Effects of two-particle two-hole configurations on particle emissions following muon capture

Nuclei are excited by muon capture reactions. If the excitation energy is above particle thresholds, the nuclei emit particles competing with the gamma-ray deexcitation. The typical examples are neutron and proton emissions. Other charged particles emissions, like deuteron-, ³He-, triton-, α-emissions, also occur if the nuclear excitation energy is very high. Information on these particle emissions is important for evaluating soft errors of semi-conductors and radioactivities of materials after irradiations of muon beams. Although experimental data of the particle emissions following muon captures have been reproduced with some phenomenological models, the detailed mechanism with respect to the nuclear structure is still unclear. In particular, experimentally measured neutron spectra after muon capture cannot be reproduced well. We worked on solving this issue with a microscopic approach of second-order proton-neutron Tamm-Dancoff-approximation and particle evaporation models considering pre-equilibrium and compound process. We found that the two-particle two-hole configurations play an important role to reproduce particle spectra obtained after muon captures.