The Gallium Anomaly

Understand

In order to test the end-to-end operations of gallium solar neutrino experiments, intense electron-capture sources were fabricated to measure the responses of the radiochemical SAGE and GALLEX/GNO detectors to known fluxes of low-energy neutrinos.

• Such tests were viewed at the time as a cross-check, given the many tests of $^{71}$Ge recovery and counting that had been routinely performed, with excellent results.
• However, the four $^{51}$Cr and $^{37}$Ar source experiments yielded rates below expectations, a result commonly known as the Ga anomaly.
• As the intensity of the electron-capture sources can be measured to high precision, the neutrino lines they produce are fixed by known atomic and nuclear rates, and the neutrino absorption cross section on $^{71}$Ga is tightly constrained by the lifetime of $^{71}$Ge, no simple explanation for the anomaly has been found.