1 Universitv of Washington Nuclear Phvsics Lab1

3He Neutral Current Detectors at SNO

S.R. Elliott, M.C. Browne, P.J. Doe, C.A. Duba, J.V. Germani, K.M. Heeger, A.W.P. Poon, R.G.H. Robertson, M.W.E. Smith,

Department of Physics, University of Washington, Seattle, WA 98 195 T.D. Steiger, P.M. Thornewell, J.F. Wilkerson

R. W.Ollerhead Guelph University, Physics Department, Guelph, ON N1G 2W1, Canada

K.T. Lesko, R.G. Stokstad Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

T.J.Bowles, S.J. Brice, E.I. Esch, M.M. Fowler, A. Goldschmidt, A. Hime, G.G. Miller, J.B. Wilhelmy, J.M. Wouters

Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Invited Talk New Era in Neutrino Physics

June 11-12, 1998 Tokyo Metropolitan University

Tokyo Japan

I Electroweak Interactions Group I

t . 7

3He Neutral Current Detectors at SNO

S.R. Elliott, M.C. Browne, P.J. Doe, C.A. Duba, J.V. Germani, K.M. Heeger, A.W.P. Poon, R.G.H. Robertson, M.W.E. Smith, T.D. Steiger, P.M. Thornewell, J.F. Wilkerson Department of Physics, University of Washington, Seattle, WA 981 95, USA R.W.Ollerhead Department of Physics, University of Guelph, Guelph, O N N 1 G 2 W1, Canada K.T. Lesko, R.G. Stokstad Lawrence Berkeley National Laboratory, Berkeley, CA 94 720, USA T.J.Bowles, S.J. Brice, E.I. Esch, M.M. Fowler, A. Goldschmidt, A. Hime, G.G. Miller, J.B. Wilhelmy, J.M. Wouters Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Abstract

The flux of solar neutrinos measured via charged and neutral current in- teractions can provide a model independent test of neutrino oscillations. Since the Sudbury Neutrino Observatory uses heavy water as a target, it has a large sensitivity to both interactions. A technique for observing the neutral current breakup of the deuteron using 3He proportional counters is described.

1. Introduction

Since the late 1960s several experiments [1,2,3,4,5] have measured fewer electron neutrinos emitted by the Sun than predicted by solar models [6,7]. 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

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heavy water:

v,+d+ p + p + e - CC

make it an ideal choice for such an experiment. The Sudbury Neutrino Observatory (SNO) [8] is being built 6800 f t un-

derground 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 PMTs. 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, MgC12 is added to the heavy water. When neutrons cap- ture 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 PMTs. The strength of this technique is its simplic- ity. The drawback is that the NC and CC signals are intertwined and must be separated during analysis.

The second technique employs 3He proportional counters (neutral current detectors or NCDs) 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 sys- tematic considerations and are being built and operated by separate subsets of the SNO collaboration, they can be considered as two independent measurements. This status report will describe the NCD project at SNO.

2. The Neutral Current Detectors

The NCDs 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 pm Cu wire is

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Laser weld Fused-silica

Resistive coupler 1 i (cable end only) Count!r body Nickel endcap

(3He-CF, gasmix) body I

Acrylic ring

Fig. 1. A drawing of an NCD string showing the various components.

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 prac- tical limit for transport underground. Thus the NCDs 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 and installed into the heavy water with a remotely operated submarine. Figure 1. shows a string.

The delay line includes an anchor which fixes the string to an attachment point on the acrylic vessel. 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 NCDs giving a total active length of 770 m. These are di- vided between 96 individual strings which are positioned on a 1 m grid throughout the vessel. The array will be calibrated by a 252Cf source.

c

f

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3. Potential Backgrounds

Because the NCDs 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 208T1 in the tho- rium chain is particularly notorious. If the chain is in equilibrium, 0.53 pgrams of 232Th distributed evenly throughout the D20, 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 NCDs is to limit the background budget to be significantly less than that from the heavy water, ie. a few percent.

The NCD array is constructed of 450 kg of CVD Ni. The measured level of 232Th in the Ni is 1 - 2 ~ 1 0 - ' ~ 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 will be deployed with the SNO calibration hardware. Once deployed, we should not see an unacceptably high Cerenkov response from this test array.

In addition to neutrons, alphas and electrons can result in an NCD re- sponse. 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 coun- ters, 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.

4. The Charged Current t o Neutral Current Rat io

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

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0.6 -

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0.0 0.2 0.4 0.6 0.8 1.0 ve flux I v, SSM flux (BP98)

Fig. 2. Shown is the ratio of the v, to vtotal fluxes as a function of v, flux normalized to the flux predicted by the standard solar model for the two hy- potheses described in the text. The shaded bands indicate an estimate of the total experimental uncertaianty in the measurement. The region labeled SuperK indicates the parameter space consistent with the SuperKamiokande experimental result.

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. Figure 2. shows the anticipated sensitivity of SNO after one year of running for a conservative background estimate of approximately 900 neutrons/year determined with a total uncertainty (statistical plus systematic) of 20%. The two hypotheses me indicated by the two shaded regions which define the uncertainty band. Intersecting these two regions is the curve resulting from the Super-Kamiokande result. It is clear from this figure that SNO can discern between the two hypotheses.

5 . References

1. 2. 3. 4. 5. 6. 7. 8.

Cleveland, B.T. et al., Astrophys. J. 496, 505 (1998). Fukuda, Y. et al., Phys. Rev. Lett. 77, 1683 (1996). Super-Kamiokande Preprint, 14 April 1998; and these proceedings. Abdurashitov, J.N. et al., Phys. Lett. B328, 234 (1994) Hampel, W. et al., Phys. Lett. B388, 384 (1996). Turck-Chikze, S. and I. Lopes, Astrophys. J. 408, 347 (1993). Bahcall, J.N., Sarbani Basu, and M. H. Pinsonneault, Preprint 24 May 1998. McDonald, A.B., Proceedings of the 9th Lake Louise Winter Institute, eds. A. Astbury et al. (World Scientific, Singapore, 1994), p. 1.