Tanaka Kenichi, Yokobori Hitoshi, Endo Satoru, Kobayashi Tooru, Bengua Gerard, Saruyama Ichiro, Nakagawa Yoshinobu, Hoshi Masaharu
Research Institute for Radiation Biology and Medicine, Hiroshima University, Hiroshima, Japan.
Appl Radiat Isot. 2009 Feb;67(2):259-65. doi: 10.1016/j.apradiso.2008.10.002. Epub 2008 Oct 17.
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 degrees C and a cycle of less than 115 ms (8.7 Hz) keeps the Cu strain below the critical value for a 230 microm thick Li target, though the values are evaluated conservatively. Against expectation, the Li temperature and Cu strain are larger for a 100 microm thick target than for a 230 microm 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 1mm depth, for both of 100 and 230 microm 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.
本研究从散热和材料强度的角度，展示了基于加速器的硼中子俘获疗法（BNCT）中锂靶组件上质子点扫描的特性。考虑到铜背板比锂具有更合适的散热性能，研究了这些特性对锂靶厚度的依赖性。本文考虑了两种情况，即点扫描的循环操作，以及质子束停留在锂靶上单个位置的停滞点扫描循环。研究发现，锂温度的最大值和铜背板的应变随循环周期的增加而增大。周期小于120毫秒（频率超过8.3赫兹）可使锂温度保持在150摄氏度以下，周期小于115毫秒（8.7赫兹）可使230微米厚的锂靶的铜应变保持在临界值以下，不过这些值是保守评估的。与预期相反，100微米厚靶的锂温度和铜应变比230微米厚靶的更大。在这种情况下，保持锂温度合理所需的周期为23毫秒（43赫兹），防止铜疲劳断裂所需的周期为9毫秒（110赫兹）。对于点扫描循环中的停滞情况，铜温度随束流关闭时间的增加而升高。对于100微米和230微米厚的靶，估计铜到达熔点的时间在表面为4.2毫秒，在1毫米深度为20毫秒。估计至少34毫秒足以在铜背板上打出一个洞。因此需要一个响应时间约为20毫秒的束流关闭机制。
