Absolute intensity measurement of pulsed muon beams using in-beam activation
The absolute number of negative muons contained in a beam is essential for many experiments at accelerator facilities, but determining it in pulsed beams has been difficult, particularly at high intensities. The method utilizing the yield of the $\beta$ delayed $\gamma$ rays from the residual nuclei after the muon nuclear capture reaction has recently been developed to determine the muon number in the pulsed muon beam. In particular, the in-beam activation method employs isotopes with short lifetimes, enabling the beam intensity to be measured over a short period with irradiating muon beams. However, only a limited number of isotopes have reliable measurements of production branching ratios (BRs), which are required to determine the absolute muon number in the pulsed beam. To search for new candidate isotopes that are suitable for in-beam activation method, the production branching ratio after the muon nuclear capture reaction was measured for natural abundance Cu, Zn, and Ag. Considering the strength of the BR, the muon capture probability, the practical detection efficiency of the detector, and rarity of the target material in the surrounding structures, the reaction $^\mathrm{nat}$Ag ($\mu^-, \nu_\mu x$) $^{107m}$Pd is found to be a useful reference for the muon number calibration.