Proportional counter response calculations for gallium solar neutrino detectors

Gallium-based solar neutrino detectors are sensitive to the primary pp reaction in the Sun. Two experiments using gallium, SAGE in the Soviet Union and GALLEX in Europe, are under construction and are scheduled to produce data by 1989. The radioactive ⁷¹Ge produced by neutrinos interacting with the gallium detector material is chemically extracted and counted in miniature proportional counters. A number of calculation have been carried out to simulate the response of these counters to the decay of ⁷¹Ge and to background events. It is shown that the use of xenon gas clearly improves the detector efficiency relative to argon in the 10-keV energy range from 35% to 53%. It appears that the disadvantage of increased conversion efficiency for background gammas in xenon is not relevant, due to the dominance of the conversion of gammas in the glass walls of the detector