Measuring the Neutral Current Event Rate in SNO Using
3He(n,p)t
Sudbury Neutrino Observatory (SNO)
SNO Physics Goals
Neutral Current Detection via 3He(n,p)3H
Water target
SNO is a high count rate detector, sensitive to e , ,
The neutrons produced by the neutral-current dissociation of deuterium
can be detected via the 3He(n,p)3H reaction. An array of 3He–filled
proportional counters is being built for installation in SNO. The
parameters of the Neutral Current Detector array are:
1.4
Search for  Flavor Change
NC rate and CC/NC ratio
1000 t
1,700 t (sensitive volume)
Energy Spectrum Distortion Due to  Oscillations
CC 8B+hep energy spectrum
e- + e  e- + e
(ES)
2H
+ e  p + p + e- - 1.44 MeV
(CC)
2H
+ x  p + n + x - 2.22 MeV
(NC)
Time Dependent Solar  Flux
Observation of 7% orbital eccentricity
Day-night asymmetry
Solar magnetic field effects
Expected Signal
pure e SK flux
BP1998 SSM
pure  SK flux
yr-1
NC
2030
4610 yr-1
13,600 yr-1
Cable endcap
with acrylic spacer
High Energy Neutrinos
0.0
Resistive coupler
(cable end only)
Counter body
(3He-CF4 gas)
Nickel endcap
body
3He(n,p)3H
Fused-silica
insulator
Delay line
termination
 Surface and Bulk Alpha Activity
232Th and 238U chains in the NCD walls, along with 210Po surface
activity, produce ’s that underlie the neutron capture peak. These
events can be rejected by event by event analysis of digitized pulses.
(see “Event Identification by Pulse Shape Analysis”)
 Electrons and Gammas
b’s and ’s from the 232Th and 238U chains can only deposit 764 keV
through extensive multiple scattering. Less than 2x10-4 fall into the
neutron window.
Vectran braid
Anchor balls
Data Acquisition and Electronics
96 NCD `strings' connect to current preamplifiers that
produce signals that go to the electronics.
The noise level is approximately 2 mV rms in a 30-MHz
bandwidth, and the largest signal the preamplifier can
deliver is 2.5 V.
NCD Event Rates are dominated by neutrons and alpha
particles. Neutrons from muon interactions and NC
events are expected to be detected at a rate of 15 per
day, and alphas 1000 - 10000 per day. The longest
duration of the signal (apart from the ion tail) is about 3
s, corresponding to the drift time across a detector.
Preamplifier Signals enter 2 parallel buffer amplifiers,
one that drives 20-m long cables to the shaper-ADCs
that reside in VME, and the other that drives a delay line
and a discriminator. The delay line provides a delay of
320 ns.
Pulse Digitization is done with two Tektronix 754A 4channel oscilloscopes. Each scope services all 96
inputs, with the equivalent scope inputs connected in
parallel to 24 multiplexed channels. Scopes provide one
level of buffering and permit digitization of pairs of
events closely correlated in time.
 High Voltage Microdischarges
HV induced surface discharge at the endcap can produce pulses.
However, all components have undergone extensive high voltage
testing and 100% discrimination is expected by pulse shape analysis.
Current
Preamp
Log Amp
~300ns
Delay
SNO MTCD
Shaper
ADC
GTID
Counter
Summing
Junction
Tek 754
DACs
& ADCs
Tek 754
32 Bit
differential
digital
I/O
Sources:
232Th chain
238U chain
56Co
e
3He
p

2H
3He
p
VME
GBIP
Controller
1.0
 290 out of 300 counters constructed
 233 counters at Sudbury in cooldown underground
 Radioassay of construction components complete
 208Pb,
214Bi  214Po,
56Co  56Fe,
208Tl

t

All Neutron Backgrounds (Estimates)
E > 2.22 MeV
E = 2.615 MeV
E = 2.445 MeV
E > 2.224 MeV (31%)
SNO Detector
 238U,232Th in water
 ’s from PMT’s and their support structure
 (,p) and (,n) at PMT’s and support structure
Neutrons/year
Photodisintegration Background
U in D2O
(20.0 fg/g)
Th in D2O
(3.7 fg/g)
U in NCDs
(4.0 pg/g)
Th in NCDs
(4.0 pg/g)
56Co in NCDs
(after 200 days u.g.)
Muons (tagged)
U Fission
D(,n)p
17O(,n)20Ne
NCD Detectors
, E=0.7-2.0 MeV

 or n parallel
to wire
238U, 232Th, 56Co
in NCD bodies
Diagnostic Techniques
Indistinguishable Backgrounds:
 Photodisintegration Background
Gamma rays with E >2.22 MeV can disintegrate
deuterons and liberate neutrons. This background
is indistinguishable from the neutral-current signal
and so must be measured and subtracted.
(see “Photodisintigration Background”)
 Observation of Cerenkov light from associated ’s
 Radioassay techniques
 Estimate from NCD  signal
GPIB
VME
ECPU
0.8
Anode
n

0.6
Photodisintegration Background
To determine the neutron capture rate on 3He it is necessary to
discriminate spurious events. A first cut can be made by measuring
the energy of the event (right). This leaves a substantial background.
Typical signals (below) look very different. Additional parameters such
as rise-time or pulse width help distinguish between pulses. A
“background free” window can be drawn in a pulse width vs. energy
parameter space. This window rejects alpha and beta events with an
approximately 50% cut in the efficiency for detecting neutrons.
NCD MUX/Trig
Controller Card
0.4
 e flux /  e SSM flux (BP98)
Event Identification by Pulse Shape Analysis
NCD String (1 of 96)
12 Multiplexer
0.2
 + 2H  p + n,
p
 Separation of charged-current (CC) and neutral-current (NC)
events in real time by use of 3He proportional counters
 Signal/Background is determined simultaneously
 Observation of secular variations and supernovae
Status of Construction (June 2000)
 or , E>2.22 MeV
Distinguishable Backgrounds:
 Tritium in 3He
3H decays deposit on average 6 keV in the gas but pile-up can produce
proportional counter signals above threshold. Low-temperature
purification of the 3He has resulted in negligible background levels.
Oscillation
Hypothesis
0.2
0.0
3H
neutron capture:
0.6
0.4
Neutrino Signal: Neutron from NC interaction
Neutrons capture via 3He(n,p)3H in the NCD and produce 573 keV p
+ 191 keV t ionization tracks.
Pinch-off
fill tube
 775 m total length
 300 Ni CVD detectors (2 inch diameter)
 96 vertical strings on a 1 m square grid
 Estimated neutron capture efficiency  37%
Motivation
SuperK
 SNO can resolve the Solar Neutrino Problem,
independent of solar models
Proportional Counter Signals
B Flux Depressed
0.8
Search for Supernova 
Flavor sensitivity
Direct neutrino mass
Relic neutrinos
 perpendicular
to wire
20
15
(20.0 fg/g U)
(20.0 fg/g U)
(3.7 fg/g Th)
(20.0 fg/g U)
(3.7 fg/g Th)
105 210Po m-2 d-1
5
14000
9
22
5
0.5
0.02
35
Antineutrinos
CCP
CCD
NCD
< 22
11
16
Total Expected Background
< 1000
MEASURING THE NEUTRAL CURRENT EVENT RATE
IN SNO USING 3He(n,p)t
R.G.H. Robertson1, T.J. Bowles4, T.V. Bullard1, S.J. Brice4,
M.C. Browne4, P.J. Doe1, C.A. Duba1, S.R. Elliott1,
E.I. Esch4, R. Fardon1, M.M. Fowler4, A. Goldschmidt3,
R. Hazama1, K.M. Heeger1, A. Hime4, K.T. Lesko3,
G.G. Miller4, R.W.Ollerhead2, A.W.P. Poon3, K.K. Schaffer1,
M.W.E. Smith1, T.D. Steiger1, R.G. Stokstad3,
J.B. Wilhelmy4, J.F. Wilkerson1, J.M. Wouters4
1Department
0
0
VME Bus
1000
2000
3000
4000
5000
of Physics, University of Washington,
Seattle, WA 98195
-9
Tim e (10 s)
2
University of Guelph, Physics Department,
Guelph, ON N1G 2W1, Canada
60
VME
Controller
160
365
< 40
180
< 150
10
DAQ
Computer
NCD DAQ is fully object-oriented, based on the same coding
structure as used in the main SNO DAQ. NCD DAQ is
currently running on a Macintosh platform, but will soon run
on Linux as well.
n perp.
to wire
microdischarge
50
Amplitude (mV)
Readout cable
pure e SK flux
BP 1998 SSM
4350 yr-1
11970 yr-1
8
1.0
e flux / total flux
Principle Reactions
CC
1.2
Current Amplitude (mV)
D2O
H2O
40
3Lawrence
Berkeley National Laboratory,
Berkeley, CA 94720, USA
30
20
10
4Los
0
0
1000
2000
Time (10
3000
-9
s)
4000
5000
Alamos National Laboratory,
Los Alamos, NM 87545, USA