O. Dössel and W.C. Schlegel (Eds.): WC 2009, IFMBE Proceedings 25/III, pp. 602–605, 2009. www.springerlink.com Convenient Method of Thermal Neutron Measurement Using Imaging Plates in Proton Therapy T. Fujibuchi 1,2 , Y. Tanabe 1 , T. Isobe 1 , H. Kawamura 1,2 , T. Terunuma 1,3 , K. Yasuoka 1,3 , T. Matsumoto 4 , J. Nishiyama 4 , H. Harano 4 , and T. Sakae 1,3 1 Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, Japan 2 Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, Ami, Japan 3 Proton Medical Research Center, University of Tsukuba, Tsukuba, Japan 4 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan Abstract— 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. Keywords— Thermal Neutron fluence, Imaging plate (IP) dosimetry, Fading characteristics, Prompt gamma-ray, Monte Carlo simulation I. INTRODUCTION In proton therapy, the evaluation of secondary neutron generation is important, because such secondary radiation may lead to tumor formation in patients, increase radiation levels in the treatment room, and so on [1, 2]. In this study, we investigate the use of an imaging plate (IP) for convenient evaluation of neutron dose. An IP is a radiation detector, which utilizes a photostimulable phosphor of BaFBr:Eu 2+ . Latent images on the IP created by ionizing radiation are scanned by a stimulating laser beam and are read by a photomultiplier as photo-stimulated luminescence. Such devices have been widely used for the quantitative measurement of two-dimensional intensity distributions of various types of ionizing radiation. In this study, the application of IPs for neutron detection has been investigated with the aim of realizing a practical dosimetry system for therapy. The linearity between intensity and thermal neutron fluence has been measured to identify the practical upper limit of measurable dose. Fading characteristics have also been studied for quantitative dose evaluation. It was found that thermal neutrons and other forms of radiation could be detected using IPs and metal plates. Fig. 1 Arrangement for the measurement of linearity characteristics of the IPs