Convenient method of thermal neutron measurement using imaging plates in proton therapy

In proton therapy, measurement of secondary neutron emission is important, because such radiation can influence the occurrence of secondary cancers and the radioactivity of the treatment room. In this study, we investigated the possibility of neutron detection using a neutron imaging plate (IP) and a general IP. The upper limit of measurable neutron fluence by an IP system is primarily controlled by the readout range of the scanner that is used. Within this limit, a reasonable linear response of the IP to a neutron fluence was maintained. To separate thermal neutron from other radiation, we covered a part of the IPs with 1.5 mm thick cadmium plates and 1.0 mm thick lead plates. The neutron IP was shielded from thermal neutrons by the cadmium cover and the measured intensity was reduced. However, in the general IP, the intensity increased slightly in the region covered with cadmium because of prompt gamma-ray production. A further increase in intensity occurred in the lead-covered region due to interaction with photons. Moreover, the neutron field was calculated by PHITS Monte Carlo simulations, and the elements of secondary radiation which were produced in the neutron IP and metal plates were confirmed. From the results of this study, thermal neutrons and other radiation could be detected by using IPs and cadmium plates, suggesting the potential of such devices as practical neutron detectors.