Verification Tests Of The Gallex Solar Neutrino Detector, With Ge-71 Produced In-Situ From The Beta-Decay Of As-71

Previously, it was demonstrated that the GALLEX solar neutrino detector responds properly to low energy neutrinos, by exposing it to two intense Cr-51-neutrino sources; the recovery yield of the product Ge-71 was reported to be 93% +/- 8%. New experiments, in which known amounts of radioactive As-71 have decayed to Ge-71 in the full-scale gallium detector, strongly support this evidence. In several experiments, the gallium detector has been spiked with similar to 10(571)As atoms, under varying conditions of how the As-71 was added (either carrier free, or with Ge carrier), how the gallium solution was mixed, and how long the Ge-71 remained in the gallium. As-71 decays by electron capture and positron emission to Ge-71, with a half life of 2.72 d. Hot atoms are produced by these decay modes with kinematics that mimic solar neutrino capture, although the Cr-51 neutrino source provided a more perfect match. This relative disadvantage is offset by the much better statistics obtainable with the As-71. In all As-71 experiments, the recovery of Ge-71 from the gallium was 100%, with uncertainties of only +/- 1%. The combined results from the Cr-51 sources and the As-71 spikes rule out any loss mechanisms for Ge-71, including hot-atom chemical effects. Chemical processes in the aqueous gallium trichloride - hydrochloric acid solution guarantee that the Ge-71 atoms formed in the GALLEX target will be quickly converted to extractable, volatile GeCl4. (C) 1998 Published by Elsevier Science B.V. All rights reserved.