I-127(N,GAMMA) I-128 REACTION IN NAI(TL) DUE TO COSMIC-RAY MUON-CAPTURE BY PB

A gamma-ray from a neutron-capture reaction on I-127 in an NaI(TI) det ector was observed in background measurement. The neutrons were consid ered to be produced by cosmic-ray muon captured by Pb, which was used for shielding the spectrometer against gamma-ray background. The obser ved gamma-ray yield of the reaction was compared with the value calcul ated by assuming that the fast neutrons produced in the Pb were therma lized while they passed through a plastic scintillation counter, which surrounded the NaI(Tl) detector, and these thermal neutrons were capt ured by I in the detector. They agree with each other, supporting the present interpretation. The gamma-ray energy from the capture state to the ground state in I-128 was different from the previously measured value using thermal neutrons; the difference could be due to a nonunif ormity of the detector. The present result has important implications for developing a low-background gamma-ray spectrometer for studying ra re physical phenomena.