Characteristics of proton beam scanning dependent on Li target thickness from the viewpoint of heat removal and material strength for accelerator- based BNCT Kenichi Tanaka a,Ã,1 , Hitoshi Yokobori b , Satoru Endo a,2 , Tooru Kobayashi c , Gerard Bengua c,3 , Ichiro Saruyama b , Yoshinobu Nakagawa d , Masaharu Hoshi a a Research Institute for Radiation Biology and Medicine, Hiroshima University, Japan b Advanced Reactor Technology Co., Ltd., Japan c Research Reactor Institute, Kyoto University, Japan d National Kagawa Children’s Hospital, Japan article info Article history: Received 25 May 2007 Received in revised form 18 September 2008 Accepted 2 October 2008 Keywords: Accelerator-based BNCT Li target abstract This study demonstrates the characterization of proton spot scanning on a Li target assembly for accelerator-based BNCT from the viewpoint of heat removal and material strength. These characteristics are investigated as to their dependence on the Li target thickness, considering that the Cu backing plate has more suitable heat removal properties than Li. Two situations are considered in this paper, i.e. the cyclic operation of the spot scanning, and a stalled spot scanning cycle where the proton beam stays focused on a single position on the Li target. It was found that the maximum of the Li temperature and the strain of the Cu backing increase as the cycle period increases. A cycle period less than 120 ms (over 8.3 Hz of frequency) enables the Li temperature to be kept below 150 1C and a cycle of less than 115ms (8.7Hz) keeps the Cu strain below the critical value for a 230 mm thick Li target, though the values are evaluated conservatively. Against expectation, the Li temperature and Cu strain are larger for a 100 mm thick target than for a 230 mm target. The required cycle period in this case is 23 ms (43 Hz) for maintaining a reasonable Li temperature and 9 ms (110 Hz) to prevent Cu fatigue fracture. For a stall in the spot scanning cycle, the Cu temperature increases as the beam shutdown time increases. The time for Cu to reach its melting point is estimated to be 4.2 ms at the surface, 20 ms at 1 mm depth, for both of 100 and 230 mm thick targets. At least 34 ms is estimated to be enough to make a hole on Cu backing plate. A beam shutdown mechanism with a response time of about 20 ms is therefore required. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Accelerator-based neutron sources (ABNS) for boron neutron capture therapy (BNCT) have been actively investigated since the beginning of the 1980s. ABNS are considered to be more advantageous when compared to uranium-fission neutrons from nuclear reactors because of the ease in commissioning new accelerator-based irradiation facilities near urban hospitals, the controllability of maximum neutron energy by selecting the type of incident particle, its energy and the target material (Brownell et al., 1986; Kobayashi et al., 2000; Yonai et al., 2003; Endo et al., 2004; Martin and Abrahantes, 2004; Binns et al., 2005). One of the promising nuclear reactions that can be used for neutron production is the 7 Li(p,n) 7 Be reaction (Q-value: 1.644 MeV, threshold: 1.881 MeV) at an incident proton energy of 2.5 MeV (Wang et al., 1989; Yanch et al., 1993; Tanaka et al., 2005; Bengua et al., 2006). This is because the required proton current will be around 10–20 mA, depending on the assumed geometry and neutron beam evaluation index applied (Wang et al., 1989; Yanch et al., 1993; Tanaka et al., 2001, 2006; Lee et al., 2001; Culbertson et al., 2004). For the aforementioned neutron production para- meters, however, the heat deposition in the Li target is as much as 25–50kW. This heat load can be removed by means of liquid cooling systems that make use of H 2 O or liquid Hg and Ga ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.10.002 Ã Corresponding author. Tel.: +81116112111; fax: +81116442001. E-mail address: tanakaken@sapmed.ac.jp (K. Tanaka). 1 Present address: Department of Physics, School of Medicine, Sapporo Medical University,17, Minami 1 Jo, Chuo-ku, Sapporo 060-8556, Japan. 2 Present address: Quantum Energy Applications, Graduated School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739- 8527, Japan. 3 Present address: Medical Physics Department, Hokkaido University Hospital, Japan. Applied Radiation and Isotopes 67 (2009) 259–265