﻿ 基于ICRU模型的辐射剂量计算

# 基于ICRU模型的辐射剂量计算Radiation Dose Calculation Based on ICRU Model

Abstract: Tissues and organs which show various radiosensitivity are in the specific depths of human body, so they suffer different levels of damage caused by the complex space radiation. Proton with wide spectrum is the main particle type in space and it is necessary to acquire the relationship between the energy of proton and the absorbed dose of tissues and organs for radiation-risk evaluation before space missions. The tissue-equivalent model recommended by International Commission on Radiation Units and Measurements (ICRU) is light and regular compared to the phantoms so that it is more suitable in practice for space radiation monitoring and evaluation. The ICRU model was established through simulation in this work. The mechanism of the interaction between ICRU model and protons/gamma-rays with different energies was analyzed and the absorbed dose of tissues and organs in the different depths of human body was calculated. The calibration system for dose monitoring based on ICRU model and the calculation system for dose evaluation was established.

1. 引言

2. 研究方法

2.1. ICRU模型

Figure 1. Sphere (a) and slab (b) model recommended by ICRU

2.2. 辐射源

1) 质子

${\left(-\frac{\text{d}E}{\text{d}x}\right)}_{ion}=\frac{4\text{π}{z}^{2}{\text{e}}^{4}ZN}{{m}_{0}{v}^{2}}\left[\mathrm{ln}\left(\frac{2{m}_{0}{v}^{2}}{I}\right)-\mathrm{ln}\left(1-{\beta }^{2}\right)-{\beta }^{2}\right]$

Bethe-Block公式中m0是电子静止质量，ν是入射粒子的速度，z是带电粒子电荷，Z是阻止介质原子序数， $\beta =v/c$c是光速，I是介质原子的平均电离电势。

2) γ射线

γ射线本身不带电，因而用相互作用截面、能量转移和能量吸收的几率来描述其在物质中的行为。γ射线与物质相互作用最主要的方式为光电效应、康普顿效应、电子对效应等，γ射线能量主要转换为次级电子能量，通过计算次级电子能量沉积可得到γ射线能量沉积。

Figure 2. The range of protons in the tissue-equivalent materials

2.3. 方法程序

Figure 3. Main module structure diagram of GEANT4

GEANT4是欧洲核子中心(CERN)开发的一种蒙特卡洛程序，在核物理实验模拟、空间辐射、加速器模拟、放射医学等领域均有广泛应用。该程序程序代码开放，采用面向对象的C++语言编写，具备完整的粒子与物质相互作用过程的材料库，包括几何跟踪，探测器响应，可视化和用户界面等模块的软件包，是粒子物理与核物理实验模拟最好的工具之一。本工作选用GEANT4程序基于ICRU模型对吸收剂量进行模拟计算。GEANT4主要模块图如图3所示。其中Global模块包含了程序所需要的常数、单位、数值运算以及随机数产生等，Particle和Material模块描述粒子和材料的物理特性，Tracking和Track模块与粒子的运输过程有关，Process模块描述粒子与物质相互作用，可设置粒子的能量、动量以及种类、对参与过程的物质等，Run是对模拟事件的控制指令，Visualization模块可以将探测器、粒子径迹等转化为图形。在模拟时，首先要明确粒子会与物质会发生哪些重要的物理过程。在各种粒子与物质的相互作用的过程中，Geant4中主要有7种物理过程包括电与磁相互作用、强子之间相互作用过程、原子核的衰变过程、光轻子与强子之间的相互作用过程、光学的光子的过程、设置参数过程以及粒子输运过程。

2.4. 输入条件

1) 基于空间质子能谱及地面质子加速器试验能段范围，质子能量选取10 MeV、20 MeV、40 MeV、50 MeV、60 MeV、80 MeV、100 MeV、120 MeV、150 MeV及200 MeV；基于空间舱内屏蔽后的γ能谱分布、地面钴60的γ平均能量1250 keV，γ射线能量选取1000 keV、1173 keV、1320 keV、1500 keV、2000 keV、2500 keV、3000 keV及3500 keV；

2) 质子、γ射线均采用入射角为cos分布、度数0~90度之间照射；

3) 质子注量选取SPE典型恶劣环境注量107 cm−2且为归一化常数，γ射线注量与质子一致；

4) ICRU模型深度选取0.07 mm、3 mm、10 mm、30 mm及100 mm，探测在该深度下的吸收剂量。

3. 结果与讨论

3.1. ICRU球模型中辐射剂量分布

Table 1. The normalized absorbed dose (Gy·cm2) in sphere model of different detecting depths caused by protons of different energies

Figure 4. The proton energy (a) and material thickness (b) dependence of the normalized absorbed dose (Gy·cm2) in sphere model

Table 2. The normalized absorbed dose (Gy·cm2) in sphere model of different detecting depths caused by γ ray of different energies

Figure 5. The γ-ray energy (a) and material thickness (b) dependence of the normalized absorbed dose (Gy·cm2) in sphere model

3.2. ICRU平板模型中辐射剂量分布

Table 3. The normalized absorbed dose (Gy·cm2) in slab model of different detecting depths caused by protons of different energies

Figure 6. The proton energy (a) and material thickness (b) dependence of the normalized absorbed dose (Gy·cm2) in slab model

Table 4. The normalized absorbed dose (Gy·cm2) in slab model of different detecting depths caused by γ-rays of different energies

4. 结论

NOTES

*通讯作者。

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