Radiative muon capture on O, Al, Si, Ti, Zr, and Ag

The photon spectra from radiative muon capture (RMC) on O, Al, Si, Ti, Zr, and Ag have been measured for photon energies greater than 57 MeV using a cylindrical pair spectrometer at the TRIUMF cyclotron. Values of ${R}_{\ensuremath{\gamma}},$ the ratio of the radiative rate for photon energies above 57 MeV, to the nonradiative rate, are $1.67\ifmmode\pm\else\textpm\fi{}0.18,$ $1.40\ifmmode\pm\else\textpm\fi{}0.11,$ $2.09\ifmmode\pm\else\textpm\fi{}0.20,$ $1.30\ifmmode\pm\else\textpm\fi{}0.12,$ $1.31\ifmmode\pm\else\textpm\fi{}0.15$, and $1.12\ifmmode\pm\else\textpm\fi{}0.13$, respectively, in units of ${10}^{\ensuremath{-}5}$. The Al/Si rate difference confirms an earlier result. The Ti/Ca rate difference and the rate suppression in Zr and Ag are new results which confirm that the RMC rate is a much smoother function of neutron excess than of atomic number. This suggests that Pauli blocking is relatively more important for radiative capture than for nonradiative capture. The value of the ratio of the induced weak pseudoscalar to axial coupling constants, ${g}_{p}{/g}_{a},$ for O was found to be $4.9\ifmmode\pm\else\textpm\fi{}0.6,$ $6.3\ifmmode\pm\else\textpm\fi{}1.1$, or ${8.1}_{\ensuremath{-}2.1}^{+1.8}$, depending on the theory used to extract it. These values are in good or fair agreement with the partially conserved axial-vector current (PCAC) hypothesis. For the other nuclei studied, large model dependences or a lack of detailed RMC calculations made tests of the PCAC hypothesis difficult.