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 detecto r responds properly to low energy neutrinos, by exposing it to two int ense Cr-51-neutrino sources; the recovery yield of the product Ge-71 w as 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 d etector, strongly support this evidence. In several experiments, the g allium detector has been spiked with similar to 10(571)As atoms, under varying conditions of how the As-71 was added (either carrier free, o r with Ge carrier), how the gallium solution was mixed, and how long t he 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 neutri no capture, although the Cr-51 neutrino source provided a more perfect match. This relative disadvantage is offset by the much better statis tics 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 rul e out any loss mechanisms for Ge-71, including hot-atom chemical effec ts. Chemical processes in the aqueous gallium trichloride - hydrochlor ic acid solution guarantee that the Ge-71 atoms formed in the GALLEX t arget will be quickly converted to extractable, volatile GeCl4. (C) 19 98 Published by Elsevier Science B.V. All rights reserved.