Neutron Asymmetries and Energy Spectra from Muon Capture in Si, S, and Ca

The energy dependence of the neutron asymmetry parameter for nuclear capture of muons has been measured in Si, S, and Ca for neutron energies of 4-53 MeV. Contrary to most recent experiments, the asymmetry parameter is strongly positive over much of the energy range, with values as much as 9 standard deviations from zero. Because of the unusual nature of these results the experiment was performed twice and with excellent agreement between the two sets of data. The integrated asymmetry parameter for $E_n>15.6\mathrm{}$ MeV is +0.316 ± 0.023 for Si and +0.290 ± 0.034 for Ca, values which are in direct conflict with the "standard" $V-A$ theory. Recent theoretical calculations by Bogan and by Piketty and Procureur obtain positive asymmetries, though not yet in good quantitative agreement with these data. The neutron energy spectra from the three targets are all quite similar. They are consistent with a simple exponential falloff in $E_n$ with a decay constant of 7 MeV. The theoretical calculations cited above are in good agreement with higher-energy regions of the spectra.