Photomultiplier Tube Calibration for the Cubic
Meter Dark Matter Time Projection Chamber
by
Kevin Burdge
Submitted to the Department of Physics
in partial fulfillment of the requirements for the degree of
Bachelor of Science in Physics
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Department of Physics
May 8, 2015
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. . . . .. . . . . . . .
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C ertified by ...............................
Peter Fisher
Department Head, Professor
Thesis Supervisor
Accepted by .................
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Associate Department Head for Education
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2
Photomultiplier Tube Calibration for the Cubic Meter Dark
Matter Time Projection Chamber
by
Kevin Burdge
Submitted to the Department of Physics
on May 8, 2015, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Physics
Abstract
This thesis concerns measurements I performed on photomultiplier tubes (PMTs)
and lenses to be used in the Cubic Meter Dark Matter Time Projection Chamber
(DMTPC) experiment. DMTPC is a new generation of detector, which takes the
idea of a standard time projection chamber and adds in some additional optical
elements, such as CCDs and PMTs. The goal of DMTPC is the directional detection
of the dark matter. During the course of my measurements, I characterized both
the absolute gains of DMTPC's eight PMTs, as well as the dark currents exhibited
by each of the PMTs. Seven of the eight PMTs demonstrated gains on the order of
106-107, and one PMT did not function at all. Of the seven working PMTs, six of
them had dark currents under 10 kHz, and one had an excessively high dark current
over 10 kHz. These gain values for the PMTs will give DMTPC the means to measure
the Z dimensions of the particle tracks it intends to image, and thus when combined
with the information from the CCDs will allow for full track reconstruction. DMTPC
will use lenses on their CCD cameras, and I also measured the transparency of these
lenses, and discovered that they are opaque below approximately 350nm. These
measurements will be essential for DMTPC, because they will provide information
about the relative amounts of light the PMTs and CCDs on the detector will register,
and thus provide key information for track reconstruction.
Thesis Supervisor: Peter Fisher
Title: Department Head, Professor
3
4
Acknowledgments
I would like to thank Professor Peter Fisher for his incredible mentoring, guidance,
and ability to inspire. Additionally, I would like to thank Ross Corliss for helping edit
this thesis, and all of his feedback and guidance on the research presented here. Also,
I would like to thank Hidefumi Tomita and Michael Leyton for their mentorship in the
laboratory. Finally I would like to thank Cosmin Deaconu for his explanations and
assistance with my work, and Gabriela Druitt, Natalia Guerrero, and Evan Zayas for
directly assisting in some of the measurements and research described in this thesis.
5
6
Contents
. . . . . . . . . . . . .
13
1.2
Weakly Interacting Massive Particles
. . . . . . . . . . . . .
15
1.3
Dark Matter Wind . . . . . . . . .
. . . . . . . . . . . . .
16
1.4
Dark Matter Detection . . . . . . .
. . . . . . . . . . . . .
16
.
.
.
.
.
Time Projection Chambers . . .
. . . . . . . . . . . . . . . . . . .
19
2.2
DMTPC's Chamber . . . . . . .
. . . . . . . . . . . . . . . . . . .
20
2.3
PhotoMultiplier Tubes . . . . .
. . . . . . . . . . . . . . . . . . .
21
.
.
.
2.1
.
25
Calibrating PhotoMultiplier Tubes
25
3.1.1
Light-Tightness . . . . . . . . . . . . . . . . . . . . . . . . .
26
3.1.2
PicoQuant Trigger . . . . . . . . . . . . . . . . . . . . . . .
26
3.1.3
Gain Results
. . . . . . . . . . . . . . . . . . . . . . . . . .
28
Dark Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
Dark Current Measurement Results . . . . . . . . . . . . . .
31
.
.
.
.
.
. . . . . . . . . . . . . . . . . . . . . . .
PMT Gain Measurements
3.2.1
.
3.2
33
Optical Response to Electron Avalanches
The CF 4 Secondary Scintillation Spectrum . . . . . . . . . . . . . .
33
4.2
The PMT response . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
4.3
The CCD Response . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
4.4
Total Responsivity
. . . . . . . . . . . . . . . . . . . . . . . . . . .
35
.
.
.
.
4.1
39
Results and Conclusions
5.1
U ncertainties
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
5.2
PMT Results
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
.
5
19
The Dark Matter Time Projection Chamber
3.1
4
.
Dark Matter .................
.
3
1.1
.
2
13
Introduction
.
1
7
5.3
Transparency Results . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
5.4
Conclusions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
43
A Code
8
List of Figures
1-1
Galactic Rotation Curve of Milky Way . . . . . . . . . . . . . . . . .
14
1-2
Example of gravitational lensing in Abell 1689 . . . . . . . . . . . . .
15
2-1
Sketch of a time projection chamber . . . . . . . . . . . . . . . . . . .
20
2-2
Sketch of a PM T . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
3-1
Dark Current of PMT PADM with room lights off . . . . . . . . . . .
26
3-2
Dark Current of PMT PADM with room lights on . . . . . . . . . . .
27
3-3
Block diagram of gain and dark current measurement setup . . . . . .
28
3-4
PMT PGEJ responding to light from PicoQuant . . . . . . . . . . . .
29
3-5
Example of baseline electronic noise in PMT PGEJ . . . . . . . . . .
30
3-6
Histogram of Peak Amplitudes of PMT . . . . . . . . . . . . . . . . .
31
3-7
Dark current in PMT PGEJ . . . . . . . . . . . . . . . . . . . . . . .
32
4-1
Secondary Scintillation Spectrum of CF4
. . . . . . . . . . . . . . . .
34
4-2
Pressure Dependence of the Secondary Scintillation Spectrum of CF 4
35
4-3
Quantum Efficency of Hamamatsu R1408 PMTs . . . . . . . . . . . .
36
4-4
Transmission of Light through X-ray and Optical Lenses
. . . . . . .
37
5-1
PMT PACI not responding to light from PicoQuant . . . . . . . . . .
40
9
10
List of Tables
Table of PMT gains and dark current . . . . . . . . . . . . . . . . .
.
5.1
11
40
12
Chapter 1
Introduction
Of the four forces that scientists observed in the universe, gravity was the first to
be described, through Newton's law of gravitation. Because of gravitational influence, astronomers understood the motion of heavenly bodies, and consequently made
discoveries such as the planet Neptune. After using gravity's predictive power to document the motion of the planets, astronomers observed orbits that did not exactly
agree with predictions, such as the precession of Mercury. This particular observation
was ultimately accounted for in a new, revised theory of gravity-Einstein's general
relativity. Then, astronomers, in their effort to understand the cosmos, encountered
a new gravitational anomaly-one that is still an active topic of research. We call this
anomaly dark matter.
1.1
Dark Matter
Quantum mechanics allowed physicists to understand atomic spectra in great detail.
In this discussion, one of the most important spectral lines is the 21cm line-produced
by the hyperfine transition in hydrogen. The line is incredibly common in the galaxy
because of the presence of interstellar hydrogen gas throughout it. Because the transition is forbidden by selection rules, it has a very long lifetime, and photons emitted
by the transition are unlikely to interact with other matter they pass through (except
for man-made antennas). This spectral line, with sources distributed throughout the
13
galaxy, allowed astronomers to observe the Milky Way in new ways.
By observing 21cm photons emitted from interstellar hydrogen gas, and their
Doppler shift, physicists such as Vera Rubin measured galactic velocity distribution
curves
[8].
Figure 1-1 on page 14 illustrates such a curve. After analyzing galactic
W. W. SHANE AND
o.
P. SIEGER-SMITH
30'
1.0
SO-
60'
339*
320*
310-
300*
km/secl
260
240
220
5
7
65
9
R, Ikpc)
9
Figure 1-1: This is an example of a rotation curve of the Milky Way. The curve
is a plot of velocity in km/s vs orbital radius in kpc. Taking into account only the
visible mass in the galaxy, Newtonian gravity and general relativity both predict a
decreasing velocity of stars as the galactic radius increases, but as seen in the figure,
observations suggest the opposite. Adapted from [9].
velocity distribution curves from numerous galaxies, physicists confirmed that both
classical Newtonian gravity, and Einstein's general relativity do not account for the
observed behavior if the only sources of mass are those consisting of visible matter.
Moreover, several extra-galactic observations of galaxy clusters and gravitational lensing suggest sources of gravitation that cannot be accounted for by visible galaxies.
One such example observed by the Hubble photographing Abell 1689 is illustrated in
figure 1-2 on page 15. Because these observations both point to an invisible source of
mass, physicists proposed one, and named it dark matter. Quantum mechanics has
demonstrated that mass comes in a quanta-what we call particles. Thus, physicsts
are searching for the quanta of dark matter: the dark matter particle.
14
LIMOUSIN ET AL
Figure 1-2: In this photograph taken by Hubble of Abell 1689, a distant galaxy cluster.
Notice the rings in the image around the center of the cluster. These distortions are
caused by the mass of the cluster gravitationally lensing the light passing through
it. GR can only explain the apparent degree of lensing if there is a large additional
source of mass- dark matter. Adapted from [5].
1.2
Weakly Interacting Massive Particles
In our understanding of how particles interact , there are four forces. The Standard
Mode very successfully describes the electromagnetic, weak , and strong forces. The
other major force , gravity, is the subject of Einstein 's theory of general relativity.
Dark matter has only been observed as a result of its gravitational effects, and our
observations indicate that it does not couple to electromagnetism.
Current searches for dark matter focus on finding a second interaction it exhibits
other than gravitation. DMTPC seeks to detect weakly interacting massive particle,
15
or WIMPs, which are just one candidate for a type of dark matter particle. This model
for dark matter proposes that it interacts via the Weak force, which is motivated by
the WIMP miracle-a mathematical coincidence in which the density of dark matter
in today's universe is consistent with the predicted density that a self-annihilating
weakly interacting massive particle would have in the modern universe [2].
1.3
Dark Matter Wind
We believe that dark matter aggregates in a halo1 around galaxies, binding them
together gravitationally and accounting for their rotation curve. In the halo model
for dark matter, we expect to observe a dark matter wind. This wind simply refers to
the fact that as massive bodies such as the Earth orbit the galactic center, they pass
through the cloud of dark matter that permeates the galaxy, and thus dark matter
particles that pass through the Earth create a wind in the direction opposite of our
orbit. In our orbit, we follow the constellation Cygnus, and thus expect this wind to
come from its direction.
1.4
Dark Matter Detection
Many efforts to detect dark matter focus on searching direct production of the particle in accelerators, or photons emitted by the particle self-anhilating. These forms of
detection are known as direction detection. The experiment in this thesis, DMTPC,
seeks to detect the presence of dark matter through directional detection.
Direc-
tional detection simply refers to a measurement seeking to find dark matter collisions
consistent with the location of Cygnus in the sky. While there are several other
non-directional collision based detectors, DMTPC has an advantage because it seeks
to correlate the collisions with the location of Cygnus in the sky-something which
would be distinguishable from other potential sources of noise. Ultimately, DMTPC
seeks to verify that collisions observed in higher density detectors in fact represent
'This is known as the Halo Model
16
dark matter collisions, and not some other source.
17
18
Chapter 2
The Dark Matter Time Projection
Chamber
2.1
Time Projection Chambers
Time projection chambers (TPCs) are a general class of detectors used in experimental
physics; a sketch of one is illusrated in figure 2-1 on page 20. They consist of a vessel
filled with a gas that releases electrons as a result of a particle passing through it.
An applied electric field then causes these electrons to drift across the chamber in a
region referred to as the drift region. The electrons then reach a second region of an
even higher electric field at the end of the chamber, called the amplification region,
where they liberate even more electrons. The electrons then produce image charges
on a plate, where the charge is then read out. The experimentalist can then describe
the particle tracks by studying this charge readout.
Studying the temporal characteristics of the charge readout on the plate carries
information about the trajectory of the particle track. If the charge readout is very
narrow in time, it indicates that the track entered the detector parallel to the plate,
and because all of the electrons produced by it had a very similar drift time. Alternatively, if the charge readout occurs over a larger amount of time, it indicates
that the track came in with a z component, and thus the electrons were liberated
varying distances from the plate, and took different amounts of time to get through
19
the drift region and reach the amplification region. The charge readout consists of an
array of pads, and thus gives an XY resolution of the particle tracks. Combining the
information from the charge readout and the temporal characteristics of the charge
readout , TPCs obtain 3 dimensional resolution.
High Polenuar
Figure 2-1: This is a sketch of a time projection chamber. Note the electrons liberated
from the particle track, and the arrows pointing away from the direction of higher
potential, indicating the acceleration of these electrons by the chamber's applied
electric field. Adapted from [6].
2.2
DMTPC's Chamber
The DMTPC's cubic meter chamber consists of a vessel filled with carbon tetrafluoride
as a gaseous scintillator 1 . We use CF 4 because it scintillates in response to nuclear
recoils- which is what we expect WIMPs to produce. Previous iterations of DMTPC
detectors have been calibrated using neutrons and alpha particles coming from strong
directional sources
[l]. Using a low density gas scintillator comes with the benefit that
the tracks of nuclear recoils manifest themselves in the scintillation medium because of
the large mean free path of the recoiling particle. By tracking the paths of the ejected
flourine in the collision (through the electrons it produces), we hope to reconstruct
the incoming momentum vector of the particle that produced the event. Smaller past
1
A substance which emits light as a result of interacting with certain species of particles
20
prototype detectors have illustrated this concept by successfully reconstructing the
direction of neutrons from a known source incident on the detector. An important
downside of using the gas scintillation method is that while it provides directional
detection because of the large mean free path of particles in the gas 2 , this also means
the medium has very low density, so its effective cross section for interacting with
dark matter is very small.
DMTPC differs from typical time projection chambers. Instead of reading out
the charge of the electrons produced in the amplification region, DMTPC looks at
the scintillation that occurs as a result of the amplification process-this is known
as secondary scintillation. To image the secondary scintillation, DMTPC uses charge
coupled devices, or CCDs. One can imagine a CCD as essentially an array of quantum
wells filled with electrons. As photons are incident on each of the wells, they scatter
some of the bound electrons out of the wells. The CCDs provide DMTPC with
the XY resolution, but do not give any resoultion in the Z direction because they
lack the temporal resolution of charge readout. Another challenge with using CCDs
lies in the fact that they continuously produce images, and have no way of easily
filtering out which images contain tracks from events. Thus, we need a device that
provides temporal resolution of the secondary scintillation, and also one to filter which
images are useful ones containing nuclear recoils, and this involves setting up another
device that can detect their occurrence-preferably an analog device that can be run
continuously. This instrument is the photomultiplier tube, or PMT.
2.3
PhotoMultiplier Tubes
PMTs use the photoelectric effect to detect photons.
The vast majority of pho-
toelectric events in PMTs involve one photon liberating a single electron from the
photocathode. Exposing the photocathode to some flux of photons above its work
function will produce a current proportional to the flux. The PMT allows scientists
2
The large mean free path of the recoiling flourine means that it produces drift electrons along a
longer path
21
.........................
..........
to probe small amounts of flux, by amplifying the small amount of current produced
by the photoelectrons produced in the photocathode. It amplifies these electrons by
the application of a high voltage across dynodes. The ratio of the number of electrons
produced after amplification to the number of photoelectrons before the amplification
is called the gain of the PMT. A major part of my work for DMTPC consisted of
measuring the gain of each of our eight PMTs. This value is significant because it will
allow DMTPC to look at the characteristics of the traces from each PMT and determine the flux produced in the scintillation, and thus will help distinguish different
types of scintillation events. The gain of a PMT is produced because, upon entering
the externally applied electric field, the photoelectron accelerates, and scatters into
a dynode, liberating more electrons. All of these electrons then also feel the applied
electric field, and scatter into the next dynode, repeating the process, and multiplying the number of electrons. After a series of these dynodes, single electrons can be
turned into macroscopic, measurable currents. A rough sketch of a PMT is illustrated
in figure 2-2 on page 22. In our experiment, we use Hamamatsu 1408 PMTs, which
Photocathode
Focusing electrode
Ionization track/
High energy
photon
Photomultiplier Tube (PMT)
Low energy photons
Connector
Scintillator
Primary
electron
Secondary
electrons
Dynode
Anode
pins
Figure 2-2: An exmaple of a 12 dynode stage PMT responding to an external scintillator. [10]
have a 9 dynode amplification with an applied potential difference of around 2000V
across them. This amplification produces on the order of 10' electrons per incident
photoelectron. The ratio of the number of electrons produced after amplification to
the number of photoelectrons before the amplification is called the gain of the PMT.
A major part of my work for DMTPC consisted of measuring the gain of each of our
eight PMTs. This value is significant because it will allow us to look at the character22
................
istics of the traces from each PMT and determine whether they represent a nuclear
recoil or not.
23
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24
Chapter 3
Calibrating PhotoMultiplier Tubes
3.1
PMT Gain Measurements
To determine the gain of DMTPC's PMTs, I measured the current output of the device
in response to a photon flux incident on the photocathode. The most challenging
aspect of this measurement was accurately estimating the incident flux of photons.
The challenge came from the PMTs being sensitive enough to detect extremely low
levels of light-making it necessary to only expose them to very low intensity sources
or risk damaging them. I thus had to ensure that my testing environment was light
tight, and used a dark box to accomplish this, as described in section 3.1.2. Initially,
I tried using an LED (light emitting diode) as my light source, but discovered that
the intensity of light the LED fluctuated too much. I finally successfully measured
the gain by switching to a device known as a PicoQuant, which is a pulsed LED
picosecond laser. The PicoQuant works like a standard laser, just it uses a rapidly
pulsed LED to produce the population inversion in the laser medium. The function
generator used to drive the PicoQuant is finely tuned to the LED used to excite the
laser medium, and the resultant laser has a very stable flux, and can be tuned to very
small, even single photon levels of flux.
25
_11___-__I_
3.1.1
-
I--
- - -
-
I
__
-
.
..
__
-
_-
-
i
-
_-
- -1 -
-
-
q:W-:IV
._
.
K:
-_
X *1 .
Light-Tightness
I pointed the PicoQuant directly at the center of the PMTs from about a foot above
them, and the entire setup took place in a dark box. To ensure there was no background light, I placed each PMT in a second dark box, covered by an additional black
tarp, and turned off the lights in the room. I then recorded the rate of events1 , and
compared that to the rate of events in just the outer dark box with the lights in the
room turned on. Because the rates were similar in both situations, I concluded the
single outer dark box was sufficiently light tight for the gain calibration measurements. I illustrate an example of dark current measurements for the PMT PADM
with the lights turned off, and the lights turned on in figures 3-1 and 3-2.
PADM Lights Off
iO*0
.
10
a
6
U)
U
C
4
2
0
I
-2
-4
0S
90.6
12
0
~11
1.4
I
Time (Seconds)
1.6
1.6
x 10.
Figure 3-1: One thousand 10 ps sweeps of PMT PADM in the dark box with the room
lights turned off. There are a total of eighteen events with an amplitude greather than
l mV.
3.1.2
PicoQuant Trigger
The PicoQuant had an external trigger input that was used to control when a laser
pulse was fired at the PMT. This trigger was supplied by a pulser, and the signal was
simultaneously fed the PicoQuant and to a Lecroy oscillioscope to trigger its sweeps
as well. This setup is illustrated in figure 3-3 on page 28.
'I defined events as traces with an amplitude greater than 1 mV
26
.....
.......
.....
..
..........
...__
......
. ...
.......
-
-
I
- _
'
'
-
-_ . .
PADM Light On
12n
12
10 --
a --
V
U
0
6
-
(h
4
0.8
.
1
'
1.2
'
-2 --
14
1.
1.8
X0"
Time (Seconds)
Figure 3-2: One thousand 10 ps sweeps of PMT PADM in the dark box with the
room lights turned on. There are a total of sixteen events with an amplitude greather
than 1 mV.
After one thousand sweeps, I observed the plot in figure 3-4 on page 29. The fact
that the pulser triggered these sweeps as well as the PicoQuant, and that almost all
of the peaks occurred at the same time, serves as strong evidence that these peaks
were in fact produced by the PicoQuant firing. To further verify that these peaks
were in fact produced by the PicoQuant, I blocked it from the PMT with a black
tarp, and conducted the same measurement again. It yielded no peaks, reinforcing
that I had been in fact observing light produced by the PicoQuant. I then adjusted
the intensity of the PicoQuant such that a peak was observed only one out of every
ten sweeps of the oscilloscope. This adjustment of intensity allowed me to apply the
regime of low-count Poisson statistics: Equation 3.1 describes a Poisson distribution,
where P(k) gives the probability of observing k photons, with A representing the rate
of photons, or -.
P(k) = Ak
Since ninety percent of our events are null events (k
(3.1)
=
0), we know that A is small
and can expect approximately nine to ten percent of observations to be single photon
events (k = 1), and on the order of one percent double photon events (k
=
2), etc.
I computed the gain by integrating the traces of all non-zero events, and averaging
these values, since each included trace represented on average one photon. When I
27
..
....
..
......
...
Pulser
PicoQuant
Oscilloscope
PMT
I
I HV Power Supply
Figure 3-3: This schematic illustrates a block diagram of my setup for the gain and
dark current measurements on the PMTs.
integrated these traces, I used the known impediance of the oscilloscope (50 Ohms) to
calculate the current readout from the PMT. I then performed this on all the PMTs,
and recorded my gain values in table 5.1 on page 40 in the results section.
3.1.3
Gain Results
In all of my measurements, I applied 2000 volts to the PMTs. For each PMT, I
recorded one thousand sweeps on the oscilloscope, corresponding to one thousand
trigger events from the pulser. The sweeps for one PMT, when superimposed on a
plot, are illustrated in figure 3-4:
As seen in figure 3-4, most of the events occurred at the same time relative to the
pulser triggering, and when I covered the PMT these events disappeared; this verifies
that they in fact correspond to light arriving from the PicoQuant.
I computed the gain by writing a MATLAB script which integrated the peaks
across all of the sweeps, and outputted the average integral of the peaks. I included
my MATLAB scripts in an appendix. Because most of the sweeps contain no peak,
28
--
"...
-
-
......
. ....
x 10·'
1000 Traces of PMT PGEJ
1 2~~--~--~-~--~--~--~-~-----~
10
- 2 ~~
, _ 1-2--1.~1
22--~1
. 1 2-4--1.~216--~1
. 128
_ _ _1~
_ 1 3---,
_ 1~
32 --1~
. 134
___
1 . ~1
36-~l.138
Time (Seconds)
x 10·•
Figure 3-4: One thousand sweeps of the oscilloscope observing PMT PGEJ respond
to one thousand triggers of the PicoQuant. The figure clearly illustrates that almost
all of the events happened between 1.128 x 10- 5 and 1.132 x 10- 5 seconds, indicating
that these events were produced by the PicoQuant. When I computed the gain I
integrated only the sweeps that met my trigger threshold after 1.128 x 10- 5 seconds
and before 1.132 x 10-5 to rule out the dark current events (for example, in this plot
the first two peaks).
I first told the code to isolate the sweeps with peaks exceeding a 1 mV amplitude.
I chose 1 mV because the baseline noise level in all of the PMTs varied, but never
exceeded 0.8 mV, so lmV allowed me to avoid triggering on it , but still capture all
relevant events. Figure 3-5 gives an example of the baseline electronic noise level in
PGEJ, and figure 3-6 illustrates why I chose the 1 mV cutoff for events. The code
then further isolated only those sweeps with events in the region corresponding to
events from the PicoQuant. Finally, the script then integrates all_ of the qualifying
sweeps in the region where all of the peaks occurred. Because these peaks correspond
to almost all single photons by Poisson statistics, their average represents the single
photon gain , or absolute gain of the PMT.
29
Electronic Noise in PGEJ
0.8
0.6
0.4
Ci)'
0.2
.:t:::!
0
2:Q)
C)
0
ns
~ -0.2
>
-0.4
-0.6
-0.8
-1
1.12
1.122
1.124
1.126
1.128
1.13
Time (Seconds)
1.132
1.134
1.136
1.138
5
x 10·
Figure 3-5: This is a zoomed in version of figure 3-4, and is an example of the baseline
electronic noise in PMT PGEJ , which ranges from -0.7 mV to 0.8 mV on the interval
in this figure. This noise was the reason I chose to use a lmV cutoff across my
measurements for what constituted an event.
3.2
Dark Current
In addition to determining the gain of the PMTs, I also measured their dark current.
Dark current consists of events registered by the PMTs that are not produced by
incident photons. Such events can be the result of thermionic emission either from
the photocathode, or one of the dynodes. An idea to measure the gain was to observe
dark current events, because such events represent single electron events. However ,
there is a flaw with this technique: thermionic emission can occur at any of the
dynodes, which means that although it represents single electron events, these events
do not necessarily undergo the full amplification associated with passing through all
nine dynodes in the PMT. The dark current events that are not fully amplified are
so small that they have little consequence on any measurements of the secondary
scintillation that occurs in DMTPC 's detector.
30
Histogram of Peak Amplitudes
400
-
350
-
300
-
0 250
0
-
100
60-
0
0.6
1
1.5
2
2.5
3
3.5
Voltage (Volts)
4
6
4.6
X10
Figure 3-6: This is a histogram of the peak voltage amplitude in one thousand sweeps
of the PMT PGEJ responding to the picoquant. Note that an overwhelming amount
of the events occur in the bins below 1 mV. Because this occured for all of the PMTs,
I decided that 1 mV serves as an appropriate cutoff to isolate events that are just
electronic noise.
3.2.1
Dark Current Measurement Results
I determined the dark current of the PMTs by isolating the PMT from any sources
of visible light. I accomplished this by placing the PMT in a small, black, light-tight
metal darkbox, covering this darkbox with a large black tarp, and enclosing all of
this in an even larger dark box (as opposed to the single darkbox I used for the gain
measurements). In order to ensure that everything was completely light tight, all dark
current measurements involved two runs-one with the lights in the room turned on,
and the other with all light sources in the room turned off, and all windows covered.
The rates of dark current were similar in both cases, indicating that what was being
measured was in fact dark current, and not background light. I measured the dark
current by taking ten ps sweeps on the oscilloscope repeatedly one thousand times,
and then counting the total number of peaks with an amplitude exceeding 1 mV on
this interval. After combining the thousand traces, the interval I counted events over
corresponded to ten milliseconds, so I multiplied the number of events by one hundred
to obtain the dark current in units of hertz (number of events per full second).
31
. ..........
...
....
.........
.....
In measuring the dark current, I used a much larger time scale than with the
game measurements because at the time scale of the gain measurements dark current
events are extremely rare. When I ran one thousand sweeps of the oscilloscope on
one of the PMTs, I observed the plot in figure 3-7.
10
Dark Current for PGEJ
x10"
-
6
0
0
4
0-
0
b.6
0.7
08
09
1
1
1
lime (Seconds)
X10"
Figure 3-7: This is a dark current run on the PMT PGEJ. It consists of one thousand
10 ps sweeps of the PMT while it was exposed to no light sources. The dark current
events are the numerous sharp peaks.
32
..
....
......
. ...
.. ..............
...
..........
. ....
..
..
Chapter 4
Optical Response to Electron
Avalanches
4.1
The CF 4 Secondary Scintillation Spectrum
The cubic meter detector relies on the light produced during secondary scintillation of
CF 4 to image particle tracks in the detector. Primary scintillation refers to the light
produced by the nuclear recoil in the drift region, whereas secondary scintillation
refers to the light produced by the electron avalanche in the amplification region.
This spectrum is significant to DMTPC because the PMTs and CCDs in the detector
will respond to light from this spectrum, and have different sensitivities to various
DMTPC has measured the secondary scintillation spectrum of CF4
,
wavelengths.
and observed the spectrum in figure 4-1.
DMTPC plans to operate the detector at pressures on the order of 10-100 torr.
DMTPC measured the spectrum in figure 4-1 at 180 torr. Recently, more research
has focused on studying the behavior of CF 4 's secondary scintillation, and found
a complicated pressure dependence, as illustrated in figure 4-2. The measurements
in figure 4-2 occur at pressures between 1 and 5 bar, and thus do not extrapolate
well to DMTPC's detector.
However, the complicated pressure dependence could
still manifest itself at low pressures, and thus merits future investigation because
of its consequences on the PMTs and CCDs. Specifically, because of their different
33
sensitivities across the spectrum, the pressure dependence of the spectrum will impact
the relative amount of light registered by the PMTs and CCDs.
0.016
0.014
0.012
U
I
C
0.01,
0.006
jb
*
0.006
B
-
0.004
0.002
0
200
Mo
400
110 70oo 00
M.v (rnj
M
Figure 4-1: This is the secondary scintillation spectrum of CF 4 , as measured by a
Hamamatsu H1161 PMT at 180 torr. There are two regions of high intensity-one
centered around 300nm, and another around 650nm. Adapted from [3].
4.2
The PMT response
The PMTs being used are Hamamatsu R1408s. When light enters these PMTs, it
passes through the CF 4 gas in the detector, a quartz glass window, the air in the
PMTs mount, and the face of the PMT. The quantum efficiency of these PMTs, as
illustrated in figure 4-3, makes them responsive almost exclusively to the UV peak of
CF 4 's secondary scintillation spectrum. Thus, the PMTs will be detecting only this
part of the spectrum, and very little of the red line. Various manufacturers of quartz
materials claim that quartz is over 80% transparent to wavelengths all the way down
to below 200nm, so the transparency of the quartz windows should encompasse the
entire spectrum of interest.
34
Charge gain -70
-5
bar
bar
-4
-3
bar
2 bar
1 bar
-
( .do
0.8
E
0.6
0.40-
0.4
0.20-
-0.2
0.00
200
300
600
Wavelength, nm
400
500
.
0.80-
'0.0
700
800
Figure 4-2: This is a measurement of the secondary scintillation spectrum of CF 4
across a range of pressures, from 1 to 5 bars. The two major peaks in the UV and
red part of the spectrum exhibit a complicated pressure dependence. Significantly,
the behaviors of the two peaks are not completely the same. Notice that the intervals
between the maximum height of the red peak are much more evenly spaced than
those of the UV peak across the different pressures. Adapted from [7].
4.3
The CCD Response
The CCDs are senstive to the red peak, but not the UV peak. The primary reason for
this lies in the lenses mounted on the CCDs. We conducted transparency measurements of these lenses across the spectrum, and observed their transparency drop off
completely around 350nm, as illustrated in figure. We conducted the measurement
in figure by placing our lens between a monochromator and a quartz glass mercury
spectral lamp. We chose to use this light source because of its strong UV peaks, and
the quartz's transparency to UV light. As highlighted in the figure 4-4, we found a
drop off in transparency around 350nm. The CCDs will thus have very little response
to the UV peak because of the opacity of their lenses to UV light.
4.4
Total Responsivity
After considering all of the optical components involved in imaging the electron
avalanches in the detector, the total response of the PMTs and CCDs are given
in equations ?? and 4.3, respectively.
35
..
...........
.......
. .....
.
.........
IIL
I
.
.
II II I ,
It,'
.
"11
jl
'
-
.
-
-
-
-
,
-
-
-
70
U
Monte Carlo Absorbtivity
>1 60
-~50
momooano*oooooooo n Effcincy
40
b
t
00
20
0
7
o
in
0 0~*
250
300
350
400
450
500
550
00
700
650
Wavelength (nrn)
600
Figure 4-3: This is a measurement of the quantum effiency of Hamamatsu R1408
PMTs. The relevant datapoints are the stars, which represent the measured quantum
efficiency of the PMTs across the spectrum. Note that the spectral response drops
off almost completely around 650nm, which is the center of red peak in the CF 4
secondary scintillation spectrum. The center of the UV peak occurs at 300nm, and
extends up to 400nm, which is a region the PMTs are very sensitive to. Adapted
from [4].
PMTSignal(A) =
F(XP; A) * TQuartz(A) * TcF4 (A)
* TAir(A) *
PMTQE(A) * PMTGain-
(4.1)
Where the total signal depends on the total number of photons produced by the
scintillation event that could be incident on the PMT, y(A), the transmission of the
quartz window, the CF4 gas, the air in the PMT mount, the quantum efficiency of
the PMT, and the PMT's gain. Note that the total photons produced, -y(A), factor
into two components, as illusrated in equation 4.2.
7(yQ, P; A)
7Geometric(x)
y(P; A)
*
YGeometric(X),
(4.2)
refers to the geometric cross section of light from the avalanche that
the PMT sees. This factor follows a
I
dependence, and also depends on the angle
the light strikes the window and PMT at, since it is more likely to reflect for large
angles of incidence. This part of -y is independent of the other part of gamma, -Y(P; A).
This term is the scintillation spectrum, where after specifying a pressure P, there is
a function of lambda that gives the flux at every wavelength, as illusrated in figure
36
-Uncovered
Optical Lens
_X-Ray Lens
10
10
10
10
10
19'Q0
3500
3000
4000
4500
Wavelength (Angstroms)
Figure 4-4: This is a measurement of the transmission of light from an elemental
lamp through optical and x-ray lenses to be used on the CCDs of the detector. The
figure clearly illusrates that the transmission of light through the lenses drops off at
around 350nm by comparing them to the blue line, which represents the spectrum
seen without any lenses in front of the lamp.
4-2.
Now performing a similar analysis for the CCDs, we arrive at equation 4.3.
CCDSignal(A) - y(*, P; A)
*TQuartz(A)
*TcF4 (A) *TAir(A) *TLensCCDQE(A). (4-3)
These two equations have many terms that cancel, such as transparencies of the
CF 4 , and windows. However, some terms, such as the PMT gain, quantum efficiencies,
and lens transparency do not cancel. All of these non vanishing terms in the ratio of
the signals represent significant information that DMTPC could use to determine the
relative signals the detector expects from the PMTs and CCDs for various wavelengths
and conditions.
37
.
.....
....
38
Chapter 5
Results and Conclusions
5.1
Uncertainties
I computed the uncertainty in the value of the absolute gain by calculating the integral
of the peaks of two different distributions for each PMT. One of these distributions
was the average integral of the peaks with a 1mV cutoff defining the minimum height
for the trace to qualify as an event, whereas the other was determined by setting the
threshold for an event as one increment higher than the highest value the baseline
electronic noise level reached on the oscilloscope (where the increment is the minimum
voltage resolution of the oscilloscope). I then compared these two values: The 1 mV
cutoff gave a higher value because it did not include the smallest peaks. I then took
the difference between these two values to be the uncertainty in the gain. Because I
computed the dark current from a simple Poisson process1 , I found that the greatest
source of error was in the uncertainty of the number of events occurring in such
counting processes, which goes like the square root of the number of events.
5.2
PMT Results
In measuring the gain and dark current of the PMTs, I discovered that one of the
PMTs, labeled PACI, produces no signal other than electronic noise. Regardless of
las illusrated in equation 3.1
39
PMT
PADM
PAZE
PEEW
PGEJ
PGEQ
PGMQ
PHDY
PACI
Gain
(4.86 ± 0.6) x 106
(8. 75 ± 0.28) x 10 6
(6.99 ± 0.54) x 106
(3.88 ± 0.07) x 106
(5.31 ± 0.51) x 10 6
(5.61 ± 0.39) x 106
(6.04 ± 0.2) x 106
Dead
Dark Current (kHz)
1.800 ± 4.24
5.800 ± 7.61
13.700 ± 1.170
1.600 ± 4.00
3.600 ± 6.00
2.100 ± 4.58
1.900 ± 4.35
Dead
Table 5.1: This table documents the final measured gains and dark current for all
eight PMTs, and includes the uncertainties I describe in section 4.2.
what light I exposed the PMT to , it only recorded the background electronic noise
voltage , and did not even produce dark current events. A plot of PACI 's response to
the PicoQuant 's light is shown in figure 5-1. All the other PMTs were functional and
yielded results for both measurements, as illustrated in table 5.1:
PMT PACI
Iii"
.:I:!
~
11)
2
Cl
ra
.:I:!
0
>1
...
...
~ .,
.....
.
( t~
l >• ~
"'\'
~r. ~ . ~~J
~
.
.: ... .
!'l: ~
"-1. '
'!l.9
. _'!·
1_; ,,
~
•
~.
~
:t•
1
,•
~
~
.
.••~ .
-1 ' - - - - - - - - - ' - - - - ' - - - - ' - - - - ' - - - - ' - - - - ' - - - - ' - - - _ _ , _ __
1 118
1.12
1.122
1.124
1.126
1.128
1.13
1.132
1.134
__.___
1.136
••
v ••
4\
•
~\'~ '" :t: :
. ,1
Time (Seconds)
·1·· ' t • '1,, ..
~
~
~~
·· ~(•
___.____,
1.1 38
x 10·•
Figure 5-1: One thousand sweeps of PACI being exposed to light from the PicoQuant.
The image clearly demonstrates the lack of any signal coming from the PMT other
than electronic noise. PACI's dark current test also failed to yield any events.
All the working PMTs exhibited gains on the order of 10 6 -10 7 , which is consistent
with the expected value of 107 fromthemanuf acturer. I believe my measurements
yielded results slightly lower than 10 7 because my measurement technique allowed me
to recognize extremely small low gain events. I am confident in these being actual
events, because of their temporal proximity to all the other events produced by the
40
PicoQuant, as illustrated in the figure of sweeps earlier. The dark noise of all the
functional PMTs fell in the regime of a few kiloHertz, which is consistent with previous
measurements of their dark current. However, one PMT proved an exception to this:
PEEW exhibited a dark noise of 13700 Hz, which stands noticeably out among the
PMTs.
5.3
Transparency Results
The xray and optical lenses to be used on the DMTPC CDDs are very opaque to
the UV peak of the CF 4 secondary scintillation spectrum, but transparent to the
red one. Because of the low quantum efficiency profile of the PMTs at the red peak
in the CF 4 spectrum, they will primarily respond to photons produced by the UV
peak. In contrast, because of the lenses opacity to UV, the CCDs will respond to the
red peak in the scintillation spectrum, and not the UV one. Thus, the two forms of
optical detectors on the chamber will have responses to orthogonal parts of the CF 4
spectrum.
5.4
Conclusions
Six of the eight PMTs intended for use on the cubic meter are in good working
order. The PMT PEEW exhibited one of the highest gains of the PMTs, but with an
excessive dark current of 13700
1170 Hz. Because DMTPC will not be triggering on
small, single photon events, this PMT should still be a useful instrument for use on the
detector. However, PACI exhibits no signal at all, but might be reparable if opened up
and investigated. For the six PMTs in good working order, the gains and dark currents
are all relatively similar, and thus they should exhibit relatively similar responses to
secondary scintillation events. These PMTs will provide DMTPC with information
about the z dimension of the nuclear recoils through the temporal characteristics of
their response to avalanches. Additionally, by using the well characterized gains of
these PMTs, DMTPC should be able to determine the electron avalanche's spatial
41
location in the XY plane of the amplification region because the response of each
PMT can be used to calculate the relative distance of the avalanche from each PMT
2.
Ultimately, with the gains in table 5.1 DMTPC will have the information necessary to
identify nuclear recoils from the PMT signal. Additionally, with the information about
the CF 4 spectrum, and the CCD lens transparency, DMTPC has some key information
about factors that influence the relative responses of the PMTs and CCDs, since they
are responding to opposite parts of the CF 4 scintillation spectrum. In light of the
observations in chapter 4, future work DMTPC should consider for the cubic meter
detector includes: verifying that the transparency of air in the PMT mounts does not
impact the light reaching the PMTs; verifying the transparency of the CF4 gas in the
chamber; testing the transparency of the quartz glass windows; and measuring the
pressure dependence of the CF 4 secondary scintillation spectrum at pressures close
2
Since the flux hitting each PMT originated from the same event, and decays as
42
.
to operating pressure.
-,I--
::"..::":::. ......................
-
-
:.
I
-
MMIMEW
. -
I
- -
-
-
-
--
-
.
-
.
..
. ....
..
.....
. .....
Appendix A
Code
matfiles = dir(fullfile('D:' ,
'UROP',
'PMTs',
'Tests2000V',
L=[];
for i =1:length(matfiles);
data = load(matfiles(i). name);
if max(data(:,2))>0.001
n=integ2 (data);
L=[L, n];
end
end
hist (L, 100)
mean (L) /50
(mean(L)/50)*10^(-12)/(1.6*10^(-19))
function
integ2=integ2 (A)
B=A ( :, 2);
C=B (98:129)
43
-31
'PGEJ',
'*.dat'));
W=A(2, 1) -A(1, 1);
Y=O;
for n =
1:length(C);
Y=Y+W*C (n);
end
integ2=10^12*Y
end
end
44
.............
......
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[101 Wikipedia. Photomultiplier
accessed 25-April-2015j.
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46