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. 
W. C. Haxton