10
10-4
I t i i
0
0.1
0.2
0.3
0.4
0. 5
0.6
I i i
;
0.7
; i
i i ;
i
0.8
0.9
i
1
R/RTPB
R33
(a)
SPE >80
0.9
0.8
103
0.7
102
10
0.2
0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1
0.9
3
R /RTPB
1
(b)
Figure 6-7: (a) R 3 /RjPB and (b) F, vs R 3 /R P for vacuum events with greater
than 80 PE. The high energy events are strongly peaked at the TPB radius and fall
into two Fp bands, one above and one below Fp = 0.6.
114
0
0.8
0.6
0.4
104
0.2
0
-0.2
-0.4
-0.6
103
-0.8
U
-I
-4
-,3
4
22
Figure 6-8: Reconstructed azimuthal and polar angles for vacuum runs with Na
source present. There is an excess of events at the position of the source in the upper
right corner.
Simulated gammas, 100 kHz
104
0
Simulated gammas, 400 Hz
10
E
102
10
0
10
20
30
40
50
60
70
80
90
PE
Figure 6-9: Gammas with origin at the PMTs simulated at 400 Hz and 100 kHz. The
100 kHz source does not cause pileup significant enough to shift the PE spectrum.
115
0
'S
4
3
10
2
10p
10
10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
3
RR3/RTPB
(a)
90
0
0
70 00
E
50 0
040 0030 DO
20 0010
U
50
100
150
200
250
300
350
400
PE
(b)
Figure 6-10: (a) Radial and (b) PE distributions for simulated bursts of 1000-1300
photons with origin on the surface of the reflective foil. The group of events with
~ 90 PE originate along the edge of the acrylic block, causing a lower light yield than
events originating closer to the PMT, away from the acrylic.
116
,A 300
0
250
200-
150
100
G0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
R3 3
R/RTPB
1
Figure 6-11: The measured centroid distribution for vacuum data. The distribution
is fit with a third order polynomial between 0.85 and 0.92. The extent of the polynomial beyond 0.92 defines the projected contributions of gammas (in white) and ESR
scintillation events (in blue). The blue region contains 1.4 million events, corresponding to 720 events per hour per square meter of ESR foil. The total area of the foil is
18.5 m2
117
118
Chapter 7
Analysis of MiniCLEAN Gas Data
The MiniCLEAN inner vessel (IV) was filled with room-temperature argon gas for
three separate gas runs. Besides further commissioning of the data acquisition system,
the gas runs allow verification of the purification system. Operation with warm argon
gas also provides a unique opportunity to measure the rate of alpha activity on the
wavelength-shifting surface. An alpha particle ejected into the argon gas with 59 MeV ranges out in a few centimeters, and generates a bright scintillation pulse.
These alpha events are the only high energy events in the detector, since electrons
with several MeV do not range out in the gas.
7.1
Run Summaries
The MiniCLEAN inner vessel was filled with warm argon gas during three periods
in Winter 2014, providing 133.9 hours total livetime which is considered here. The
dates of operation were: January 29-31 (runs 106-115, 17.1 h livetime), February 1418 (runs 215-232, 63.2 h livetime), and February 28 - May 21 (runs 281-473, of which
runs 281-301 with 53.6 h livetime are considered here). The photomultiplier and DAQ
configurations are as described in section 6.1, and data quality cuts in section 6.2.
A typical gas event is shown in Figure 7-1. Photoelectron, Fp, and charge centroid
distributions are compared to vacuum data in Figure 7-2. Gas events produce more
photoelectrons at late times, and reconstruct more often in the center of the detector.
119
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T~V
ITI
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II
4000
6000
II
10000
8000
Event ID: 8; Trigger Pattern:
Trigger time: 87100394873122446; WFD time: 3501636; Reduction Level: 2; ZLE: 1;
Ox2;
14000
12000
16000
ns
r
0
qWi
II
0.
0
Figure 7-1: A typical gas event, with 132 PE. The top panel shows waveform traces
for all PMTs with charge. The bottom panel is the event display, where the area of
a polygon corresponds to the charge in that PMT.
120
Gas,Hour1
0
1i
~
-_ Vacuum
0104
102
10
0
700
PE
600
500
400
300
200
Gas, Hour 1
-
5
100
0-
- Vacuum
0
30000
1000
0
0
0.1
0.3
0.2
0.4
0.7
0.6
0.5
o
070000
000 0
0.8
0.9
1
FP
- Gas, Hour 1
doo0
--
Vacuum
0
05000
40000
30000
20000
10000
0.1
,.-
.
... ...
0
0.2
I..
0.3
1-
0.4
0.5
1-
0.6
-
0.7
-
0.8
0.9
3
1
R /R TPB
Figure 7-2: PB, F,, R 3 /I4PB distributions for the first hour of gas run 281, compared
with vacuum data.
121
7.2
Triplet Quenching Overview
At room-temperature, many materials inside the IV outgas water vapor which quenches
the late triplet scintillation light of the gas. The largest contribution to the outgassing
rate comes from the acrylic, which contributes 250 mtorr/l/min. The effect may be
seen in Figure 7-3 which shows PE and Fp distributions for the first five hours of
data taking after a gas fill (runs 281 - 301). Figure 7-4 plots the mean prompt, late,
and total PE vs time. The mean late PE value decreases by more than 30% over five
The triplet quenching can also
hours, while the prompt light remains unchanged.
_
Hour 0 -1
Hour 1 - 2
Hour 2 - 3
-
Hour 3 - 4
_
j1 0.1
0-2
-
Hour 4 - 5
1
10-3
104
10-5
150
100
50
200
250
350
300
4 0
PE
&Saw
-
--
40000
0
Hour 0- 1
Hour 1 - 2
Hour 2 - 3
Hour 3-4
Hour 4 - 5
35000
-30000
25000
20000
15000
10000
5000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Figure 7-3: Photoelectron and Fp distributions for the first five hours of gas runs
281 - 301. The average PE value decreases with time, while the average Fp value for
scintillation events increases.
be seen in the decreasing value of the triplet time constant. Figure 7-5 depicts the
summed waveform for events acquired during the first 11 minutes of run 281. The
122
Total
0 Late
0
Prompt
CL bU
c5550 -+
45
40:
35
00
40
20
250
20
15
10
'
1.5
2
2.5
3
3.5
4
4.5
5
Hour
Figure 7-4: Mean total, late, and prompt photoelectron values for events with Fp <
0.6 during the first five hours of runs 281- 301.
normalized waveform is fit with the model
f(t - to) = q g(t - to,Tl) + (1 - q) g(t - to,Tr),
(7.1)
where
1
g(t, T) = -
-t
exp(-),
T
(7.2)
T
to is the time at which the summed waveform attains a maximum, and
TI
and T. are the
long and short time constants characterizing the argon scintillation time probability
density function. The model is not a good fit for the data in the intermediate region,
but the two exponential model has become a standard method of identifying the long
time constants for noble liquids [61]. Inclusion of PMT noise [97] or an intermediate
fluorescence time constant for TPB [74] have been proposed to improve the model.
The decay of the long time constant with time is plotted in Figure 7-6.
123
'
Summed Waveform
0)
E
------ Fit
10-
.Fi
0
Z
10
-
-
1000
2000
3000
4000
5000
Relative Time (ns)
Figure 7-5: Sum of PMT waveforms with Fp < 0.6 collected during the first 11
minutes of run 281, normalized to unit integral. The triplet time constant is 1.58 ps,
determined by fitting with equation 7.1.
16
00-
B 1500U)
8
14 007
4)
1300-
000 12
11 0010 00
0
900
oc -*
8 00
7
0
100
200
300
400
500
Time since fill (minutes)
Figure 7-6: Decay of the triplet time constant with time.
124
7.3
22
The tagged
Na calibration source
22 Na
source described in the previous chapter was present during gas runs
281-301. It was hoped that an energy calibration could be obtained from the tagged
data. However, it appears the tag was not working properly. Figures 7-7 show PE,
Fp, and centroid distributions for events tagged during the first hour after the gas fill,
overlaid on events collected anti-coincident with the
22Na
tag. The anti-coincident
distributions are scaled so that their integrals are equal to those for the tagged events.
The coincident and anti-coincident distributions are very similar.
In order to tell whether the presence of the source can be registered at all, data
taken when the source was and was not present can be compared. Since the gas fill
for run 281 (when the source was present) was accomplished more quickly than for
run 215 (when the source was not present), the photoelectron and Fp distributions
are difficult to compare. However, the reconstructed charge centroid coordinates in
Figure 7-8 do show variation. The data taken when the source was present are more
strongly peaked at the location of the source (cos(0) = 0.85,
125
#=
2.1).
104
:_-
22Na Tag
-_-
No tag, scaled
#A
103
0
102
10
1
3
50
_
150
200
250
300
350
400
450
50
PE
0.4
0.5
0.6
0.7
0.8
0.9
1
22Na Tag
-
600
100
No tag, scaled
-
o500
400
300
200
100
....
03
0.2
0.1
0.3
o 10
0)
_-
22Na Tag
__-
No tag, scaled
01400
0
1200
1000
800
--
-
600
400
-4'++
--
200
0
0.1
0.2
0.3
0.4
0.5
'
0.6
++
++
0.7
0.8
0.93
1
R /RTPB
Figure 7-7: PE, Fp, and R3 /RrPB distributions for tagged 22 Na events, overlaid on
events anti-coincident with the tag. The tagged and untagged distributions are very
similar, indicating there was a problem with the tag.
126
- -- Na Tag
0.05
-- --- Anti-coincident
0.045
Na not present
0.04
0.035
0.03
0.025
0.02
0.015
0.011
-1
-0.8
-0.6
-0.4
0
-0.2
0.2
0.4
0.6
1
0.8
cos(O)
0.032 -- -[-Na Tag
0.03
-Anti-coincident
Na not present
0.028
0.026
0.024
0.022
0.02
0.016
0.014
-
_-
-2
-2
-
0.018
1
0.012
Figure 7-8: Reconstructed charge centroid coordinates for data taken when the 2 2Na
source was present and not present. Distributions are normalized to unity. The source
is located in a divot in the inner vessel, drilled at (cos(O) = 0.85, 4 2.1).
127
7.4
Instrument Effects
Several instrument effects may skew the integrated charge value for an event to an
artificially high value. Since analysis in subsequent sections is focused on identifying
high-energy alpha events, these effects are considered here in the context that they
might mimic a real alpha scintillation event. The effects are PMT afterpulsing, PMT
or base discharge, and baseline sag. For each of these, the additional charge appears
in the late part of a PMT waveform, which pushes the Fp value down relative to the
expectation for alpha events. In the case of afterpulsing and discharge, it is usually a
single PMT which undergoes the effect. This pushes the charge centroid artificially
close to the wavelength-shifting surface. The three effects are described here in turn.
* Afterpulsing occurs when the electron cascade in the PMT dynode chain ionizes
a helium atom.
The helium ion then accelerates toward the photocathode,
causing a secondary electron cascade in the dynode chain and a large broad
pulse. An example of afterpulsing is shown in Figure 7-9. Note that the summed
waveform receives a dominant contribution from the afterpulsing PMT.
128
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Run ID: 282;
2000
Voltage (mW,)
I
I1111~l
1111111 1111111
16000
11111111111111111
-
WFD ,2,15 3
WF012
WFO 12.1n0
WFD 1,1.17
WED " 6 5
14000
-
-
-
--
-
WF0 10,55
WED 11,1n, 4
WED 101
-
I-
WFD 9,I5
WF0 10,1n0 4
WFDO0,1n 3
WFO41
WFD In 3
WFDBS
WEID 8,9
WO~ :5
WED 5,553
-
-
--
I-
-
WED 5,550
WF %00SWF0 6,556
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4
25
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2
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6
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I
4
-
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7
2.55.2,5523
2,5502
5,555n
WFDS.4
WED 451n53
WED 5,552
WED 1,.
2
Event 4ID: 20530rge
atr:0n
Trige time: 870422327;WDtm:620737euto
Flie 'Instleb~~
845DlnS2200408080002040 00
303010277
eslcs008
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3 -DO
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250
-F ,n30
I
1111111111 1 1 1 1 1
I 1 1 1 1
1 1 | I I I I I I ] I I I I I ] ' I 11 1 1 1 1 H l t l
10000
12000
8000
4000
6000
0
2000
Event ID: 208553; Trigger Pattern: 0Ox2;
Trigger time: 87100492128392670; WFO time: 622007713; Reduction Level: 2; ZLE: 1;
14000
16000
1 11 11 1 1 l i il
Figure 7-9: An afterpulsing event with 1000 PE, Fp = 0.08, R 3R
3
= 0.979. Top:
on top, with the brightest PMT
all PMT waveforms. Left: the summed waveform
(WFD 9, Ch. 5) below. The afterpulse occurs ~1 ps after the primary pulse. Right:
event display with brightest PMT in center foreground.
129
1
ns
e
Base discharge is a problem in gaseous argon, where the PMT bias voltage
is sufficient to cause breakdown over distances less than a centimeter. Most
bases were coated in epoxy, so the breakdown rate in gas is low. Those bases
which were not conformal-coated remained off during the gas runs. Once a base
begins to discharge, the spark path provides a preferred route for continued discharge. This can be seen in Figure 7-10 which indicates PMT 2 was discharging
frequently during run 281. Most discharge events are eliminated from consideration by the requirement that the maximum of the summed waveform occur in
the trigger window. Some appear similar to after-pulsing events, without the
broad defined pulse (Fig. 7-11).
Run 281 - 301
F < 0.3
PE > 500
A300-CD
, 250200
150-100
50--
0
10
20
30
40
50
60
70
80
90
PMT ID
Figure 7-10: PMTs with greatest charge for high-energy, low-Fp events. PMT 2 was
prone to discharge.
130
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Voltage (MWV'IIIII
IIII I1I
I11 1II 11III 1II I1
I 111111
liii
14000
16000
111111111 111111111
WFD 12,17n4
WFD 12,17 0
WFO 11,0
W D010,In7
WFD010,1706
WFD 0
In I
6706,1 7
W70 6.171
WFO 7,1n7
WFO 1I 3
47708,1725
WFD707:n7I
WF707,13
47708.1n72
477 ,l7 7
47707,1n73
4770 5.17 0
47706,1n7
47705:.n7
470417
47704,1n74
47704,13
47704,172
47704,171I
47704,170
47703, 7
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47703,1.70
47702,1n73
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--
47701I,17
1111111|111111111 111111111 111111111 11111111111111111111111111111111111111 11i1
16000
14000
12000
10000
8000
6000
4000
2000
Event ID: 235874; Trigger Pattern: Ox2;
ns
Trigger time: 87100456267531345; WFD time: 1268368403; Reduction Level: 2; ZLE: 1;
File: skim filesiskim_131021_0 1400257249
4000
0
2000
Run ID: 281;
MemBlocks
6000
000281
8000
0000 0014.root;
12000
10000
14000
ai l l i ai ei i t l i il i l a timm
11111 1111
( M
in , n t l i lill nlin
voltac
16000
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-200
-300
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.400
.500
0
*
;*
0.
-10C
-200
-300
WFD
Ijn 2
-. 7,
-400
-500
1-
1111111111111111111i1111111111111111111
11
12000
8000
10000
6000
4000
2000
0
Ewent ID: 235874; Trigger Pattern: 0x2;
Trigger time: 87100456267531345; WFD time: 1268368403; Reduction Level: 2; ZLE: 1;
14000
16000
nS
Figure 7-11: A PMT discharge event with 735 PE, F = 0.12, R 3 /R3PB =0.996. Top:
all PMT waveforms. Left: the summed waveform on top, with the brightest PMT
(WFD 1, Channel 2) below. Both summed and individual waveforms have amplitude
~ -500 mV. Right: event display with brightest PMT in center foreground.
131
* Baseline sag is an effect of the impedance of the PMT power supply. When
the photocathode is heavily depleted of electrons, the power supply takes some
time to restore the baseline to its original potential. There can also be lowfrequency baseline oscillation induced by heavy photocathode depletion. Since
the digitizer's dynamic input range is set to accept negative-voltage PMT pulses,
a positive-voltage baseline oscillation tends not to be digitized.
This causes
charge-integral values for a baseline-sag event to be artificially high, leading to
an overestimated PE value. Figure 7-12 shows a baseline sag event.
132
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Run ID: 282;
2000
4000
6000
8000
10000
12000
Voltage
(mWii~
111111111
I,,InIII III11111111
14000
1 111111111
1111111111
16000
1111111111
1111111111
111
I T 11
WFD 12,1,n3WFD 12.1n
WIFD 11 14-
0
________________
-
-
-
-
-
WED11 1-
-_
WFID 1:1
_
3-
-
_
_
_
- -_______________
-
:
-__.
-
0-
-
-
--
-
---
-
WEDBi4 I-
WFEDB:"i'
-
-
-n
-
WFD414
WIFD 1,1140
WEIFIDO
-
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WID 11,1n1-
-
-
-
-
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-
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-
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WFD7.4
WED 7.0
WF07D 2_
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-
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WED 4:142
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WED 3,143
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WED I,1I
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WF 51.11
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0m
7ii
2000
400
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74
1111 li iiiI Ilii I~ll~ ii I iiI
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100620
Even ID: 5027
Trige time: 870599114;WDtm:665334euto
F 5d
3~ssi
RonI 5O:1 22
WFO
30410277
00
40
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40
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40
40
60
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60
D
-1000n
-1500in
-2002
n
-
-2000
IF
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300371
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WFD 27,164
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B
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oe
Run 281 - 301, Hour 1.0925, PE > 500
0.9
0.8
0.70.6
0.5:-
0.30.2
0.1
0
0 .
Figure 7-13: Fp vs R 3 /R
0.2
B
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
3
R /RTPa
distribution for events with more than 500 PE. The
cluster of events with F, < 0.3 and R 3 /RPB = 1 occur outside the TPB radius and
induce one of the instrumental effects described in the text.
7.4.1
Instrument Effects Summary
The result of these instrumental effects, for a given event, is to push the PE count
to an artificially high value, Fp to an artificially low value, and the charge centroid
toward the TPB radius. The original cause of the event may be scintillation by a
surface alpha, scintillation in the light guide, or a random discharge in the PMT base.
The latter two types of events will reconstruct with charge centroid closer to the TPB
radius. Figure 7-13 shows the Fp vs charge centroid distribution for events with PE
greater than 500 during the first hour of the third gas fill, run 281. Many of the low
Fp events are bunched against the TPB radius. These are the instrument-induced
events. In the subsequent section, they will be eliminated from the surface alpha data
set by requiring that R 3 /R}PB not exceed a cut at 0.92.
134
7.5
Surface alphas
Radon progeny plate out onto the TPB during assembly and installation.
210Pb
is
of particular concern since it has a long 22-year half-life and falls out of equilibrium
with the other members of the
238U
chain. This results in higher concentrations of
ambient 210Pb than would be expected if the species were in secular equilibrium.
210 Pb decays
with a 22 year half-life to 2 1 0Po which emits a 5.3 MeV alpha. Alpha
particles with energies of a few MeV have a path length of several centimeters in
argon gas. Therefore, alphas emitted from the TPB surface into the argon range out
in the gas and produce a bright scintillation pulse. Electrons with MeV energy, on
the other hand, have path lengths of several meters. Electrons deposit at most several
hundred keV in the meter-diameter sphere contained by the TPB.
Figures 7-14 show Fp vs PE distributions for data collected during the first 6 hours
of runs 281 - 301. The high energy events (PE > 500) fall into two categories: the
circled bunch at Fp ~ 0.4 which migrates to lower PE and higher Fp values over time;
and the band below Fp = 0.3. The circled group of events are surface alphas being
ejected into the argon gas. The central Fp value during the first hour is between 0.3
and 0.4, close to the expectation of 0.3 for electrons. This is lower than the expected
value of 0.7 for alphas in liquid argon. The reason is that the alpha's energy deposition
is much less dense in gas than in liquid, so that the triplet quenching is less efficient.
A typical event display for a surface alpha event is shown in Figure 7-15.
135
Hour 1
Hour2
a
10'
i
0.9
0.8
102
102
H
)
0.7
10'
0.9
0.8
0.7
0.6
102
102
0.5
0.4
0.3
0.2
'"0..
10
0.4
'. 6.
10
1
200
Hour
400 600
0.1-
-
10
800 1000 1200 1400 1600 1800 2000
PE
200
400 600
800 1000 1200 1400 1600 1800 2000
PE
10-
Hour 4
3
----------10
-- -
10'
'
aL
U.9
0.9
0.102
0.8
0.7
0.6
0.7
12
102
0.6
(.
10
0.5
.5
10
0.3'
0.2,.
0.1
T
. .. .i i..I . I.. 1...J .
400
600
. .. .LL-
800 1000 1200 1400 1600
I ..
1800
2000
PE
1000 1200 1400 1600 1600 200
PE
10
-
ci.
1W.
Hour
-
i
. 10
10
0.1
.
'
.
200
-
0.2
'1
Hour 5
a
0.4
0.3
1n4
0.9
200
400
600
800
6
In'
I-
0.9
0.8
10,
0.8
102
0.7-
0.7
0.6
1
0.5
.
0.5
-10
0.4
0.4
0.3
0.2
0.1
,
,,.
.. , ,
'
200
400
' '
600 800
0.6
- .'6
10
0.3
0.21
,1
; '|.
102
-
-0.1
1000 1200 1400 1600 1800 2000
PE
_
200
400
600
800
1000 1200 1400 1600 1800 2000
PE
Figure 7-14: Fp vs PE distributions for the first six hours of runs 281-301. The group
of events circled in the top left panel migrate to higher Fp and lower PE values with
time, due to the increasing water vapor content in the argon gas which quenches the
late triplet scintillation light.
136
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(li
-
2:.14012
W
WFD 11
16000
43
-
WFDS 73
WF01013n4WFD 114
3
n
W 2:0
~~
2
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14000
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Trige time 8704394640 WF4ie
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.
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km11204852931B060000001.31
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WD31In00
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WFO ,1315
PMT
5a:ors
E 183
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with the brgts 5M WD4
Figur 7-1:A
IMT
waeors
amplitude3-0-0-mV,-th
poygns
-
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-
et
th0ih7cpny
mdwvfr
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hesrndwvfr
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,
2
;;; In 4l I 1I11 1
0D
200
400
02
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870436770nWI
ID
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13I6113
ee:2 L:
4088;Rdcto
-___
-
Wrge0 ie1 7': 5297740 nF 2ie
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600
40010
80300
20
40
60
I;1
oolol1312
E
ih26
lh vn
cuac.Lft
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iniida
PMT
~03,R/~
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-20m.Rgh:eetdsaywt
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l
7.5.1
Surface alpha rate
Surface alpha events, where the alpha is ejected from the TPB surface into the argon,
reconstruct with charge centroid much less than one. This can be seen in Figure 7-16,
which shows the charge centroid vs PE distribution for simulated surface events. A
real surface alpha event may also induce an instrument effect, pushing the Fp value
out of a central range.
Figures 7-17 show Fp distributions for events which have
charge centroid R 3 /PRB3 < 0.96 and PE value greater than a cut placed "by-eye",
described later in the caption of Figure 7-22. The Fp distributions are fit with the
sum of a Gaussian and constant. The means and variances of the fitted Gaussians
are shown in Figure 7-18. Events which fall outside 5- of the mean, where - = 0.024
is the mean variance obtained from the fits, are marked by solid circles in the PE
distributions of Figures 7-19 and 7-20. The average values of the uncircled events
above the PE cuts (ie those which do not produce instrument effects) are plotted in
Figure 7-21.
The surface alpha rate is determined by counting events above a PE cut, set to
1/2 the alpha PE yield in Figure 7-22. As the PE cut decreases with time, high
energy vacuum events begin to contribute to the counting rate as in Figure 7-23.
These background contributions are subtracted from the rates displayed in Figures
7-24. Each point in the figures represents at least one hour of livetime. The time
axis represents time since the fill. Events are required to have R 3 /R3
PB<
0.92. The
same PE threshold curve vs time has been used for each fill. The effect of varying the
PE threshold above or below the central value by 20% and the centroid cut between
0.88 < R 3 /R3PB < 0.96 is displayed by the inner square brackets bounding the points.
The bars represent the statistical variation, determined by summing in quadrature
the Poisson-variation of each bin above threshold. The mean alpha rate is consistent
across the runs. The livetime-weighted mean of the three runs is 42.9 t 0.7 events
per hour.
The fraction of alpha events removed by the PE and centroid cuts may be derived
for the first hour after the fill from Figure 7-16, where 75 of 2000 simulated events
138
W 112
~0.9
0.8-
10
0.7
8
0.6
0.5
IF
0.4
6
4
6
0.3
2
0.2--2
00
200
400
60
800
1000
120
S200
400
600
800
1000
1200 1400
200
400
600
800
1000 1200 1400 1600 1800 2000
PE
E
1 400 1600 1800 200
0102
10
0
PE
1600 1806
20
10
0
Figure 7-16: (Top) R3 /RrPB vs PE distribution for 2000 simulated alpha events which
deposit 5.3 MeV in the argon gas along the surface of the TPB. The argon scintillation
yield has been set to 1800 photons per MeV in order to match the PE yield of 1500
observed during the first hour of run 281 (see Fig. 7-19). All the simulated events
reconstruct away from the TPB surface with R3 /RarPB < 0.7. Of the 2000 simulated
events, 1960 cause an event trigger, and 35 generate less than 800 PEs. (Bottom)
Projection onto the PE axis. The cluster of events near 600 PE originate near or in
front of the baffle, which absorbs the UV scintillation light.
139
Hour 1.021944
= 652
p1 = 0.363
- p2 = 0.046
8- -- 2/NDF = 27.740 / 10
pfl
0
-
o
06-
4
2
0
I,,I1.,I ,I.,,I.,, ,
.
0
0.1
0.2
, ,
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
FP
Hour 12.198611
.
-
C4
0
-
o 20
0
p0 = 2.000
p1 = 10.000
p2 = 0.600
p3 = 0.024
X2/NDF = 85.426/ 7
5-
a
1 0-
0
+
0LT...T ++++
, , , ,I...I..,.
.I....
I..,
0.1
0.2
0.3
+
5--
0.4
0.5
0.6
0.7
0.8
0.9
0.4
0.5
0.6
0.7
0.8
0.9
FP
Hou r 22.075001
o 142_
0
2
0
C
p0 = 9.740
p1 = 0.700
p2 = 0.023
2
x /NDF = 18.663 / 13
1'0
Bn
8642C
0
0.1
0.2
0.3
F,
Figure 7-17: Distributions of Fp values for alpha events which satisfy the centroid
and PE cuts, for several hours of runs 281-301. The fit function is a gaussian plus
a constant. The instrumental effects described in the text cause the eve nts to fall
outside the central Fp value.
140
Run 281 - 301
C
LL0.7
0.6
0-
0
20
3
40
50*
60
10
20
30
40
50
60
Hour
40
50
60
Hour
0.5
0.4
0
Run 281 - 301
0.07
9 0.06
U.
*)
0.05
0.04
0.03
0.02
FilIif
0.01
0
1fIIfI
10
I
I
20
30
Figure 7-18: Summary of the fitted mean and variance values for the Fp distributions
shown in Figure 7-17. The mean variance, averaged over is 0.024.
141
Hour 1.09
w
PE > Cut, Good F
10C
10
-*-
PE > Cut, Bad FP
0
0104
103
102
10
1
0
500
1000 1500 2000 2500 3000 3500 4000 4500 50(
PE
Hour 2.22
PE > Cut, Good F
I 106
10
-.-
p
PE > Cut, Bad F
p
0
Q1104
10
3
102
10
,- L
1
0
500
1000
1500 2000
2500 3000 3500 4000 4500 501 0
PE
Hour 3.31
W
106
In
PE > Cut, Good F
0
-
105
PE > Cut, Bad F,
0
-
0104~
1 03
102
10
+ ~~
1
0
500
1000 1500 2000 2500 3000 3500 4000 4500 5000
PE
Figure 7-19: Photoelectron distributions for events with R3 /RrPB > 0.96 during the
first three hours of run 281. Each panel contains at least one hour of live time, and is
labeled by the time since the beginning of the gas fill. The vertical lines represent the
7.5.1. The solid -;rcles are surfacc
position of the ".y-eye" cut described in s
alpha events which induce an instrument-effect, identified by their Fp value which
falls outside a range defined in Figure 7-18.
142
Hour 4.38
w
-
106
0
PE > Cut, Bad F
-.-
104
PE > Cut, Good F
-p
105
104~
102
10
It
1
500)
0
t
1000
ttItItlH
1500 2000 2500 3000 3500 4000 4500 5000
PE
Hour 13.59
-
PE > Cut, Good F
-e-
PE >Cut, Bad F,
106
I
1o
5
0
104
103
102
10t
500
0
1000 1500 2000 2500 3000
3500
4000 4500
5000
PE
Hour 26.97
010
(0 t
E10
-
PE > Cut, Good F,
--
PE > Cut, Bad F p
0
103
102
10
t
0
500
1000 1500 2000 2500 3000 3500 4000 4500 5000
PE
Figure 7-20: Photoelectron distributions several hours after the gas fill. The hour
labels are the time since the fill.
143
Run 281 - 301
CL
ao = 792.233
tO = 0.006
al = 487.201
t1 = 0.731
a2 = 714.480
t22 = 0.126
-
0) 16W
1400-
- /NDF
43.116 /43
12001000-
800-
600
0
10
20
30
40
50
60
Hour
Figure 7-21: Average photoelectron yield for alpha events which do not induce an
instrument effect, fit with a sum of three exponentials.
either fail to cause an event trigger or reconstruct with less than the 780 PE cut.
This implies 3.9% of surface alphas fail to be counted in data during the first hour.
The efficiency-corrected alpha rate is 44.6
0.7 events per hour.
Figure 7-25 shows the distribution of PMTs with maximum charge for alpha
events, averaged across the three gas runs. The exposure times for each TPB face
during installation were recorded and are represented by the blue line, which has
been scaled so that the integral matches the measured rate. The predicted alpha
rate, based on the measured radon concentration in the assembly cleanroom and a
deposition rate of 7.4 x 105 decays/m 2 /day due to 1 min exposure to 1 Bq/m 3 , was
0.4 decays/hour along the whole 2.3 m 2 of TPB surface. (See equations 6.3 and 6.4.)
The measured surface rate is a factor of 100 larger than the prediction. It is likely
that the polymer baffles, which close the gaps between the light guides, make a contribution to this rate, as discussed in section 6.4. It is also possible that the TPB
surfaces were electrostatically charged before coating by handling with gloves. Some
evidence for this may be seen in Figure 7-26, where there is possible smudging along
the edges of several TPB faces.
144
800
70.
700
-
1/2 Alpha PE
.
By-eye points
Fit to eyes
----
500
-
.
600
*
400 -
-
300
.............................................
10
20
30
40
50
60
Hour
Figure 7-22: Photoelectron cut. The solid line is the alpha PE yield multiplied by
half and is used to determine the alpha rate. The dashed line was used to select alpha
events to accept for the PE yield determination. The dashed line is a fit to the points,
which were identified by-eye for several PE spectra where there was an obvious gap
between the falling gamma spectrum and alpha accumulation, for example the first
two hours of Figures 7-19.
145
Runs 475, 483, 488
106
105
R /RTP,
<
0.96
-t
titi
104
0102
10
1
10.
10-1
10-2
200
D
400
600
800
1000
1200
1400
1600
PE
0
6
3
R /R r<
U)
5
0.96
PE>Cut
w
4
3
fif9
2
-. 44,I
1
-10
10
20
30
40
50
60
Hour
Figure 7-23: The photoelectron distribution for vacuum events, and the integral of
this distribution above the PE cut of Figure 7-22.
146
o 70
60
0
50
40
p0 = 42.9 +/- 2.3
30
2
X /NDF
20
5
0
=
16 / 8.09
Prob = 0.946
10
15
20
25
Hour
70
06
40
pO 43.5 +-1.1
-
X2/NDF = 46 / 42.72
30
Prob = 0.610
0
10
40
30
20
50
60
70
Hour
0
80
0
70
0
-
60
50
40
h1~I~
p1 = 42.2+/- 1.1
30
X2/NDF = 48 / 60.54
Prob = 0.106
20
0
10
20
30
40
50
60
Hour
Figure 7-24: Surface alpha rates. (Top) Runs 106-115, started Jan. 29 2014, with 17.1
hours livetime. (Middle) Runs 215-232, started Feb. 14, with 63.2 hours livetime.
(Bottom) Runs 281-301, started Feb. 28, with 53.6 hours livetime. The livetimeweighted mean rate for the three runs is 42.9 0.7 events per hour.
147
-Rate
--1.2
Exposure
0
t,
0.2
0
1
10
20
30
40
50
60
70
80
90
PMT ID
Figure 7-25: Distribution of PMTs with greatest charge for alpha scintillation events.
The exposure time for each cassette during assembly is represented by the blue line,
with an overall normalization determined by the integral of the measured rate.
Figure 7-26: Photograph taken during assembly of the inner vessel, looking into the
vessel before cassette installation was complete.
148
7.5.2
Light yield
The number of photoelectrons yielded in response to a 5.3 MeV alpha energy deposition at the start of the gas fill, when the yield is greatest, may be estimated by
evaluating the PE yield curve of Figure 7-21 at time t = 0. The result is 1760 PEs.
MiniCLEAN's efficiency for converting scintillation photons into photoelectrons may
be estimated using the scintillation yield of argon gas. Reference [98] measured this
yield as a function of pressure using alpha emission from boron after neutron capture.
The measurement, shown in Figure 7-27, extends to 107 kPa, less than MiniCLEAN's
180 kPa. However, since the scintillation yield plateaus beyond 60 kPa, it is reasonable to expect this extends to MiniCLEAN's gas pressure. The measured light yield is
5600
1200 photons per decay, where 2.79 or 2.31 MeV of kinetic energy is deposited
with branching ratios of 6% and 94%, respectively. This corresponds to 2400 + 500
photons per MeV of energy deposition, or 13000
alpha emission. Thus 7.2
3000 scintillation photons per
2 10
Po
1.5 argon scintillation photons produce one photoelectron
at the beginning of MiniCLEAN's gas run.
This efficiency may be converted into a photoelectron yield for liquid argon operation by using the PMT efficiency which is 20% lower [99] and the liquid scintillation
yield 40 photons per keVee. The result is 9.0 t 1.9 scintillation photons per PE or
4.5
4.85
0.9 PE/keVee. Light yields obtained in the past include 2.8
0.08, and 7.9
0.1, 1.26
0.15,
0.4 PE/keVee by DEAPI [78], WARP [100], MicroCLEAN [61],
and DarkSide-50 [51], respectively.
149
8000
7
1%
6000
oi
C
nm
+-900 nm
1200 nm
-600
2000
.....-
0
30
60
90
120
pressure (kPa)
Figure 7-27: Argon gas light yield for alpha emission following neutron capture on
boron film, from [98]. The emitted alpha has energy of 2.79 or 2.31 MeV, with branching ratios of 6% and 94%, respectively. The measurement does not extend to Mini-
CLEAN's 180 kPa pressure. However, the plateau beyond 60 kPa suggests the light
yield remains constant at 5600 photons per decay above the limit of the measurement.
This corresponds to 2400 photons per MeV of energy deposition.
150
Chapter 8
Sensitivity Outlook and Conclusion
Acrylic
Class I
206
35pm
2TPb
Pb.
Class II
a
I
77-TPB 1.5pim
Figure 8-1: Schematic of 2 0Po decays to alpha particle and 2 0 6Pb near the wavelengthshifting surface, drawn to approximate scale. The measured alpha rate, represented
on the left, receives contributions from parent nuclei on the TPB-argon interface,
in the TPB, on the TPB-acrylic interface, and up to ~ 10 pm depth in the acrylic.
Class-I and II events can mimic a dark matter signal.
Surface events impact MiniCLEAN's sensitivity to a dark matter signal by leaking
into the the fiducial volume of argon, in which the background rate would best be zero.
The previous chapter presented a measurement of the rate of alphas being ejected with
several MeV from the wavelength-shifting surface into the active argon region of the
151
MiniCLEAN detector. The result was 44.6
0.7 events per hour. When the majority
of the alpha's energy is deposited in the acrylic, rather than the TPB and argon,
the event may fall in the dark matter energy region of interest, mimicking a WIMP
signal. These events must be discriminated against using position reconstruction and
scintillation time information in the variables Fp, F,, and L,.
Surface events which contribute to the dark matter background may be divided
into two classes based on the position of the parent nucleus. Here it is assumed the
parent is
2
10Po
which decays to a 5.3 MeV alpha and 100 keV
206Pb
nucleus, although
other radon progeny may also contribute. In the first class (I), the
210 Po
is on the
TPB surface which contacts the liquid argon. The alpha is ejected into the acrylic,
depositing ~ 100 keV in the TPB which scintillates (- 900 photons/MeV), and the
206Pb
nucleus enters the liquid argon.
nucleus are
-
The path lengths for the alpha and
20 6Pb
35 pm and - 0.07 pm, respectively.
In the second class (II), the parent
210Po
nucleus is embedded in the TPB or
acrylic, or directly on the TPB-acrylic surface. In these events, the recoiling
206
Pb
nucleus deposits all its energy in the TPB (thickness - 1.5 pm) or acrylic, and the
alpha deposits its energy in some combination of the acrylic, TPB, and argon. This is
represented schematically in Figure 8-1. The lateral straggling length for the 5.3 MeV
alpha is
-
6 pm. This class of event can mimic a dark matter signal when just enough
of the alpha energy is deposited in the TPB and argon scintillators to fall in the WIMP
energy region of interest, 75-150 PE. This occurs, for example, when the path length
of an alpha in the TPB is several times the thickness of the TPB, and the alpha
deposits negligible energy in the argon.
MiniCLEAN's zero-background fiducial volume may be determined by simulating
these two classes of surface events, and identifying the fiducial radius within which
zero events per year reconstruct.
Unfortunately, the collaboration's Geant-based
simulation RAT does not implement ionization on ~ pm length scales correctly. The
standard software package implementing this ionization is SRIM (Stopping Power of
Ions in Matter, by James Ziegler)
[711,
which contains tables of stopping power vs
energy for ions in various materials. Proper simulation of the straggling of the alpha
152
particle for class-II events may be accomplished by porting SRIM for use in RAT,
which has not been done. For class-I events, where the thickness of TPB traversed by
the alpha particle is
-
5% of the alpha's total range, it is sufficient to determine the
alpha's energy deposition in the TPB by assuming straight-line propagation. This
energy deposition is used to calculate the TPB scintillation yield, and the simulation
of these optical photons and the recoiling
20
Pb nucleus is left to RAT.
The procedure for simulating class-I events is as follows:
" Draw a starting position for the event, chosen uniformly along the TPB surface.
" Draw an azimuthal angle 0 with respect to the TPB surface, uniform in cos(O)
within a hemisphere, for alpha emission through the TPB into the acrylic.
* Determine the alpha path-length in TPB, assuming straight-line propagation
and TPB thickness 1.5 pm.
" Use SRIM tables to determine alpha energy loss in TPB.
* Generate 420 nm optical photons following reference [75], which reported a TPB
scintillation yield of 882
time constants 11
210 photons per MeV of alpha energy deposition with
5 and 275
10 ns.
" Pass the optical photons and recoiling
201Pb
nucleus to the Geant-based RAT
simulation, where argon scintillation, TPB fluorescence, photon propagation,
and generation of photoelectrons is accomplished.
Figure 8-2 shows the reconstructed radius distribution for simulated class-I surface
events, where the parent nuclei is
2 10 Po.
The radius coordinate R, was obtained with
the likelihood method Shellfit, described in section 4. The effects of the dark matter
region-of-interest (ROI) cuts are also displayed: 75 < PE < 150, Fp > 0.7, Fc, > 0.55,
L, > -0.1, and reconstructed charge centroid R, < 29.5 cm. The Fp cut has been chosen to accept 50% of nuclear recoils. The nuclear-recoil acceptance efficiencies of the
F,, and L, cuts are both > 99%. In the radius distribution, there is a long tail extending into the center of the detector. The position origin for events with reconstructed
153
No Cut :Frac =1.00
E
0
10-
W
102
75<PE<150: Frac = 0.31
Frac = 0.07
75<PE<150 F,>0.7
75<PE<150 F,>0.7 f >0.55 L,>-0.1
4)
-----
-
75<PE<150 F >0.7
f
Frac = 0.014
>0.55 Lr>-0.1 Rr<295mm :Frac =1.295e-04
W10-
10.-
10
M
101
0
50
100
150
200
250
300
350
400
450
R,
Figure 8-2: Reconstructed radius distribution for simulated class-I surface events,
displaying the effects of several cuts.
radius R, < 29.5 cm is shown in Figure 8-3. The reconstruction performs poorly for
event originating along the edges of the polygon-shaped wavelength-shifting surfaces.
In some cases the charge centroid method does reconstruct events closer to the TPB
radius than Shellfit, as seen in Figure 8-4 which plots R, vs R,. Improvement of the
Shellfit algorithm's operation on edge events should be an emphasis of future work.
154
E00
E
300
200
100
0
-100-200-300
-400
-400
-300
-200
-100
0
100
200
300
400
x (mm)
x (mm)
Figure 1-3: (Left) Event origins along the TPB surface for simulated type-I events.
(Right) Origins for events which reconstruct with R < 29.5 cm.
-.
E
E*400
.
350
U) 300
250
200
150
100
50
'0
50
100
150
200
250
300
350
400
450
Charge Centroid RC (mm)
Figure 8-4: Reconstructed radius using the Shellfit algorithm vs the charge centroid
method.
155
The fraction of simulated class-I events which satisfy the other ROI cuts and
reconstruct within the target 29.5cm fiducial radius is 1.30 x 10- 4 . Unfortunately,
in the case that the surface rate 44.6 + 0.7h- 1 contributes class-I events exclusively,
this implies 48.3
0.8 background events per year leak into the WIMP ROI. In
order to obtain a fiducial volume with zero background events per year, the lower PE
threshold may be increased, since the position reconstruction performs better with
more photoelectrons. Figure 8-5 displays the zero-background fiducial mass obtained
as a function of PE threshold for several surface rates. The required threshold for the
measured surface rate is 360 PE. Within a target fiducial radius 29.5 cm, the number
of expected surface leakage events per year vs PE threshold is depicted in Figure
8-6. An energy threshold of 120 PE gives 15 surface leakage events in 150 kg - y of
exposure.
10k /year
1 /rn / h
10/h
44.6/ h
-
500
0 400300200100-
50
100
150
200
250
300
350
PE Threshold
Figure 8-5: PE threshold required to ensure zero-background fiducial mass, for various
class-I type surface rates.
156
50
.
..................... .................................................. ......................................... ........ ........................
CL
40
'"11- 111,11, ................ .................................................. .................................
CD
4)
.... ................. I.................*................
30
.........
20
................ .....
................................................. .........................
0
ACM
10
.
........ ................................. ................................. ................ ................ .......
80
100
........ .................I .................I ................ ................
.................
.... ................ .........
120
140
160
180
200
220
240
PE Threshold
Figure 8-6: Number of class-I surface events per year which leak into the WIMP ROI
with 150 kg fiducial volume.
h...................... ...................... .............................................................................................
25
C
C
2
0
h-
20
...............................................................................................
15
. .........*................
...................... ............................................................ ................................
10
.
......
5.
................................
............................... .........
..............
.....
......... ...................... ......................
80
100
...................
120
140
..............
.................
..........................................
160
200
180
PE Threshold
Figure 8-7: Number of class-11 surface events per year which leak into the WIMP
ROI with 150 kg fiducial volume.
157
As an approximation to type-II events, the collaboration has used a strategy
similar to that for class-I, but with the parent nuclei embedded in the TPB or acrylic
and providing the origin for straight-line alpha propagation. For events simulated in
this way with origin inside the TPB, Figure 8-7 displays the number of events per
year leaking into a 150 kg fiducial mass. A PE threshold of 95 gives 15 background
events per year.
MiniCLEAN's projected SI WIMP-nucleon cross-section sensitivity with 150 kg -y
exposure, 15 leakage events, and 120 PE threshold is displayed in Figure 8-8. With the
,
target light yield of 6 PE/keVee, the limit is strongest for a WIMP mass of 166 GeV/c 2
attaining us, < 1.5 x 10-44 cm 2 . With the 4.5 PE/keVee light yield projected in the
previous chapter, the limit is weakened to -s, < 3.6 x 10-44 cm 2 . The target limit,
which projected zero background for 150 kg -y exposure and 75 PE threshold, is
displayed for comparison. The dotted blue curve represents the effect of increasing
the PE threshold to 140, reducing the background rate to 11 per year. The green
curve shows the much weaker limit attained when the PE threshold is increased to
360, so that zero background events appear in 150 kg -y exposure. The dotted black
curve represents the results for simulation of class-II events, with 95 PE threshold, 15
background events, and 6 PE/keV.
8.1
Conclusion
This thesis has presented analysis of the first commissioning data taken with the
MiniCLEAN dark matter experiment.
MiniCLEAN is a novel single-phase liquid
argon detector, which uses the wavelength-shifting fluor tetraphenyl butadiene and
cryogenic photomultiplier tubes arranged spherically for light detection. The commissioning data taken in the beginning of 2014, when the inner vessel was evacuated
and when filled with warm argon gas, provide an opportunity to measure the surface
background ratc, wh
1
i gne
by th apha
e
O
radon progeny on or near
the wavelength-shifting surface. The requirement for cleanliness of the wavelengthshifting surface is driven by the challenge of position reconstruction in MiniCLEAN,
158
10-42 --
43
0-
sac-%.
-
61
1
10
10
102
GeV/c
2
Figure 8-8: Projected 90% confidence SI WIMP-nucleon cross-section limits for MiniCLEAN. All except the red curve assume 6 PE/keV. All except the green curve
assume 150 kg fiducial volume. The blue and red curves assume a PE threshold of
120; the black-dashed curve assumes 95 PE threshold and 15 background events.
where the scintillation photons generated by an energy deposition in the liquid argon
are not directly detected by the photomultipliers.
The measured surface rate, 19.0
0.4 /h/m 2 , is a factor of 40 greater than the
collaboration's target rate, indicative of the technical difficulty of clean assembly.
It is likely the deposition rate of radon progeny on the acrylic and reflective foil
surfaces was accelerated by electrostatic charge buildup, attracting particles at a rate
exceeding expectation.
This effect is confirmed in
[101], a measurement motivated
by health-safety, which found deposition rates onto charged surfaces increased by
orders of magnitude compared with the uncharged condition. Another reference
[102]
measured a factor of up to 18 increase in deposition rates in the vicinity of main
power cables.
Liquid noble detectors have come into use in rare-event searches because they
are bright scintillators, easy to purify, and, since they are inert and non-flammable,
conducive to use in underground confined space. The driving concepts behind the
single-phase, rather than dual-phase, approach, have been the high light yield obtain-
159
able with 47 photomultiplier coverage and ease of scaling to increased size, due to
the lack of electric field applied in dual-phase detectors. The experience of designing,
fabricating, and assembling MiniCLEAN has shown that achieving 47r coverage of the
argon target with a clean, gapless wavelength-shifting surface is a primary challenge
for single-phase detectors. MiniCLEAN's larger sister experiment, DEAP3600 [103],
is currently pursuing a different approach to effecting a wavelength-shifting surface,
using a monolithic spherical acrylic shell rather than MiniCLEAN's geodesic assembly.
The future for single-phase liquid noble detectors will be informed by the performance
of both experiments. Upcoming construction of the multi-ton dual-phase liquid xenon
detectors LZ [104] and XENON1T [105] ensures the continuing importance of noble
liquids in the terrestrial search for dark matter.
160
Appendix A
Select Topics in Detector Design
A.1
Validation of photomultiplier and base
The signal response of bialkali PMTs has been known for decades to drop precipitously at temperatures below about 170 K, due to decreased electron mobility in the
photocathode (Fig. A-1). At low temperatures, electrons cannot arrange themselves
to dissipate space charge which eventually prevents efficient movement of photoelectrons to the tube's dynode chain. The formation of space charge depends sensitively
on the tube's geometry, uniformity of photocathode, etc., so two different tubes of the
same model can exhibit response degradation at different temperatures. Multialkali
photocathodes do not exhibit this steep response-drop but are undesirable because of
their higher noise rates.
In the mid-2000s Hamamatsu developed an 8-inch tube, the R5912-02MOD, with
a thin platinum layer between the PMT bulb and its bialkali photocathode.
The
platinum retains conductivity sufficient to produce workable signal response down to
liquid helium temperature. The trade-off is decreased tube efficiency above 170 K,
since the platinum forms a barrier that signal photons must penetrate on their way
from the PMT glass to the photocathode. Hamamatsu attempted to mitigate this
deficiency through use of a sand-blasted ("frosted") bulb. The frosting provides a
diffuse scattering surface for signal photons which penetrate the tube bulb but reflect from the platinum, failing to produce a photoelectron in the photocathode. The
161
.( 300
4
0
-
-
W41642
o - -.*a0- R31o
Figure A-1: Signal response of PMTs vs temperature from [106]. The R,1221 and R,649
(triangles) have multialkali photocathodes, while the rest have bialkali photocathodes
and exhibit steep response degradation below about -100 C.
frosting reflects the photon back toward the photocathode, giving it another chance
to excite a photoelectron.
Although the frosted tube appears opaque (Fig. A-2),
tests done by Austin Jackson et al. at LANL in 2008 [107] are consistent with the
claim that sand-blasting improves efficiency for tubes with platinum underlay. Besides the platinum, the cold-operating R5912-02MOD differs from its warm-operating
R5912 cousin through its four additional dynodes. The R5912-02MOD's 14 dynodes
allow lower operating voltages at cold temperatures where power conservation can be
important.
The PMT base design for MiniCLEAN was adapted from the design used in MicroCLEAN, which demonstrated tube operation in liquid argon and neon [99]. MiniCLEAN's larger size and requirement for low radioactivity informed several design
changes. MiniCLEAN uses a single cable for signal and bias voltage, compared to
MicroCLEAN's two. This reduces by half the number of cryogenic feedthroughs and
the amount of cable which must run the 15 m from detector to power supply and
DAQ. Within the outer and inner vessels, the cable is a low-mass type provided by
W L Gore and Associates; outside is RG58. MiniCLEAN's base also uses low-mass,
surface-mount circuit components (ceramic rather than silver mica capacitors) and
162
"Dead"
"Dead"
Figure A-2: Photograph of several 8-inch Hamamatsu photomultipliers. The two
tubes on the left have lost vacuum, which causes evaporation of the brown photocathode and the tubes' clear appearance compared to the SNO (R1408) tube. The
increased opacity of the platinum-coated tube can be seen in the duller appearance
of the dead R5912-02MOD compared to the R5912. MiniCLEAN uses the frosted
variety of R5912-02MOD.
kapton (rather than fiberglass) laminate substrate, to limit radioactivity. Use of single cable and ceramic capacitors, which are less suited for high-frequency application
than silver mica, degrades signal quality compared to the MicroCLEAN design.
MiniCLEAN's base initially lacked back-termination, the addition of which suppressed ringing seen during characterization.
The final base schematic is shown in
Fig. A-3, along with scope traces with and without back-termination in Fig. A-4.
Several tubes were tested in a dedicated cryostat at LANL. A helium compressor
provided cooling to a copper cold-head in contact with the inner vessel of a dewar,
where the PMT could be covered by nitrogen or neon gas and exposed to light via
a fiber-optic feedthrough (Fig. A-5). The cold head was coupled with Arctic Silver
(typically used for bonding a CPU to a heat sink) to a copper flange, which was sealed
to the stainless inner vessel with indium wire. Single photoelectron gain, along with
dark rate, was determined at each temperature with a 532 nm laser as light source.
Pulse digitization was accomplished either by a CAEN V1720 digitizer controlled by
DCDAQ 1 , or by a Lecroy Waverunner 6Zi with faster digitization (40 GS/s) than the
'Written by the talented programmer Dan Gastler at BU, first as a graduate student and then
as a post-doc.
163
K
F2
F1 F3
D11
D12
1k
Dl
D13
510
220
D14
50
P
PMT
iM
iM
iM
100K
_
_
1K
510
220
50
250K
.
.
.7
50
T4.7nF
Unmarked resistors: 400K
Unmarked capacitors: 1 nF
Figure A-3: MiniCLEAN R5912-02MOD base schematic. Positive bias voltage
(~
1100 V at
TLAr)
pulses (~
100 mV) on the bias voltage. Back termination can be seen in the 50 Q-
is applied across the PMT lead to ground. Signal appears as negative
4.7nF path from PMT lead to ground. Not shown is the bias-tee circuit which
baseline-subtracts the signal for input to the digitizers.
V1720's 250 MS/s.
The procedure for determining gain and dark rate as a function of temperature is
as follows: at each temperature, the bias voltage is scanned, while the laser is pulsed
at an intensity that produces approximately 1% occupancy. The prompt charge is
determined by integrating the waveform in the prompt time window, in which the
laser pulse is expected to produce a single photoelectron in 1% of events. Prompt
charge values are placed in a histogram and fit with the model described in section
5.2.2 equation 5.1 with the number of dynodes
Ndnoe
here set to 14. The noise rate
is then found by using a 40 ns sliding integration window along the waveform outside
the prompt window, registering pulses with charge greater than 3/4 PE. An operating
voltage for the PMT is defined as that voltage which produces 5 pC gain. Results of
this procedure are illustrated in Figure A-6.
Gain and noise vs temperature for PMT 108 are shown in Figure A-7.
Note
that the noise increases with decreasing temperature, contrary to expectation in
case the noise mechanism is thermal electron emission from the photocathode. The
aCtaLI mechani ism fOr spontaneous electron emission from cryogenic
uialkali
metal
is
unknown [108]. Another consequence of this unknown mechanism is that dark pulses
arrive in bursts rather than Poisson-distributed time-separation intervals (Fig. A-8).
164
-0.2
-with
back termination
-
-0.4
04
Withot back
termtination
-0.6-
-0.8-
170
180
175
185
205
200
Time (ns)
195
190
Figure A-4: Single photoelectron pulses at ~ TLAr for bases with (blue) and without (red) back termination. Ringing seen in the red trace is damped by the back
termination.
2
In order to simulate the PMT response in Monte Carlo, Tom Caldwell developed
a model for the pulse shape based on the calibration data taken at LANL. The model
is a sum of two or three lognormal distributions:
n
I(t) =
*(A.1)
(t/22
Q-
In 2 (
_e
i-_
(t + to)
mO) /2U2
n
27rcri
where to is the fixed trigger time, defined relative to the laser pulse time. Pulses
(examples shown in Figure A-9) are fit with the two and three component models,
and classified as either triple or double lognormal (TLN or DLN) according to their
reduced chi-squared values (Figure A-10). In the calibration data, 81.2% of pulses
are DLN, and the rest are TLN. The pulse time tp is defined as the geometric mean
of the first log normal, and pulses are classified as early (tp < -18ns), prompt
(-18ns < tp < 18ns), or late (tp > -18ns). Double pulsing is also considered. The
time and charge distributions weighted by relative probability are shown in Figures
A-11 and A-12
2
Tom put most of MiniCLEAN together himself, as a graduate student at U. of Pennsylvania.
165
optical fiber
R659
Nd:YAG (532 nm)
HV/V1720
Vacuuam
(IO 'mbar)
N/Ne
Indium wire seal
Copper flange
Arctic silver
Cold head
He compressor
Figure A-5: Diagram and picture of cold gas cryostat for PMT testing. The picture
shows the closed setup, with only the outer vessel of the dewar visible. The PMT is
housed in the stainless inner vessel where it can be covered by nitrogen or neon gas.
Low-mass cable from Gore is used for electrical connections inside the dewar, while
RG58 provides the rest of the connection to the DAQ (CAEN V1720 digitizer) and
power supply (HV). Light from a YAG laser is sent to the PMT with optical fiber.
Not depicted in the diagram is the piping for gas handling.
166
PMT: 108 at 293 K
j
at 950 V]
26
4
--
-
as
X Gain
Nierae S bove
0.25s
0
detalo]
z
Noisc rate, 3/4 PE
-
threshold
0.2
4
3
0.15
2
0.1
.
A
26
5
10
15
800
20
.-
.
850
--
9
950
1000
Charge (pC)
at 1I000V
PM
-
0.05
1
1050
Voltage (V)
r: 108 at 90K
1.2
0
z
-
a*
0.8
4
-l
S
0.6
3
.*
I
0.4
2
Charge (PC)
0.2
1
x asm.,01 V .........
800
850
900
9
1000
1050
Voltage (V)
Figure A-6: (Left) Single photoelectron charge spectrum for PMT 26 at 950 V (top)
and 1000 V (bottom), at room temperature. (Right) Gain and noise measurements
vs voltage for PMT 108 at 298 K (top) and 90 K (bottom). The bias voltages which
produce 5 pC gain are 1043 V and 1031 V
167
>104 5
104 00
103 5
103 0
-
102 5
0
-
102 05-
101 01
101
1. 2
-
N
I
0e
0. 80. 60.
0.
2L
2.
2. 3
0
2
0
2
8
100
150
200
250
300K
Figure A-7: Gain and noise results vs temperature for PMT. The top panel shows
the operating voltage required to obtain 5pC gain. The middle panel shows the rate
of noise pulses with charge greater than 3/4 single photoelectron, at the operating
voltage from the top panel. The bottom panel shows the gain at 950 V operating
voltage.
168
~1
*
Time between dark pulses
T-95K
102
10
0
2
10
8
6
4
14
12
16
20
18
A t (inS)
Figure A-8: Time separation between dark pulses. The non-Poisson distribution, with
a peak near time zero, suggests a mechanism other than thermal emission causes dark
pulses at cryogenic temperatures.
20W0
2045
2040
190
1960
2020S
Z400
2 )0
~2070
1tw0
100
2000
200
2200
2300
2150
2196
2
Th.- (01
220
2200
2250
ZZSO
2350
2350
2300
Z"
TI2
-
120
(0.1
2030
2
50
20
20
30
30
40
4
51
12010
1000
1"0
2000
Z100
2200
230
2400
2500
Figure A-9: Examples of single photoelectron pulses at liquid argon temperature.
The left (right) panels are classified as DLN (TLN). An ADC count corresponds to
2 V/2 2 = 4.88 mV.
169
5
z
4.
4-
3 .5
3
DLN
2 .5
1
0
.5
TLN
DLN
x!
Figure A-10: Reduced X' distributions for Eqn. A.1 fit to PMT calibration data
taken at liquid argon temperature. Each pulse is classified as DLN, TLN, or multiple
PE (upper right).
0
Prepulses
10-
Prompt Puses
Double Pulses
--
(1)
Double Pulses (2)
-
10-
Late
Pulses
10
-40
0
-20
20
40
60
80
100
140
120
14I,
Ime (ns)
Figure A-11: Pulse time distributions for the different catagories of pulses, weighted
by their relative probability.
Double Lag-Normal
Triple LoC-Normal
Earl Pubes
10-1
Double Pubes
---Late
Pubses
Nei" Pubes
-
10-1
2
4
6
8
10
12
Charge (pC)
Figure A-12: Charge distributions for the different categories of pulses, weighted by
their relative probability.
170
A.2
Selection of light guide reflector
Visible light fluoresced by the TPB must be guided 29.5 cm to the PMT faces for
detection. The light yield depends sensitively on the reflectivity of the light guide
walls (Fig. A-13). 3M ESR, a reflective polymer film [109], was chosen over a physical
vapor deposition (PVD) or electroplating as the reflective surface, despite evidence
that ESR wavelength shifts light below 380 nm with 30%-50% efficiency to 420 nm
[110], and concern that the foil is not 98% reflective as advertised (Figures A-14 and
A-15). (A wavelength shifting reflector is to be avoided, since scattering events in the
argon contained by the light guide become visible.) Depositions of silver or protected
silver by several companies 3 were also considered and rejected due to high cost or
poor performance. Particular difficulties of PVD, beyond achieving 98% reflectivity,
included coating the stainless steel light guides (requiring a nickel flashing) rather
than aluminum, depositing uniformly along the inner surface of a tube, and, for bare
silver, protecting against tarnishing during detector assembly. Typical cost estimates
for PVD on the full set of 92 light guides, amounting to 200 square feet of surface,
were a factor of 20 more than for ESR film.
6
.2
2
so10
60
LG reflectivity (%)
Figure A-13: Simulated light collection vs reflectivity of light guide walls, based on
10 eV betas at detector center.
3
Silvex (Westbrook, ME), JDecker Industries (El Cajon, CA), Flexbrite (Houston, TX), Newport
(Irvine, CA)
171
Total Reflectivity
--- -.
.
-
- 00
Reflectivity at 45 deg.
- ----
-
190 -
-
.....
...
s9o
90
80
80
..
70-
70
65
....
.....
-..
60
--
I
55
50 -
380 400 420 440 460 480 500 520 540 560
Wavelength (nm)
......
-
50
Decker
.Silvex
.
.
60
-
75
380 400 420 440 460 480 500 520 540 560
Wavelength (nm)
Figure A-14: Measured reflectivities of coatings from two companies, JDecker Industries (red) and Silvex (blue). These are calibrated measurements performed by
Angstrom Sun Technologies (Acton, MA) using a Varian Cary-500 spectrometer and,
for total reflectivity, an integrating sphere. The silver-based coating from JDecker is
proprietary; the Silvex sample is plated with silver per ASTM B700 Type 2 Grade C
Class S.
172
Angle = 20
Angle = 30
45
45
40
40
35
35
30
-
23 0
ts
25
25
0
-
0
-
-
20
20
_
-
5
15
0
10
5
5
400
0-
500
600
700
0
800
Wavelength (nm)
0
0
0
0
400
500
600
700
Angle = 40
45
0
40
35
35
723 0
30
25
25
0
20
15
15
10
10
2
5
-
0
S
:
*
f 1
0
5
400
500
600
700
800
Wavelength (nm)
400
I
t
500
600
Angle = 60
Angle = 70
45
40
40
35
35
30
E 30
25
25
20
20
15
15
10
10
5
5
500
600
800
700
Wavelength (nm)
45
400
800
Wavelength (nm)
Angle = 50
H4 5
4
0
700
*
Decker
*
3m ESR
*
AI/SiO reference
qI"
400
800
Wavelength (nm)
500
600
700
800
Wavelength (nm)
Figure A-15: Measured reflectivities at several angles of JDecker coating, 3M ESR
film, and an Al/SiO front-face mirror. These uncalibrated measurements were made
with a Cary-500 and VASRA attachment for angular manipulation. The result is
that the JDecker coating performs better than the ESR and Al/SiO at all angles.
The lack of calibrated reference mirror prevents a quantitative statement from being
made.
173
A.3
Acrylic length and PMT neutron simulation
Acrylic plugs, affixed in the detector-center ends of the light guides, provide the
surface for TPB deposition and shielding of the argon target against neutrons. A
longer plug would do a better job shielding neutrons, but attenuate more signal
photons and reduce the light yield. Here is described a simulation quantifying the
effect of plug length on the neutron background, which resulted in a change of the
acrylic design length from thin (5 mm) to 10 cm.
About 40k neutrons per year are generated in (o, n) reactions in the borosilicate
glass of the PMTs, the largest source of neutron background (see Section 3.2.3). In
this simulation, neutrons are generated uniformly in the volume of the PMT glass,
with isotropic flux and energies drawn from the distribution of Figure 3-3. The Monte
Carlo events are passed through the DAQ simulation and to the energy and position
reconstruction algorithms described in Section 4. Then the selection cuts are applied:
events are required to have reconstructed energy between 75 and 150 photoelectrons,
position within the fiducial volume contained by radius 29.5 cm, and Fp > 0.70. Fp
(also called Fprompt) is defined as in Lippincott et al. [70]:
Fp -ff0
V(t)dt
fff V (t)dt
where T
=
(A.2)
to -- 50 ns, Tf = to +9 ps, to is the trigger time obtained when the summed
waveform is maximum, and the integration time o is set to minimize electronic recoil
contamination in the high Fp region. The requirement F, > 0.70 is set to allow ~ 50%
nuclear recoil acceptance.
Determination of o is the first task. For this, electron and WIMP events are
generated uniformly in the liquid argon volume, and F, is determined in each 5 PE
energy bin while scanning across values of the integration time . Figure A-16 shows
an exam ple F, calculation for events with 100-105 PEs and - = 100 us, w1th thi11
(5 mm) acrylic. The median Fp' and Fpx values are determined for the electron and
WIMP data sets along with the statistical variance -of the electron distribution. The
174
electronic recoil contamination (ERC) is defined as the fraction of electron events with
Fp greater than the median value of Fpx, assuming a Gaussian distribution for Fj with
o is chosen to minimize ERC. Figure A-16 shows the ERC and difference
variance o-.
between median Fe) and median Fp in the energy window 100-105 PEs, as a function
of
o is determined in each energy bin for a given acrylic thickness (Figure A-17),
.
o is used in the neutron simulation for that acrylic thickness.
and then the average
F100-1OPE 10-5P
---------
-LU
A
0o.35
......
-
-
200
10"o
OA
0.4
......
-V
O
Contamination
J25020
10-
F;; - <F>
0.4s
.
SA
C
150-
0.3-
-
0.25-
1000
100-
0.2
10
11
-4'
50
0
1
.
5-0-
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
F
2i
200
-t10
I
300
400
,4
500
(ns)
Figure A-16: Example determination of o for thin acrylic and energy 100-105 pho= 100 ns.
toelectrons. (Left) Fp for simulated electron and WIMP events, using
as a
(Right) Contamination and difference between median values of Fe and F,
which minimizes elecfunction of . The left (right) arrow indicates the value of
tronic recoil contamination (maximizes the difference between median Fp values). o
is chosen to minimize contamination, and for this energy window takes the value
140 ns.
Figure A-18 shows neutron backgrounds for thin and 10 cm acrylic as a function
of energy after application of Fp and radius cuts, and Figure A-19 plots the neutron
background as a function of acrylic thickness along with the percentage reduction
in light yield for each thickness.
The reduction in light yield is determined from
transmittance measurements taken at UNM
[111], used to select the acrylic vendor
RPT over Spartech since RPT's acrylic was more transparent. Presented with Figure
A-19 at a meeting in January 2011, the collaboration reached consensus to use 10 cm
acrylic, accepting the predicted 27 neutron background events per year and 6% light
loss relative to thin acrylic. Besides light loss, longer acrylic requires more rigid light
guides for support, which also disfavors use of longer acrylic.
175
C
F..
170
-
C"ItSmttIo.
"3
40
so
<F;
160
150
140
130
20
60
100
120
140
160
180
200
Upper bound of PE window (PE)
Figure A-17: o for thin acrylic as a function of energy expressed in photoelectrons.
The average value 135 ns minimizes contamination.
-No
~102
Com
-F,_.,
cut
-No
MRcut
-P,
0 0
102
10
Cu
10
Eve~n
703.5
In PIE Wm
9.5
S+
w
Everd
10-1
40
60
60
100
20.
0,
Thin acrylic
20
120
140
160
160
onbPE
WWW
10I
24Sh. 2.0
200
Reconstructed PE
0.9
10cm acrylic
[
0
20
40
60
6
100
120
140
160
160
200
Reconstructed PE
Figure A-18: PMT neutron background as a function of reconstructed photoelectron
number, before cuts, then with Fp > 0.7 and R < 29.5cm cuts. After cuts, 60.0
and 26.5 neutrons per year reconstruct in the energy region of interest 75-150 PEs
for thin (5 mm) and 10 cm acrylic. The results are normalized to 40000 neutrons per
year generated in the PMT glass.
176
-14
60---
-
Neutrons
Light yield
- 12
50-
-10
-o
Z40CD
-8
Cr
30-6
20-
-4
10-
-2
0
I
0
5
20
10
15
Acrylic Plug Length (cm)
I
-
z
25
Figure A-19: (Red) Number of neutrons per year which reconstruct in the energy
region of interest 75-150 PEs after Fp and R cuts, as a function of acrylic length.
(Blue) Percentage light loss relative to thin acrylic as a function of acrylic length.
The collaboration chose to use 10 cm length acrylic, coincident by chance with the
cross on this plot.
177
A.4
Inverted conflat validation and bolt selection
In order to maximize the radius of the fiducial argon volume, a non-standard conflat design was implemented for the junction of the IV ports with the spools. This
"inverted" design has the knife-edge outside the bolt pattern, so that the bolts themselves are sealed within the IV volume (Fig. A-20). This configuration allows the
ports to have the largest possible radii, given the dimensions of the IV. Since the light
guides must be inserted through the ports during assembly, the diameter of the port
sets the maximum diameter of the light guide face. The irregular hexagon light guide
has the largest face, and fits through the port with 0.149 inches clearance along the
guide's long axis.
Safety guidelines at SNOLAB require the IV be ASME-certified as a pressure
vessel. The relevant code is Section VIII Division 1, which limits the allowed torque
applied to the fastening bolts. Stainless steel bolts torqued to the standard were found
not to seat the flanges. This was the case for copper gaskets manufactured by both
Lesker and MDC, although those from MDC were more easily crushed and eventually
used underground. Therefore, custom bolts were fabricated by Vegas Fastener (Las
Vegas, NV) with Inconel alloy 718, which obtains a higher torque rating than stainless
steel. Inconel is a nickel-chromium alloy that is difficult to machine due to rapid workhardening. Correct annealing should be done in a reducing environment to prevent
oxidation.
Because the inverted conflat is non-standard, its function was verified in an apparatus at LANL, shown in A-21. In this test, a canister with an attached inverted conflat
was cooled to liquid argon temperature and pressurized with helium to 3200 mbar. A
helium leak detector was arranged to pump on the vacuum shroud to determine the
leak rate, which was found to be 8.9 x 10- mbarl/s.
A maximum allowed leak rate Lma, of argon from the IV into the vacuum shroud
can be estimated
Lmax = Pac * Ctube
where Pvac = 10- mbar is the pressure of the shroud, and Ctube
178
(A.3)
is the conductance
of the tube connecting the vacuum pump to the OV [112]:
Ctube = --
12
(A.4)
VD
L
D and L are the diameter and length of the tube, v = (8kT/7rm)1 / 2 is the mean velocity for a particle with mass m at temperature T, and k is the Boltzmann constant.
The equation is valid in the molecular flow regime, where the mean free particle path
is much larger than the pipe diameter. Ctube is also assumed to be much smaller than
the pump speed, which for MiniCLEAN's Edwards EXT turbo pump is 540 1/s. For an
argon particle at
TLAr=
87 K, a 3 m long pipe with inner diameter of 3 inches has con-
ductance Ctube= 8.2 l/s, so that the maximum leak rate Lmax is 8.2 x 10-5 mbarl/s.
This corresponds to Lma/1 8 4 = 4.5 x 10-7 mbarl/s for a single seal. The measured
leak rate for the inverted conflat fulfills this requirement by more than an order of
magnitude.
ASME certification of the inner vessel was performed at the fabricator Winchester's facility in New Hampshire. Unfortunately, the 6000 Inconel bolts arrived from
Vegas Fastener with a heavy black scaling, and schedule demanded assembly of the IV
with the dirty bolts. Later when the IV had arrived underground at SNOLAB, effort
was made to clean the bolt holes of the inverted conflats using wire brushes attached
to a drill and various wipes. Figure A-22 shows this process in progress, along with
a photograph of bolts before and after electropolishing by Waterloo Electroplating
in Ontario. Electropolishing was done after attempts to scrub the scaling off failed.
The scaling is likely high-temperature oxidation formed during the annealing process.
An x-ray fluorescence measurement done by the author at SNOLAB confirmed the
presence of alloy metals in the scaling (Figure A-23).
The collaboration's hope is
that the scaling does not leach from the imperfectly-clean bolt holes into the liquid
argon and degrade scintillation. This is reasonable since the scaling is relatively heavy
particulate and captured to some extent by the bolts in their threads.
179
S0 0
0 0
00
0
00
0
00
0
0
0
Q)
0
00
Q)
0
o Q
Figure A-20: (Top) Jeff Griego's (LANL) mechanical drawing of the spool from two
perspectives, showing a standard conflat on the left and inverted conflat on the right.
(Middle) Photograph of an IV port's inverted conflat, without a spool attached. The
inside of the IV can be seen through the port. (Bottom) Close-up photograph of
spool attached to IV. In the foreground (toward the outside edges of the photo) is
shown a standard conflat, with bolt-holes outside the radius of the knife-edge, where
the tophat is to be attached. The black heads of bolts fastening the spool to the IV
via the inverted conflat are in the photo's middle ground. The pictures were taken at
Winchester during test-assembly for pressure vessel certification.
180
)j
IG
OVC
RGA
Nitrogen
VN2
RN3
PR
N2
Cyl
II
MKS3
V N4
NI
N
VN5
640
VN3
V6
B1
RN2
Condenser
U-
0
9
~L~L
INVERTED SEAL TEST
221)
Figure A-21: Cassette test stand schematic (top). This stand was conceived to benchmark the optical properties of a full cassette in liquid argon, with light guide, reflector,
PMT, and TPB-coated acrylic. The schematic shows the configuration used to purify gaseous argon and condense it with liquid nitrogen providing cooling via a small
condensing canister attached to the main vessel. Fine temperature control of the
condenser is accomplished by pressurizing with nitrogen gas. During the inverted
seal test, assembled as depicted on the lower right, helium was substituted for argon,
and a helium leak sniffer was attached to the vacuum shroud.
181
Figure A-22: (Left) Alistair Butcher (RHUL) cleaning bolt holes. The blue and white
wipes were used to keep track of which holes had been marginally cleaned. (Right)
Comparison of bolts before and after electropolishing.
-
Scaling
-EP
Fe
bolt
-Blank
Cr
--
Co
Ni
Mn
*0
Zn
C
1
1r
o1y3
.1
Ti
CU,
,1,
...
keV
Figure A-23: X-ray fluorescence measurement of tape lifts from an Inconel bolt with
scaling and from an electropolished Inconel bolt. The listed metals are all present
in Inconel 718 alloy. The scaling is likely high-temperature oxidation which occurred
during annealing.
182
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