区域γ辐射定向监测仪准直器的设计

为区域γ辐射分布定向监测实验相机系统设计了一个针孔准直器。应用针孔准直器的解析公式,根据探测器内在分辨率和实验相机系统的预期分辨率,考虑探测器尺寸,确定了相机的焦距及准直器的张角;然后根据预期角分辨率计算有效孔径,得出孔径和孔深的范围;再利用蒙特卡罗方法模拟面源入射,在各种孔径和孔深组合中,挑选出透射和散射光子份额最小的组为最优组,对应孔径和孔深参数即为最优参数。     

γ射线测距中散射光子影响因素的蒙特卡罗模拟

基于散射光子的γ射线测距技术,具有测距精度高、响应速度快、可靠性高、体积小、重量轻等特点,适用于在苛刻空间环境中实现近距离高精度的高度测量。本文采用蒙特卡罗程序MCNP建立模型,模拟不同条件下散射光子的能量、强度的变化规律,分析了探测距离、源 探距离、γ射线能量、靶目标厚度以及靶目标材料的变化对反散射峰光子能量与强度的影响,得出以下结论：反散射峰光子能量与靶目标厚度（>7 cm）、靶目标材料无关,与γ射线能量、源 探距离正相关,与探测距离负相关;反散射峰光子强度与靶目标厚度（>7 cm）无关,与探测距离正相关,与γ射线能量、源 探距离、靶目标材料负相关。对于不同靶目标材料,模拟计算的反散射峰光子能量分布区间与理论计算结果一致,证实本文γ射线散射光子测距技术的仿真方法可行、结果可信。     

X射线工业CT散射修正

X射线工业CT（ICT）依赖准直器压低散射光子存在实际困难和不足。为解决这一问题，需采用相应的修正软件加以校正。文章从理想的模型出发，近似推导透射式ICT中散射X光子的理论计算公式，并把计算结果与实验结果进行比较。比较结果表明，两者吻合较好。据此，文章给出了散射光子软件修正的可行性方案。      

CSNS/RCS粒子散射束流损失研究

在中国散裂中子源快循环同步加速器（CSNS/RCS）中,质子束流在加速过程中会与一些器件（如剥离膜、准直器、散射引出膜等）相互作用,产生粒子散射并导致束流损失。本工作首先利用ORBIT模拟RCS束流注入过程,并用FLUKA模拟注入束流穿过剥离膜的粒子散射过程,计算剥离膜散射所造成的束流损失。其次,模拟质子束流与准直器相互作用的粒子散射,计算质子束流与不同尺寸的次级准直器相互作用的吸收效率,作为对次级准直器优化的依据。最后,研究CSNS/RCS膜散射引出方案,利用FLUKA对不同引出方案进行模拟并比较,得到最佳的可行性方案。     

基于Geant4的γ-CT系统优化设计

基于蒙特卡罗方法的粒子输运模拟软件包Geant4构建低能γ射线多相流CT系统仿真模型,通过散射光子比、图像相关系数及相对图像误差等优化指标分析,对γ-CT系统的准直器、射线源数及探测器阵列单元数进行优化设计.实验结果表明,优化后系统散射光子比小于5%,明显提高检测信号灵敏度的信噪比.     

高能γ光子成像针孔准直器的解析设计方法

在对高辐射环境进行射线源分布成像时,需要为γ相机设计专门的用于高能光子成像的针孔准直器。为了简化针孔准直器的设计过程,使用几个新的解析公式来描述准直器各参数与性能间的关系。首先,采用解析方法推导了更通用的有效孔径公式、角分辨率公式和几何相对效率公式;然后,与采用蒙特卡罗方法模拟计算得到的数值进行比较。结果表明,解析方法提供的计算结果在一定范围内准确可靠。因此,本工作使用的针孔准直器解析设计方法简单方便,物理图像清晰,具有一定的实用价值。     

干式储藏~(137)Cs源反散射份额的计算研究

干式储藏 ~(137)Cs源由于其自身结构特点,放射源发射的γ射线中含有反散射光子,采用此类型的储藏源校准剂量计,会存在一定的偏差。本文首先建立了多源照射器实验室三维模型,利用该模型理论计算并分析了实验室墙壁对反散射峰计数的影响。然后测量 ~(137)Cs能谱,采用两种方式扣除康普顿坪并计算出反散射峰与全能峰的计数之比。实验解谱结果表明,在实验室内离源6m处反散射光子与661.66keV光子计数之比为0.090 9,实验解谱数据与蒙卡模拟数据相对偏差为-13.8%。     

蒙特卡罗方法选择 TGS 准直器的最佳形状

采用蒙特卡罗方法和技术，模拟ＴＧＳ连续扫描模式，对不同形状的准直器，计算了被测量废物桶中各个像素内的纵向效率的最大百分差，从中选取了棱形准直器和调整后的棱形准直器作为ＴＧＳ装置的扫描准直器。     

蒙特卡罗方法选择TGS准进器的最佳形状

采用蒙特卡罗方法和技术，模拟了ＴＧＳ连续扫描模式，对不同形状的准直器，计算了被测量废物桶中各个像素内的纵向效率的最大百分差，从中选取了棱形准直器和调整后的棱形准直器作为ＴＧＳ装置的扫描直器。     

医用直线加速器主要部件对X射线能谱的影响

研究了医用直线加速器的主要部件对X射线能谱的影响。利用蒙特卡罗软件包EGSnrc/BEAMnrc针对Varian600C医用直线加速器,模拟完整及分别去除初级准直器、均整器和次级准直器条件下的相空间文件,通过相空间文件分析程序Beamdp对相空间文件进行分析,分别得到相应条件下的X射线能谱。均整器对能谱的影响非常大,对比于无均整器的情况,有均整器时,平均能量上升了48.9%;但初级准直器、次级准直器对能谱的影响小,平均能量的变化小于1%。在射野内,对比于无均整器的情况,有均整器时,能量注量降低了52.4%,但其空间分布变均匀,非平坦度从15.09%变为1.82%;对比于无初级准直器的情况,有初级准直器时,能量注量上升了1.58%;而对比于无次级准直器的情况,有次级准直器时,能量注量下降了2.00%。均整器对X射线的能谱的影响起主要作用,它能够使X射线的平均能量上升和射野内能量注量的分布变均匀。初级准直器对X射线能谱影响小但对X射线的散射不可忽略,而次级准直器对X射线能谱的影响小而且对X射线的散射作用可以忽略,这些结果对加速器X射线束流模型的建立有一定的参考意义。     

Some angular properties of the scattered photons for 1.43 MeV and 2.75 MeV source photons penetrating aluminium,steel and lead shields

Experimentally determined angular flux spectra for 1.43 and 2.75 MeV source photons from disc geometry penetrating shielding slabs of lead, steel and aluminium were analyzed. For both source energies, the proportion of photons scattered in a forward direction increases with increasing penetration thickness. Also, at any polar angle, the scattered photon energy decreases experimentally with increasing shield thickness. Changes in the scattered photon energy spectrum with polar angle and shield thickness are discussed. For the steel shields, angular contributions to the scalar build-up factor are presented.     

Angular properties of 6.13 MeV source photons penetrating lead,steel and concrete

Experimentally determined angular flux spectra for 6.13 MeV source photons from disc geometry penetrating shielding slabs of lead, steel and concrete are analysed. For all three shield materials, the proportion of photons scattered in a forward direction increases with increasing penetration thickness. At any given polar angle, the scattered photon properties decrease exponentially with increasing shield thickness. Angular exposure dose build-up factor is defined and angular contributions to the scalar build-up factor are presented. Changes in the distribution of photon energies relating to shield penetration thickness and polar angle are discussed. The influence of bremsstrahlung and positron annihilation photon sources are considered.     

编码孔技术用于X射线前向散射的成像

简要介绍了编码孔成像技术的概念和原理,并通过蒙特卡罗程序的模拟结果,说明这种成像技术可以用于X射线前向散射的成像,而且成像效果较为理想。     

Some angular properties of 6 MeV source photons penetrating steel

The properties of angular flux densities for 6.2 MeV source photons penetrating up to 4.7 m.f.p. of steel are analysed. Results show that the energy flux distribution can be represented by a cosine function varying with penetration thickness. Above 2 m.f.p. penetration, the angular distribution of the scattered photons are forward peaked and similar for all photon energies. The angular energy spectrum hardens with increasing penetration.     

Photon energy absorption and exposure buildup factors for deep penetration in human tissues

Using photons in therapeutic and diagnostic medicine requires accurate computation of their attenuation coefficients in human tissues.The buildup factor,a multiplicative coefficient quantifying the ratio of scattered to primary photons,measures the degree of violation of the Beer-Lambert law.In this study,the gamma-ray isotropic point source buildup factors,specifically,the energy absorption buildup factor (EABF) and exposure buildup factor,are estimated.The computational methods used include the geometric progression fitting method and simulation using the Geant4 (version 10.4) Monte Carlo simulation toolkit.The buildup factors of 30 human tissues were evaluated in an energy range of 0.015-15 MeV for penetration depths up to 100 mean free paths (mfp).At all penetration depths,it was observed that the EABF seems to be independent of the mfp at a photon energy of 1.5 MeV and also independent of the equivalent atomic number (Zeq)in the photon energy range of 1.5-15 MeV.However,thebuildup factors were inversely proportional to Zeq for energies below 1.5 MeV.Moreover,the Geant4 simulations of the EABF of water were in agreement with the available standard data.(The deviations were less than5%.) The buildup factors evaluated in the present study could be useful for controlling human exposure to radiation.     

电子辐照加速器屏蔽墙中预埋管道倾角对辐射防护性能影响的蒙特卡罗计算

电子辐照装置屏蔽墙中总存在一些较大口径的管道如辐照电缆传输孔道,这些管道的存在势必会降低屏蔽体的局部屏蔽能力。为定量评估管道对屏蔽效果的影响,本文以3 MeV电子辐照加速器为例,采用蒙特卡罗MCNP程序分别模拟计算了电子打靶以后产生的能谱中1 MeV以下和1 MeV以上不同单能光子入射同一屏蔽体时,管道以不同角度穿透屏蔽墙时管道出口处及屏蔽墙外辐射场分布;并比较了在不同墙厚下这两个能段对管道出口处辐射场的影响程度。计算结果表明:不同单能光子入射屏蔽体,在管道穿墙角度≥45°时,管道出口处辐射场变化不再明显,对于1 MeV以下的低能光子在墙外的透射辐射场基本可忽略,对于1 MeV以上的能量光子,其墙外光子透射辐射场随角度增大影响显著;随着墙厚增加,低能光子衰减越显著,高能光子在管道穿墙夹角设计中越占据了主导地位。     

Cross-Sectional Imaging of Large and Dense Materials by High Energy X-Ray CT Using Linear Accelerator

A prototype high energy X-ray CT (computed tomography) system has been developed which employs a linear accelerator as the X-ray source (max. photon energy: 12MeV).

One problem encountered in development of this CT system was to reduce the scattered photons from adjacent detectors, i.e. crosstalk, due to high energy X-rays. This crosstalk was reduced to 2% by means of detector shields using tungsten spacers. Spatial resolution was not affected by such small crosstalk as confirmed by numerical simulations. A second problem was to reduce the scattered photons from the test object. This was done using collimators. A third concern was to realize a wide dynamic range data processing which would allow applications to large and dense objects. This problem was solved by using a sample and hold data acquisition method to reduce the dark current of the photo detectors. The dynamic range of this system was experimentally confirmed over 60 dB. It was demonstrated that slits (width : 2 mm) in an iron object (diameter : 25 cm) could be imaged by this prototype CT system.     

Energy distributions of multiple backscattered photons in materials

Backscattering of gamma photons from a material is of fundamental importance in radiation shielding,industrial and medical applications, radiation dosimetry,and non-destructive testing. In Compton scattering, incident photons undergo multiple scatterings within the material(target) before exiting. Gamma photons continue to soften in energy as the number of scatterings increases in a thick target; in other words, the energy of gamma photons decreases as the scatterings increase in case of a thick target and results in the generation of singly and multiply scattered events. In this work, the energy distribution of backscattered gamma photons with backscattering intensity and energy probabilities were calculated by using the Monte Carlo method for metallic, biological, and shielding materials with various thicknesses of slab geometry. The materials under study were targeted with gamma photons of 0.279, 0.662, 1.250, and 2.100 Me V energies. In addition, the energy distributions of multiply scattered gamma photons were studied for materials with infinite geometry.The results are presented and discussed in detail by comparing with other Monte Carlo calculations.     

A successful experimental observation of double-photon Compton scattering of γ rays using a single γ detector

The phenomenon of double-photon Compton scattering has been successfully observed using a single γ detector, a technique avoiding the use of the complicated slow-fast coincidence set-up used till now for observing this higher-order process. Here doubly differential collision cross-sections integrated over the directions of one of the two final photons, the direction of other one being kept fixed, are measured experimentally for 0.662 MeV incident γ photons. The energy spectra of the detected photons are observed as a long tail to the single-photon Compton line on the lower side of the full energy peak in the recorded scattered energy spectrum. The present results are in agreement with theory of this process.