127I(n, γ)128I reaction in NaI(Tl) due to cosmic-ray muon capture by Pb

A γ-ray from a neutron-capture reaction on 127I in an NaI(Tl) detector was observed in background measurement. The neutrons were considered to be produced by cosmic-ray muon captured by Pb, which was used for shielding the spectrometer against γ-ray background. The observed γ-ray yield of the reaction was compared with the value calculated by assuming that the fast neutrons produced in the Pb were thermalized while they passed through a plastic scintillation counter, which surrounded the NaI(Tl) detector, and these thermal neutrons were captured by 127I in the detector. They agree with each other, supporting the present interpretation. The γ-ray energy from the capture state to the ground state in 128I was different from the previously measured value using thermal neutrons; the difference could be due to a nonuniformity of the detector. The present result has important implications for developing a low-background γ-ray spectrometer for studying rare physical phenomena.