Cross Section Uncertainties in the Gallium Neutrino Source Experiments

The 51Cr neutrino source experiments play a unique role in testing overall operations of the GALLEX and SAGE solar neutrino experiments. Recently Hata and Haxton argued that the excited-state contribution to the 71Ga cross section for 51Cr neutrino absorption might not be known reliably, despite forward-angle (p,n) measurements. A large-basis shell model calculation reported here indicates that the unusual situation they envisioned - destructive interference between weak spin and strong spin-tensor amplitudes - does occur for the transition to the first excited state in 71Ge. The calculation provides a counterexample to procedures previously used to determine the 51Cr cross section: the predicted (p,n) cross section for this state agrees with experiment, while the BGT value is well outside the accepted 3-standard-deviation limit. The results argue for a shift in the interpretation of the source experiments: they become more crucial as measurements of the 71Ga detector response to 7Be solar neutrinos, and less definitive as wholly independent tests of 71Ge recovery and counting efficiencies.