Hunting Neutrinos in Arizona

Neutrinos are very fundamental, yet little known elementary particles produced in radioactive beta decay. Among the things we do not know is whether or not they have a finite mass (if so, it would be thousands of times smaller than the electron mass). If they have a small mass, it is predicted that there would be "oscillations" between different neutrino species. If discovered, neutrino oscillations would shed light on some of the most essential issues of modern particle physics, ranging from a better general understanding of lepton masses to the exploration of new physics beyond the Standard Model of Elementary Particles. In addition, the pheonomenon of oscillations would have important consequences in astrophysics and cosmology. The neutrino associated with the electron was discovered in the 1950's by Frederick Reines at the Savannah River Reactor. Reines, along with Martin Perl of SLAC, who discovered the tau lepton in the 1970's, was awarded the Nobel Prize last year. The neutrino associated with the muon was discovered in the 1960's at Brookhaven National Laboratory by Leon Lederman, Mel Schwartz and Jack Steinberger, who received the Nobel Prize for their work in 1988.

Experiments using both particle accelerators and nuclear reactors have been carried out extensively in the last twenty years, without finding evidence for neutrino oscillations. However, recent evidence has been collected on two effects that could point to oscillations: the solar neutrino puzzle and the anomaly observed in atmospheric neutrinos. While in the first case, an interpretation in terms of neutrino oscillations would lead to mixing parameters too small to be directly observable in an Earth-based experiment, the atmospheric neutrino anomaly points to a region in parameter space that is accessible using MeV-energy neutrinos and a baseline of about one kilometer.

The Palo Verde experiment will use low energy electron (anti)neutrinos from the 11 GW nuclear reactors. The detector will be located at about 1 km distance from the reactors in order to explore very long oscillation lengths and hence, small neutrino masses. At this distance, the neutrino flux is relatively modest, and in order to make a measurement, a large detector is needed. A 12 ton liquid scintillator detector will be used to detect about 25 neutrinos per day from the reactor. Since the signal is so low, good shielding is required to protect from cosmic rays, which can create a false neutrino signature. This requirement makes it imperative that the detector be placed underground, shielded by the earth.

A 25 meter deep underground vault has been built, which is large enough to contain the detector surrounded by pure water shields and active cosmic ray veto counters. The lab and its access tunnel are shown in these photographs. Toward year end, the first part of the detector will be installed under the Arizona desert. Some preliminary answers to this quest for neutrino oscillations should be found by the summer of 1998...stay tuned!

Inside the access tunnel to the neutrino lab.

Underground structures of the Alabama-Caltech-Stanford Neutrino Laboratory. The detector will sit in the concrete bunker on the left. The long pipe on the right of the photo is the tunnel used to access the lab.

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