Chemistry of the Gallex Solar Neutrino Experiment

Abstract

The international Gallex collaboration has observed for the first time the low energy solar neutrinos from the main proton–proton fusion reaction in the solar interior using a radiochemical gallium neutrino detector in the Gran Sasso underground laboratory, Italy. This confirms basic theories about stellar energy production proposed more than half a century ago. The neutrino detector is a tank containing 30 t of gallium in the form of 100 t of a highly concentrated aqueous GaCl₃ solution, acidified with hydrochloric acid. Capture of solar neutrinos in the tank produces only about one ⁷¹Ge atom each day. Every 3–4 weeks, an average of about 10 atoms of radioactive ⁷¹Ge (halflife 11·4 days) have been chemically separated together with 1 mg of stable germanium carrier in a high and reliably known yield. After conversion to GeH_4, the few ^7lGe atoms have been counted in miniaturised low level gas proportional counters. In this extremely low level experiment, special attention must be paid to chemical and radiochemical trace impurities and all potential background sources of undesired activity. The chemical, radiochemical and process engineering aspects of the experiment are described and operating experience and preliminary results obtained during the first ⁷¹Ge measurement period are outlined.