GALLEX

Detector

The 54-m³ detector tank was filled with 101 tons of gallium trichloride-hydrochloric acid solution, which contained 30.3 tons of gallium. The gallium in this solution acted as the target for a neutrino-induced nuclear reaction, which transmuted it into germanium through the following reaction:

ν_e + ⁷¹Ga → ⁷¹Ge + e⁻.

The threshold for neutrino detection by this reaction is very low (233.2 keV), and this is also the reason why gallium was chosen: other reactions (as with chlorine-37) have higher thresholds and are thus unable to detect low-energy neutrinos. In fact, the low energy threshold makes the reaction with gallium suitable to the detection of neutrinos emitted in the initial proton fusion reaction of the proton-proton chain reaction, which have a maximum energy of 420 keV.

The produced germanium-71 was chemically extracted from the detector, converted to germane (⁷¹GeH₄). Its decay, with a half-life of 11.43 days, was detected by counters. Each detected decay corresponded to one detected neutrino.

Results

During the period 1991-1997, the detector measured capture rate of 73.1 SNU (Solar neutrino units). The follow-up GNO experiment found the capture rate 62.9.

The rate of neutrinos detected by this experiment disagreed with standard solar model predictions. Thanks to the use of gallium, it was the first experiment to observe solar initial pp neutrinos. Another important result was the detection of a smaller number of neutrinos than the standard model predicted (the solar neutrino problem). After detector calibration the amount did not change. This discrepancy has since been explained: such radiochemical neutrino detectors are sensitive only to electron neutrinos, and not to muon neutrinos or tau neutrinos, hence the neutrino oscillation of electron neutrinos emitted from the sun and travelling to the earth accounts for the discrepancy.

Results from GALLEX together with SAGE and later confirmed by the BEST experiment have reported a deficit in the expected decay of ${\displaystyle {}^{71}\text{Ga}+{\nu }_e\to e^{-}+{}^{71}\text{Ge}}$ that has been named the gallium anomaly.

Other experiments

The first solar neutrino detection, the Homestake Experiment, used chlorine-37 to detect neutrinos with energies down to 814 keV.

After the end of GALLEX its successor project, the Gallium Neutrino Observatory or G.N.O., was started at LNGS in April 1998. The project continued until 2003.

A similar experiment detecting solar neutrinos using liquid gallium-71 was the Russian-American Gallium Experiment SAGE.