SNOElectroweak Interaction Research at the University of Washington
SNO Neutral Current Detectors
The flux of solar neutrinos measured via charged and neutral current
interactions
can provide a model independent test of neutrino oscillations. Since the
Sudbury Neutrino Observatory (SNO)
uses heavy water as a target, it has a large
sensitivity to both
interactions. The
Neutral Current Detector Group of the SNO collaboration has developed a
technique for observing the neutral current breakup of the
deuteron using 3He proportional counters. The counters were fabricated and tested in the NCD Lab at the
University of Washington
.
The Neutral Current Detector project has been described in a number of
publications and conference talks. The access to
internal NCD information
is restricted to members of the Neutral Current Detector Group.
Since the late 1960s several neutrino experiments have measured
fewer electron neutrinos emitted
by the Sun than predicted by solar models. A number of particle and solar
physics solutions have been proposed to explain this discrepancy. However,
the data are
best described by matter enhanced neutrino oscillations. Such oscillations
would result
in electron neutrinos changing flavor. Since the previous experiments were
predominantly sensitive to charged current (CC) interactions, they
registered too
few neutrinos.
However, the total flux of active neutrinos from the sun can be determined
by a neutral
current (NC) interaction. Thus an experiment with a good sensitivity to
both types
of interaction could give definitive evidence for neutrino oscillations if
the two
flux measurements were unequal. The two neutrino reactions with heavy water
make it an ideal choice for such an experiment.
The
Sudbury Neutrino Observatory (SNO)
is being built 6800 ft
underground in the
INCO
nickel mine in Sudbury, Ontario, Canada. It consists of a 12 m
diameter acrylic
sphere which will be filled with 1000 tonnes of heavy water. This sphere
will be
suspended inside a cavity containing 7000 tonnes of light water.
Surrounding the
acrylic vessel is a geodesic structure supporting 9500 photomultiplier
tubes (PMT).
In the above reactions, the Cerenkov light emitted by the recoil
electrons in the CC
reaction are observed by the PMT's. For the NC
reaction however, the goal is to observe the free neutron. Because the NC
reaction is so critical to understanding the solar neutrino problem, the
SNO
collaboration has developed two techniques for its measurement. In both cases the process of
neutron capture on an additive nuclide provides a signature indicating the presence
of a neutron.
In one technique, MgCl2 is added to the heavy water. When neutrons capture onto
35Cl, several gamma rays totaling 8 MeV are emitted. The Compton
electrons produced in the heavy water from these gamma rays are observed via their
Cerenkov light by the PMT's. The strength of this technique is its simplicity. The
drawback is that the NC and CC signals are intertwined and must be statistically separated during
analysis.
The second technique employs 3He proportional counters (neutral current detectors
or NCD's) installed into the heavy water. When the neutrons capture on the
3He, the recoil proton-triton pair ionize the counter gas. Although this technique
requires complex hardware, its strength is that the CC and NC signals are detected in
separate systems. Because the two techniques have different systematic
considerations and are being built and operated by separate subsets of the SNO
collaboration, they can be considered as two independent measurements.
The Neutral Current Detectors
The NCD's are made of 2-inch diameter chemical-vapor deposited (CVD) nickel
tubing. Each counter is either 200, 250, or 300 cm long with a shaped CVD Ni endcap
closing the ends. Penetrating each endcap is a 2-inch long, 0.2-inch diameter Suprasil
quartz tube high-voltage feedthrough. A 50 micrometer Cu wire is tensioned between the ends
of these quartz feedthroughs. The gas is 85% 3He and 15% CF4 operated at
a gas gain of roughly 100.
The maximum length of a counter was set at 3 m because that is the practical limit
for transport underground. Thus the NCD's are grouped into strings of 2-4
counters varying in length between 4 and 11 m. Each of these strings have a delay
line at their bottom and a cable connection at the top. These parts will be
welded together underground into a NCD string
and installed into the heavy water with a remotely operated submarine.
The delay line includes an anchor which fixes the string to an attachment
point on the acrylic vessel. A schematic
sideview of the NCDs in the acrylic vessel illustrates this arrangement.
The cable is a custom design which will float in heavy water. Thus the
cable will rise from the string and approximately follow the acrylic vessel
contour on its route to and up the neck.
There are 300 NCD's giving a total active length of 770 m. These are divided
between 96 individual strings which are positioned on a 1 m grid throughout the
vessel to give an array of NCD strings inside the acrylic vessel.
The array will be calibrated by a 252Cf source.
Potential Backgrounds
Because the NCD's reside in the heavy water, the radioactivity constraints
are stringent. Any gamma ray with energy greater than 2.2 MeV can
photo-disintegrate the deuteron resulting in a free neutron which would be a background to
the solar neutrino signal. The 2.6-MeV gamma ray from 208Tl in the thorium
chain is particularly notorious. If the chain is in equilibrium, 0.53 micrograms of
232Th distributed evenly throughout the D2O, will produce about 50
photo-disintegration neutrons/year. This is equivalent to 1% of the
anticipated neutron flux arising from solar neutrinos. For comparison, the design goal
for the heavy water purity is to limit the photodisintegration background to less than
about 10% of the SSM. The design goal for the NCD's is to limit the background budget to be
significantly less than that from the heavy water, i.e. a few per cent.
The NCD array is constructed of 450 kg of CVD Ni. The measured
level of 232Th in the Ni is 1-2 x 10-12 by wt. or 1-2% SSM
equivalent. All other materials are much less massive and have been assayed to ensure
radiopurity. Because the cables and delay lines reside near the acrylic vessel, their
radioactive burden is less critical. The radioassay studies indicate a total
photo-disintegration neutron flux of about 5-6% of the SSM neutrino induced neutron rate. To
verify that the assembled counters do indeed meet this level, a test array was deployed
with the SNO calibration hardware. When deployed, an unacceptably high
Cerenkov response was not seen from this test array.
In addition to neutrons, alphas, and electrons can result in an NCD response.
These processes can be eliminated by employing energy and pulse shape
discrimination. Each event in the NCD array is digitized and the waveform can be studied
off-line. Because neutron events in the proportional counter are defined by back-to-back
highly-ionizing events, their character is very different than alpha or electron
events. For a given energy deposit in the counter gas, the duration of the pulses will
differ. In this two-parameter space, there is a region in which only neutrons will
appear. This region will contain about 45% of all neutron events.
Since the signal rate per counter is so low, electrical discharge in the counters,
cables and preamps must also be taken into consideration. All parts are tested at an
elevated voltage to ensure that the microdischarge rate is sufficiently low. Such
events can also be rejected by analyzing the pulse shape.
The Charged Current to Neutral Current Ratio
It is useful to express fluxes as determined by the CC and NC rates as a
ratio because the uncertainties in the cross sections are correlated and therefore
cancel. If the ratio is 1, then neutrinos do not oscillate into active flavors and the
solar flux is simply depressed (or there are oscillations into sterile neutrinos).
However if the ratio is less than 1, then oscillations do occur.
SNO will be able to discern between the two hypotheses.
