Measurement of production branching ratio after muon nuclear capture reaction of Al and Si isotopes

Background: Muon nuclear capture is a reaction between a muon and a proton inside a nucleus through weak interactions. This reaction results in the formation of an excited nucleus, which subsequently deexcites by emitting several particles. Examination of the excited state allows for an investigation of the properties of nuclear excitation and particle emission in highly excited nuclei.Purpose: This study investigates muon nuclear capture of $^{27}\mathrm{Al}$ and $^{28,29,30}\mathrm{Si}$, focusing on determining the absolute production branching ratio (BR) following muon nuclear capture and subsequent particle emissions. By measuring the absolute production BR, we can collect valuable information on the excitation energy distribution of muon nuclear capture.Methods: Measurements were conducted using the in-beam activation method at two pulsed muon facilities: RIKEN-RAL beamline, ISIS Neutron and Muon Facility at Rutherford Appleton Laboratory and Materials and Life Science Experimental Facility at Japan Proton Accelerator Research Complex. Absolute BRs were determined by measuring the number of muons irradiating the target using a plastic scintillator and the $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays emitted from the produced nuclei using germanium detectors.Results: The absolute production branching ratios of muon nuclear capture on $^{27}\mathrm{Al}$ and $^{28,29,30}\mathrm{Si}$ were obtained with the highest accuracy to date. Predominant neutron emissions, even-odd atomic number dependence of particle emission probabilities, and influence of the neutron excess were observed. These results were compared with previous measurements and theoretical models and discussed regarding the excitation energy distribution, particle emission mechanism, and nuclear properties, such as resonance in the isovector transition.Conclusion: This study emphasizes the importance of considering nuclear structure effects, even-odd effects of proton and neutron numbers, neutron excess, nucleon pairing effect, and particle emission mechanisms, in the context of the muon nuclear capture reaction.