Measurement of the response of a gallium metal solar neutrino experiment to neutrinos from a51Crsource

The neutrino capture rate measured by the Russian-American Gallium Experiment is well below that predicted by solar models. To check the response of this experiment to low-energy neutrinos, a 517 kCi source of ${}^{51}$Cr was produced by irradiating 512.7 g of 92.4%-enriched ${}^{50}$Cr in a high-flux fast neutron reactor. This source, which mainly emits monoenergetic 747-keV neutrinos, was placed at the center of a 13.1 ton target of liquid gallium and the cross section for the production of ${}^{71}$Ge by the inverse beta decay $\mathrm{reaction}{\hspace{0.5em}}^{71}\mathrm{Ga}({\nu }_e{,e}^{-}{)}^{71}\mathrm{Ge}$ was measured to be $[5.55\pm 0.60(\mathrm{stat})\pm 0.32(\mathrm{syst}\left)\right]\times {10}^{-45}{\mathrm{cm}}^2.$ The ratio of this cross section to the theoretical cross section of Bahcall for this reaction is 0.95 ±0.12 ${\mathrm{}\left(\mathrm{expt}\right)\mathrm{}}_{-0.027}^{+0.035\mathrm{}}$ (theor) and to the cross section of Haxton is 0.87±0.11 (expt)±0.09 (theor). This good agreement between prediction and observation implies that the overall experimental efficiency is correctly determined and provides considerable evidence for the reliability of the solar neutrino measurement.