Verification tests of the GALLEX solar neutrino detector, with 71Ge produced in-situ from the beta-decay of 71As

Abstract

Previously, it was demonstrated that the GALLEX solar neutrino detector responds properly to low energy neutrinos, by exposing it to two intense 51Cr-neutrino sources; the recovery yield of the product 71Ge was reported to be 93%±8%. New experiments, in which known amounts of radioactive 71As have decayed to 71Ge in the full-scale gallium detector, strongly support this evidence. In several experiments, the gallium detector has been spiked with ∼10 5 71 As atoms, under varying conditions of how the 71As was added (either carrier free, or with Ge carrier), how the gallium solution was mixed, and how long the 71Ge remained in the gallium. 71As decays by electron capture and positron emission to 71Ge, 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 51Cr neutrino source provided a more perfect match. This relative disadvantage is offset by the much better statistics obtainable with the 71As. In all 71As experiments, the recovery of 71Ge from the gallium was 100%, with uncertainties of only ±1%. The combined results from the 51Cr sources and the 71As spikes rule out any loss mechanisms for 71Ge, including hot-atom chemical effects. Chemical processes in the aqueous gallium trichloride - hydrochloric acid solution guarantee that the 71Ge atoms formed in the GALLEX target will be quickly converted to extractable, volatile GeCl 4.