Number 84, June 15, 1992 by Phillip F. Schewe and Ben Stein GALLEX OBSERVES SOLAR NEUTRINOS at a rate of 83 Solar Neutrino Units (SNU). The multi-nation collaboration (U.S. contact: Richard Hahn at Brookhaven, 516-282-4337) detects solar neutrinos in a 50,000 liter bath of gallium chloride installed in the Gran Sasso tunnel under the Abruzzi Mountains in Italy. The neutrinos, arising largely from proton-proton fusion reactions in the Sun, but also from the decay of beryllium and boron in the Sun, convert a gallium-71 nucleus into a germanium-71 nucleus. The radioactive Ge is extracted every three weeks and monitored closely in a separate vessel. The calculated production rate of 83 SNU (1 SNU = 10**-36 neutrino captures per atom per second) is to be compared with theoretical estimates that range from 124 to 132 SNU. For the 132-SNU estimate, 74 SNU should come from (relatively low energy) pp reactions, 34 SNU from Be-7 decays, 14 SNU from B-8 decays, and 10 SNU from N-13 and O-15. The Gallex results are much closer to the theoretical estimates than those from the South Dakota or Kamiokande (Japan) detectors, which are sensitive only to the higher energy B and Be neutrinos; these two earlier detectors typically observed only one third the neutrinos prescribed by the Standard Model of solar physics. The Gallex results are in stark contrast with those from SAGE, a fourth detector group, located in the Former Soviet Union. SAGE last year reported finding essentially no neutrinos from pp reactions. (Paper give last week at the Neutrino 1992 meeting in Grenada, Spain. Also P. Anselmann et al., upcoming article in Physics Letters B.) MAGNETORESISTANCE is a process, widely employed in the recording industry, in which magnetic fields are used to change the resistance of a conducting medium. In certain "magnetic superlattices," stacked materials in which thin layers of magnetic atoms (e.g., iron) and non-magnetic atoms (e.g., chromium) alternate, an even larger, "giant," magnetoresistance (GMR) effect can be produced, as of four years ago, although theorists largely cannot explain how it happens. Now two research teams have observed GMR effects in non-layered materials, a development which may make the materials easier to use in magnetic recording heads. In one experiment, for example, a UCSD-LBL-NYU collaboration studied thin films containing cobalt-rich particles in a copper-rich matrix. (J.Q. Xiao et al. and A.E. Berkowitz et al., Phys. Rev. Lett., 22 June.) GROUND-BASED GAMMA-RAY ASTROPHYSICS looks at physics in energy doses up to the PeV (10**15 eV) and even the EeV (10**18 eV) range. The sources for such stupendous energies are believed to be compact objects such as black holes or neutron stars. The messengers are high-energy gamma rays which, having no electrical charge, can follow a straight path through the galactic magnetic field. The evidence for the gammas here on Earth are the huge showers of particles engendered by the gammas when they hit our atmosphere. To search for PeV gammas, a Michigan-Chicago-Utah collaboration uses a combination of 1089 scintillation detectors, muon detectors, and Cherenkov telescopes. The University of Utah's Fly's Eye Detector looks for the fluorescence of air molecules arising from very-high-energy gammas; they observe about 20 events per year with energies above 10 EeV. (Beam Line, Spring 1992; published by SLAC.)