Understanding the properties of neutrinos ranks as one of the major unresolved issues in nuclear and particle physics. In particular, we do not know if neutrinos have mass or not. Neutrinos with mass would provide conclusive evidence of new physics beyond the minimal Standard Model of elementary particles and fields while possibly also comprising a significant portion of the dark matter thought to abound in the universe. Increasingly, the "solar neutrino problem", in which all four of the existing experiments observe far fewer neutrinos from the Sun than predicted by solar model calculations, appears to point to neutrino oscillations. Neutrino oscillations convert one "flavor" of neutrino into another, and can only occur if neutrinos have mass. Our experimental program to resolve this question includes a collaboration with the Russian Academy of Sciences, Los Alamos National Laboratory, and the University of Pennsylvania on a {EMBED EQUATION |} radiochemical measurement (SAGE) of the pp neutrino flux and participation in the Sudbury Neutrino Observatory (SNO), a joint Canadian, US, UK effort to measure the spectral distribution and flavor composition of the flux of the higher-energy, {EMBED EQUATION |} neutrinos from the Sun.

The Sudbury Neutrino Observatory will be the first solar neutrino detector capable of registering and distinguishing both the flux of electron neutrinos and the total flux of all left-handed neutrinos from the Sun. As such, it can make an unambiguous statement that neutrino oscillations are occurring, if the total flux is found to be larger than the {EMBED EQUATION |} flux. The conclusions will be essentially independent of solar models.

The sensitive medium of the SNO laboratory is 1000 tonnes of ultra-pure {EMBED EQUATION |}. The charged-current interaction of electron neutrinos on deuterium produces a fast electron that emits {EMBED EQUATION |} radiation in the water. An array of 9500 photomultipliers records the amount of light and, therefore, the energy of the electron. Our group is involved in developing the data acquisition system for the readout of the SNO detector, directing the research and development effort for the acrylic vessel that separates the heavy water from the light water shield, coordinating the SNO detector turn-on and commissioning efforts, and providing an independent detector array for recording the neutral-current signal of the SNO detector. Neutral-current interactions of all flavors of neutrino disintegrate the deuteron into a proton and a neutron, and the rate of these interactions can be determined by measuring the rate of neutron production.

A compact 20 MeV gamma-ray source is being developed for energy calibration at the Sudbury Neutrino Observatory (SNO). The gamma-rays are produced through the radiative capture reaction 3H(p,g)4He.

There are several constraints in designing the source. It has to be compact and portable, as it will be lowered to the center of the SNO detector through the 57" diameter deck of the acrylic vessel. The neutron production rate has to be low to ensure SNO's neutral current sensitivity to supernova neutrinos during the scheduled high energy g calibration. Even though the Q-value of 3H(p,n)3He is -0.763 MeV, which corresponds to a reaction threshold of 1.02 MeV, impurities in the beam and the target will give rise to undesired neutrons through the 2H(t,n)4He, 3H(d,n)4He, and 3H(t,nn)4He reactions. So the discharge hydrogen gas and the target tritium must be of very high purity. The solid tritium target must also have a high thermal stability to minimize tritium gas mixing into the discharge gas.

Fig. 2.3-1 shows the design of the source. It can be divided into two sections: a cold cathode Penning ion source and a target chamber. The anode E2 is biased at +2 kV, while the cathodes E1, and E3 are grounded. Pyrex(tm)-stainless steel couplings (C) are used to isolate high voltages. Permanent magnet rings M provide a ~1 kG axial magnetic field causing the electrons to spiral around the field lines, generating more ions. A SAES® getter pump is attached to E1. The getter pump serves as the dispenser of hydrogen gas. Ions (H+, H2+ and H3+) are accelerated through a -15 to -20 kV bias towards the target mounted at the end of the beam line. In this scheme, the construction of complicated accelerating and focusing electrodes is avoided, thus keeping the length of the source to a minimum. In fact, the length of the source is only 16".

The target selected for this source is a solid scandium tritide (Sc3H2), primarily because of its good thermal stability. It will be prepared at the Tritium Laboratory of Ontario Hydro Technologies in Toronto, Canada. It is clear from the drawing that the source is a sealed one, minimizing the hazard of tritium contamination.

The electro-optics of the ion source were first computer simulated and subsequently tested experimentally. The beam current has been measured using two independent methods: by measuring temperature change of a copper target and by integrating the beam profile measured using a Faraday cup. In the thermal measurement, the temperature of the copper target was monitored by a type T thermocouple. The beam power was later calibrated by an electric heater embedded in the target. We designed and constructed a special Faraday cup for measuring the beam profile. The cup was biased such that the secondary electron effects were minimized. Fig. 2.3-2 shows the results of the beam current measurements.

We also designed and constructed a mass spectrometer to measure the mass composition of the ion beam. Ions of different masses are separated by a magnetic field perpendicular to the ion beam propagation. Our experiment showed that protons compose (63±15)% of the beam, the rest being H2+ and H3+ molecular ions. We are now building a target evaporation chamber, similar to that used by Kherani et al.,1 to test fully the target fabrication procedure of Sc3H2 before using the tritium facility at Ontario Hydro Technologies.

Neutrons can be detected with high efficiency in {EMBED EQUATION |}-filled proportional counters without interfering significantly with the Cerenkov light from charged-current processes. We plan to install in SNO an array of such counters, with a total length of 800 meters, and constructed of highly purified materials.

Our rationale follows: {EMBED EQUATION |} proportional counters offer fundamental advantages. Neutral-current and charged-current events are recorded separately and distinguished event by event. The effective CC rates are doubled and the NC rates quadrupled in comparison with the dissolved-salt method. For the basic dissolved-salt method, the rate of neutron capture is obtained by subtraction of data without salt, after correcting for capture in deuterium. Recently, however, it has been suggested that gamma and electron events can be distinguished to some degree on-line. The secular variation in the NC rate due to the Earth's orbital eccentricity would become observable at the 95% confidence level. Time variations in the neutrino flux could be followed simultaneously in the NC and CC channels on the time scale of milliseconds to years. All signals and backgrounds can be determined at the same time, and there is no need to compare and subtract data taken at different times and under different conditions. The method is fully compatible with the dissolved-salt approach, allowing, by two different techniques, a valuable systematic check of important physics results. The duty factor for full-efficiency NC detection rises from 50 to 100%, and the possibility of missing NC data from a supernova or other interesting event is correspondingly diminished. Even if salt is present in the water, the conversion of NC events to {EMBED EQUATION |} light makes inferences about which events are CC and which NC indirect. Event-by-event NC detection offers the prospect of determining a {EMBED EQUATION |} or {EMBED EQUATION |} mass (especially in the cosmologically interesting range of 20-100 eV) if a supernova should occur.

We are collaborating with scientists at Los Alamos National Laboratory and Lawrence Berkeley Laboratory to design, fabricate, and deploy this array of {EMBED EQUATION |} proportional counters. The key issue in building such an array is to select materials that will minimize the amount of radioactive U and Th chain elements. A special chemical vapor deposition (CVD) process to produce ultra-pure nickel has been found that meets these requirements. The research and design phase is nearly complete and preparations are underway for full scale construction of the array.

Development is underway to permit modest, ~1 meter resolution, position readout of the {EMBED EQUATION |} filled proportional counter detector strings that will be used as neutral-current detectors in the SNO experiment. Position readout along the length of the proportional counters is desirable for several reasons. The identification of backgrounds internal to the counters, as well as localized concentrations of Th or U in either the counters or the acrylic vessel, will be easier to identify with position information. Furthermore, the ability to construct spherical radial distributions of neutron events will improve our ability to identify, and correct for, backgrounds from Th and U uniformly distributed in the acrylic vessel.

Single-ended readout of the detectors is highly desirable in order to minimize the amount of material in the heavy water and to keep the mechanical design and string deployment as simple as possible. The method presently under development relies on leaving the remote end of the counter unterminated. The pulse that propagates toward the remote end is reflected back with the same sign and adds with the pulse that heads directly to the preamp. The maximum delay time between the leading edge of the direct and reflected pulses is approximately 100 ns for an 11 m string of counters. A delay line of approximately 25 ns will be used at the remote end to define the minimum time delay for pulses originating close to that end. Thus, the time delays will range from 50 to 150 ns. Since the widths of current pulses range from 20 ns to 4 ms, the position signal in the combined pulse shape will vary from a double-peaked shape to a kink in the leading edge.

The acquisition of the 10 Kevlar@ suspension ropes has begun following successful completion of the selection, R&D and radioassay program established and overseen by LANL.

The coming year will be a challenging one. Installation of the vessel chimney begins in early June '95 followed by the upper hemisphere in August '95.

The suspension ropes will be required in October '95. Once the upper hemisphere is suspended, the lower hemisphere along with the neutral current detector anchors will be installed. The vessel is expected to be complete in May 1996. Ways of improving the schedule are being investigated.

The Russian-American Gallium Experiment (SAGE) is a radiochemical solar neutrino flux measurement based on the inverse beta decay reaction {EMBED EQUATION |}. The threshold for this reaction is 233 keV which permits sensitivity to the p-p neutrinos which dominate the solar neutrino flux. The target for the reaction is in the form of 55 tonnes of liquid gallium metal stored deep underground at the Baksan Neutrino Observatory in the Caucasus Mountains in Russia. About once a month, the neutrino-produced Ge is extracted from the Ga. {EMBED EQUATION |} is unstable with respect to electron capture (t1/2 = 11.43 days) and, therefore, the amount of extracted Ge can be determined from its activity as measured in small proportional counters. The experiment has measured the solar neutrino flux in 31 extractions between January 1990 and October 1993 with the result; 69{SYMBOL 177 \f "Symbol"}10 (statistical) +5/-7 (systematic) SNU. Additional extractions through the end of 1994 are awaiting analysis.

The collaboration is currently using a 500-kCi {EMBED EQUATION |} neutrino source to test the experimental operation. The energy of these neutrinos is similar to the solar {EMBED EQUATION |} neutrinos and the source thus makes an ideal check on the experimental procedure. The extractions for the Cr experiment took place in January and February of 1995 and the counting of the samples will continue through the summer. The University of Washington plays a major role in the statistical analysis of the data and in the determination of systematic uncertainties. We are very active in the planning and analysis of the Cr experiment data.