Measurements of radioactive nuclides produced by positive and negative muon in the J-PARC Neutrino Experimental Facility

We have measured the radioactivity of Al, Fe and Cu samples irradiated by high-energy positive and negative muons at the Muon pit in the J-PARC Neutrino experimental facility. Irradiation was performed in two operational modes (neutrino mode and anti-neutrino mode) of the magnetic horns which were used to supply neutrinos and anti-neutrinos toward the Super-Kamiokande detector by focusing the charged pions before the decay as π⁺ → μ⁺ + v and π^- → μ^- + ν-bar. The effective cross-sections-of Al, Fe and Cu induced by positive and negative muons in the neutrino and anti-neutrino mode, respectively, were estimated using the muon flux characterized by the muon monitor installed in front of the irradiation samples. Negative muons generally produced higher yield of radioactive nuclides than positive muons. The effective cross-sections experimentally obtained are compared to the semiempirical charge-distribution and mass-distribution (CDMD) formula of Rudstam which can describe spallation yields and cross sections with a few parameters. The measured data of positive muons agrees well with the CDMD estimation using parameters of photonuclear spallation reaction, but the measured data of negative muons don't agree with the CDMD estimation. The discrepancy between measured data and the CDMD estimation for negative muons indicates the possibility of other reaction mechanisms than spallation reaction to contribute to activation. (author)