S.R. Elliott and J.F. Wilkerson

The Russian-American Gallium Experiment (SAGE) is a radiochemical solar neutrino flux measurement based on the inverse beta decay reaction, ⁷¹Ga(_e,e^-)⁷¹Ge. The threshold for this reaction is 233 keV which permits sensitivity to the p-p neutrinos that comprise the dominant contribution of 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 induced Ge is extracted from the Ga. ⁷¹Ge is unstable with respect to electron capture (t_1/2=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±10 (statistical) +5/-7 (systematic) SNU. This is well below the standard solar model expectation of 138 SNU. Additional extractions through the end of 1994 are awaiting analysis.

The collaboration has used a 518-kCi ⁵¹Cr neutrino source to test the experimental operation. The energy of these neutrinos is similar to the solar ⁷Be neutrinos and thus makes an ideal check on the experimental procedure. The extractions for the Cr experiment took place between January and May of 1995 and the counting of the samples lasted until fall. We have finished the bulk of the analysis of this data and presented preliminary reports at several meetings this past year. The result, expressed in terms of a ratio of the measured production rate to the expected production rate, is 1.0±0.15. This indicates that the discrepancy between the solar model predictions and the SAGE flux measurement cannot be an experimental artifact.

The University of Washington is playing a major role in the statistical analysis of the SAGE data and in the determination of systematic uncertainties. We are very active in the remaining analysis of the Cr experiment data as well as the solar neutrino data.