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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 tonne target of liquid gallium and the cross section for the production of 71 Ge by the inverse beta decay reaction 71 Ga(νe, e − ) 71 Ge was measured to be [5.55 ± 0.60 (stat) ± 0.32 (syst)] × 10 −45 cm 2 . The ratio of this cross section to the theoretical cross section of Bahcall for this reaction is 0.95 ± 0.12 (expt) +0.035 −0.027 (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. PACS number(s): 26.65.+t, 13.15.+g, 95.85.Ry

An intense source of 37 Ar was produced by the (n, α) reaction on 40 Ca by irradiating 330 kg of calcium oxide in the fast neutron breeder reactor at Zarechny, Russia. The 37 Ar was released from the solid target by dissolution in acid, collected from this solution, purified, sealed into a small source, and brought to the Baksan Neutrino Observatory where it was used to irradiate 13 tonnes of gallium metal in the Russian-American gallium solar neutrino experiment SAGE. Ten exposures of the gallium to the source, whose initial strength was 409 ± 2 kCi, were carried out during the period April to September 2004. The 71 Ge produced by the reaction 71 Ga(ν e , e − ) 71 Ge was extracted, purified, and counted. The measured production rate was 11.0 +1.0 −0.9 (stat) ± 0.6 (syst) atoms of 71 Ge/d, which is 0.79 +0.09 −0.10 of the theoretically calculated production rate. When all neutrino source experiments with gallium are considered together, there is an indication the theoretical cross section has been overestimated.

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