Charge exchange reactions and the efficiency of solar neutrino detectors

The efficiencies of solar neutrino detectors are often based in part on weak interaction strengths determined by (p,n) and other charge exchange reactions. Although the (p,n) determinations are surprisingly good, it is shown that they may be inaccurate for important Gamow-Teller transitions whose strengths are a small fraction of the sum rule limit. This emphasizes the importance of direct calibration with ν sources for detectors such as ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ and ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ where direct β-decay information cannot be obtained. It may also bear on recent attempts to compare charge exchange and beta decay in the mass-37 system.