CT靶物质的辐射特性研究

Study of X-Rays Generated by Target in CT

采用Monte Carlo模拟程序MCNP4B对电子-光子在CT靶物质中的耦合输运进行了模拟．对几种靶的光子效率和X射线前向性随靶厚度变化进行了计算和分析．模拟计算结果表明,与电子加速器中常用的Au,W等高Z靶相比,Cu靶具有特殊的辐射物理性质:1)光子效率对Cu靶厚度不敏感．通常,对于高Z靶,光子效率在靶厚度为某特定值时达到最高值,然后随靶厚度增加而迅速减小;而对于Cu靶,光子效率在达到最大值后,靶厚度再增加时,光子效率仅略有减小,且变化趋于平缓．2)电子束轰击Cu靶时,虽然光子效率较低,但是角分布具有很好的前向性,且前向剂量率可与Au靶、W靶相当．实验结果表明,用能量为20MeV的电子轰击Cu靶时,前向0°附近的X射线剂量率达到并超过了用W靶时的结果,而且剂量率角分布比W靶的前向性好．

The X-rays generated and transported in target bombarded by electron beams in CT have been simulated with Monte Carlo code MCNP4. Photon efficiencies and forward directional characteristics of X-rays for several kinds of often-used targets were calculated and analyzed. Calculations show that as the thickness of the target increases, the photon efficiency increases and reaches a maximum, and then drops sharply for gold and tungsten target, while drops gently for cooper target. Also, the photon efficiency for cooper target is relatively small, but the angular distribution shows good forward directional characteristics, with the dose rate in the forward direction equivalent to those of tungsten or gold target. Experiment results show that the dose rate in the forward direction of a copper target is higher than that of a tungsten target for 20MeV electrons, while the angular distribution presents better forward directional characteristics compared with a tungsten target.