In the recent radiation therapy, it is pointed out to be careful about neutrons caused by photonuclear reaction. In this work, we studied a method of thermal neutron measurement by using NaI(Tl) scintillator with the self-activation. Thus the NaI crystal served as both target and 4π counter of activation method. After the neutron irradiation of 30 minutes, the activation of NaI crystal was measured by NaI(Tl) detector itself with a photomultiplier tube. In the energy spectra obtained, a significant contribution of beta-rays from I-128 was dominant for several hours after the termination of neutron irradiation. During that period, the total count-rate decreased exponentially with a half life of 25 minutes, which corresponded to that of I-128. No prominent peak of gamma-rays was observed in the spectra. Owing to the good detection efficiency of such beta-rays, it is expected that high sensitive neutron-detection can be achieved. By the precise determination of I-128 activity, a quick evaluation of thermal-neutron fluence may be possible through the online measurement.
