The Gallium Anomaly and the BEST Experiment

Steve Elliott

Los Alamos National Laboratory

At the end of the last century, large volumes of gallium metal were used as targets to detect electron neutrinos (n_e) from the Sun. A small number of ⁷¹Ge atoms, produced in many tons of gallium through the charged current reaction ⁷¹Ga(n_e,e^–)⁷¹Ge, were subsequently counted to determine the solar neutrino flux. ⁷¹Ge atoms are radioactive and their decays were observed in small proportional counters. The radiochemical technique procedures were extensively studied through auxiliary measurements, and tests with strong radioactive sources were performed. These source tests, however, found fewer ⁷¹Ge atoms than expected given the cross section and measured efficiencies. This, as yet unexplained result, is commonly referred to as the gallium anomaly.

The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate this anomaly, which has been frequently interpreted as evidence for oscillations between n_e and sterile neutrino (n_s) states. The BEST measurement consisted of a 3.414-MCi ⁵¹Cr n_e source placed at the center of two nested Ga volumes and measurements made of the ⁷¹Ge production at two different average distances. The measured rates in both volumes were low and hence consistent with oscillations. However, the rates in the two volumes were similar providing for other interpretations. This talk will summarize the history of the measurements, the BEST experiment and its results, the motivating physics, and possible interpretations.