Nucleon pairing in ${\mathrm{\mu }}^{\mathrm{-}}$ capture by ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$

Spectra of energetic protons above 35 MeV have been measured following negative muon capture from rest in Ca. The spectrum extends to the kinematic limit near 93 MeV, with a branching ratio of (2.3±0.3)×${10}^{\mathrm{-}4}$ per capture above 40 MeV. Nuclear cascade calculations of the proton and neutron spectra in this energy region are presented and are consistent with the measured proton spectrum when capture on correlated pp and np pairs in the nucleus is included. The ratio of capture on np to pp pairs is 6.7±1.6, which is consistent with results from pion capture.