Beam Quality Determination of Ultra High Energy Neutrons

作为在筑波大学PARMS建造的超高能量中子装置，在剂量分布方面产生的中子束优于现代直线加速器产生的X射线束。这是利用高能物理实验室的助推器同步加速器发射的500 MeV质子束在厚铀靶上的反应实现的。该中子束的百分比深度剂量几乎相当于x射线的剂量，在20毫伏左右，剂量率为每分钟15cGy。利用HMV-I细胞株，估计了该中子束的相对生物学效应。中子辐照后的细胞存活曲线为肩部，n和Dq分别为8 Gy和2.3 Gy。目前中子在10 ^2生存水平的RBE值，与^<137> Cs伽马射线相比为1.24。结果表明，当中子的深度剂量分布优于高能直线x射线时，高能中子的生物效应实际上并不大。

As ultra high energy neutron facility was constructed at PARMS, University of Tsukuba, to produce a neutron beam superior to an X ray beam generated by a modern linac in terms of dose distribution. This has been achieved using the reaction on a thick uranium target struck by 500 MeV proton beam from the booster-synchrotron of High Energy Physics Laboratory. The percent depth dose of this neutron beam is nearly equivalent to that of X-rays at around 20 MV and the dose rate of 15cGy per minute. Relative biological effectiveness of this neutron beam has been estimated on the cell killing effect by the use of HMV-I cell line. Resultant survival curve of cells after the neutron irradiation shows the shoulder with n and Dq of 8 and 2.3 Gy, respectively. RBE value at 10 ^2 survival level for the present neutron, compared with ^<137> Cs gamma -rays is 1.24. The result suggests that the biological effects of high energy neutrons are not practically large enough whenever the depth dose distri bution of neutrons becomes superior to high energy linac X-rays.