The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos ${\nu }_e$ observed in previous gallium-based radiochemical measurements with high-intensity neutrino sources, commonly referred to as the “gallium anomaly,” which could be interpreted as evidence for oscillations between ${\nu }_e$ and sterile neutrino (${\nu }_s$) states. A 3.414-MCi ${}^{51}\mathrm{Cr}$ ${\nu }_e$ source was placed at the center of two nested Ga volumes and measurements were made of the production of ${}^{71}\mathrm{Ge}$ through the charged current reaction, ${}^{71}\mathrm{Ga}({\nu }_e,e^{-}){}^{71}\mathrm{Ge}$, at two average distances. The measured production rates for the inner and the outer targets, respectively, are [${54.9}_{-2.4}^{+2.5}(\mathrm{stat})\pm 1.4(\mathrm{syst})$] and [${55.6}_{-2.6}^{+2.7}(\mathrm{stat})\pm 1.4(\mathrm{syst})$] atoms of ${}^{71}\mathrm{Ge}/d$. The ratio ($R$) of the measured rate of ${}^{71}\mathrm{Ge}$ production at each distance to the expected rate from the known cross section and experimental efficiencies are $R_{\mathrm{in}}=0.79\pm 0.05$ and $R_{\mathrm{out}}=0.77\pm 0.05$. The ratio of the outer to the inner result is $0.97\pm 0.07$, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20%–24% lower than expected, thus reaffirming the anomaly. These results are consistent with ${\nu }_e\to {\nu }_s$ oscillations with a relatively large $\mathrm{\Delta }{m}^2$ ($>0.5{\mathrm{eV}}^2$) and mixing ${\sin }^{2}2\theta$ ($\approx 0.4$).

• Received 23 September 2021
• Revised 18 January 2022
• Accepted 28 February 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.232501