Reconstruction of a radiation point source’s radial location using goodness-of-fit test on spectra obtained from an HPGe detector

High purity germanium (HPGe) detectors are ubiquitous in nuclear physics experiments and are also used in numerous low radioactive background detectors. The effect of the position of ⁶⁰Co and ¹³⁷Cs point sources on the shape of spectra were studied with Monte Carlo and HPGe detector measurements. We briefly confirm previous work on the position dependence of relative heights of peaks. Spectra taken with the radiation sources placed at locations around the detector were then compared using the Kolmogorov–Smirnov (K–S) goodness-of-fit test. We demonstrate that with this method the Compton continuum spectral shape has good sensitivity to the radial location of a point source, but poor angular resolution. We conclude with a study of the position reconstruction accuracy as a function of the number of counts from the source.     

Compton recoil electron tracking with silicon strip detectors

The application of silicon strip detectors to Compton gamma ray astronomy telescopes is described. The silicon Compton recoil telescope tracks Compton recoil electrons in silicon strip converters to provide an unique direction for Compton scattered gamma rays above 1 MeV. With strip detectors of modest positional and energy resolutions, of 1 mm FWHM and 3% at 662 keV, respectively, true imaging can be achieved to provide an order of magnitude improvement in sensitivity to 1.6×10 ^-6 γ/cm²-s at 2 MeV. The results of extensive Monte Carlo calculations of recoil electrons traversing multiple layers of 200 μm silicon wafers are presented. Multiple Coulomb scattering of the recoil electron in the silicon wafer of the Compton interaction and the next adjacent wafer is the basic limitation to determining the electron's initial direction     

MV级EPID成像系统中滤过板的应用

与kV级X射线成像相比,MV级电子射野影像系统(EPID)成像质量较差,图像模糊,影响患者图像定位和摆位验证的精确性。本文结合Monte Carlo模拟计算的方法,在不改变治疗靶的现有MV级EPID系统中,提出增加滤过板进行康普顿散射线的滤过,通过Monte Carlo模拟、Matlab作图、临床实验,得出在6 MV级EPID系统中,增加1层2 mm厚铜材料的滤过板,有助于成像质量的改善。     

Spatial distribution of reflected gamma rays by Monte Carlo simulation

In nuclear facilities, the reflection of gamma rays of the walls and metals constitutes an unknown origin of radiation. These reflected gamma rays must be estimated and determined. This study concerns reflected gamma rays on metal slabs. We evaluated the spatial distribution of the reflected gamma rays spectra by using the Monte Carlo method. An appropriate estimator for the double differential albedo is used to determine the energy spectra and the angular distribution of reflected gamma rays by slabs of iron and aluminium. We took into the account the principal interactions of gamma rays with matter: photoelectric, coherent scattering (Rayleigh), incoherent scattering (Compton) and pair creation. The Klein-Nishina differential cross section was used to select direction and energy of scattered photons after each Compton scattering. The obtained spectra show peaks at 0.511^∗ MeV for higher source energy. The Results are in good agreement with those obtained by the TRIPOLI code [J.C. Nimal et al., TRIPOLI02: Programme de Monte Carlo Polycin?etique à Trois dimensions, CEA Rapport, Commissariat à l’Energie Atomique. [1]].     

Design of an Anti-Compton Spectrometer for Low-Level Radioactive Wastes using Monte Carlo Techniques

An anti-Compton spectrometer with semi-2π Compton suppression is designed to identify the photons emitted from low-level radioactive wastes from radioisotope usage and nuclear research laboratory. Since the objective sample is massive and large, the system has a full opening toward the sample position. The characteristics and features of the system concerning Compton suppression and reduction of the background component due to natural radioactive source are estimated by the Monte Carlo simulations. The anti-Compton technique is shown to be quite advantageous for the reduction of the surrounding natural background radiation, as well as the suppression of the background for the higher energy photons.     

惰性气体β-γ符合测量系统探测器能量及分辨率刻度

β-γ符合法是全面禁止核试验条约(CTBT)放射性核素核查中惰性气体氙测量的一种重要方法,探测器能量及分辨率刻度是其首要解决的关键技术。本工作详细介绍了β-γ符合测量系统NaI(Tl)闪烁体和塑料闪烁体探测器能量及分辨率刻度的方法和结果,采用γ放射性核素点源刻度NaI(Tl)γ射线能量及分辨率,利用137Cs661.66keVγ射线康普顿散射电子刻度塑料闪烁体β射线能量及分辨率,并与131Xem内转换电子刻度的β射线能量分辨率结果进行了比较。结果表明:用137Cs康普顿散射电子刻度塑料闪烁体β射线能量是一种简便可行的方法,但用其刻度的β射线分辨率比实际的大。     

Some Studies on the Time Resolved Energy Spectrum Reconstruction for Pulsed Neutron Source

The purpose of this article is to present some new numerical results concerning the time resolved energy spectrum reconstruction of a short pulsed neutron source, like one created in plasma focus devices. An MCNP code is used to simulate a time dependent neutron source, plastic scintillator detectors, and their recorded signals. A Monte Carlo program reconstructs spectrum using the signals produced by MCNP code. By the numerical computations we determined the optimum number of four detectors which are needed to be placed at 0, 10, 18 and 30 m from the source, respectively. A time resolution about 12 ns and an energy resolution of 40 keV are obtained. Neutron scattering in the air is considered, and it is found that the intensity of spectrum is increased by 6%.     

Determination of spatial resolution of plastic scintillation fiber array with a simple method

The spatial resolution of a position sensitive gamma-ray detector configuration based on plastic scintillation fiber array was measured using a Monte Carlo simulation method. Both point spread function and modulation transfer function （MTF） were presented. The factors that influence the spatial resolution were also discussed. The results of the simulation showed that the intrinsic spatial resolution was consistent with the size of the physical pixels and a few centimeters spatial resolution could be obtained under certain circumstances.     

Effects of internal scattering on X-ray microtomography imagereconstruction

A computer simulation is used to study the problem of internal scattering for targets that are imaged using X-ray microtomography. A strategy is outlined for selecting X-ray energy and image resolution based on properties of the material being imaged. The X-ray scanning process is simulated by applying a Monte Carlo technique to a modeled target that emulates the material properties of a microelectronic device. The X-ray photons are subject to photoelectric absorption, Rayleigh scattering, and Compton scattering. The simulation applies a method of high-resolution image reconstruction based on discrete Fourier transforms. Examples of reconstructed images that have 0.5-μm spatial resolution are shown for images of simulated lead and aluminum targets     

线极化γ射线康普顿散射的蒙特卡罗模拟

本文利用蒙特卡罗程序Geant4模拟了不同能量、不同线极化度γ射线的康普顿散射过程,通过记录散射γ射线方位角上计数的不对称性模拟计算了入射γ射线的极化度。比较了模拟计算的极化度与原入射射线的极化度,发现两者符合很好。结果表明,利用蒙特卡罗方法能模拟极化光子与物质的相互作用,可为γ极化仪的设计提供指导。     

Image reconstruction from Compton scattering data

A new image reconstruction method was developed for a Compton camera. A simulation to determine a γ-ray source position was performed by using the simulation tool, GEANT4. An image reconstruction was made in two steps. First, a three dimensional image was constructed and projected in one selected plane, then the points from each ellipse was picked up by taking the peak points of a density distribution of crossing points between the ellipse and the first step image. The second step procedure improved the accuracy and the spatial resolution of a position determination significantly, comparing with the image obtained by only the first step. The accuracy and the resolution for a point source were obtained to be about 0.02 mm and (1.35 0.15) mm, respectively. The same procedure was applied to an imaging of the distributed y-ray source.     

康普顿背散射成像中的有机物加亮效应研究

康普顿背散射成像技术与透射成像相比具有使有机物自动加亮的特点,因此更适合于对爆炸物、毒品等违禁物品的检测.本文通过蒙特卡罗仿真方法,分析了影响有机物加亮效应的若干因素(包括入射射线能量、被检物体尺寸、探测器大小与位置、探测器工作方式),为背散射成像系统设计提供了参考.     

基于模拟点源测量的多针孔SPECT精确迭代重建

高分辨率单光子发射计算机断层成像（SPECT）是临床和小动物影像的重要技术之一。对SPECT的系统传输矩阵进行建模,并结合于迭代重建算法中,是提高系统空间分辨率的有效手段。本文基于实验室自主研发的多针孔小动物SPECT系统,研究了一种蒙特卡罗模拟和解析模型相结合的系统传输矩阵精确计算方法。该方法是基于多点源模拟的投影数据,结合数据再估计方法和高斯拟合方法提取点扩展函数（PSF）特征参数,并结合解析形式的系统物理模型,从而计算任意位置的精确PSF。通过对四点源模拟投影数据分别用纯解析方法和蒙特卡罗模拟与解析模型相结合的方法进行计算,并对所得的系统传输矩阵进行重建对比,验证了蒙特卡罗模拟与解析模型相结合方法对图像分辨率的改善效果。进一步比较了不同点源排列的重建效果,获得了优化的点源排布方案。相对于基于实验方法测量系统传输矩阵,本方法能以更合理的代价实现系统传输矩阵的精确建模,并能有效提高系统空间分辨率。     

HPGe探测器死层厚度及点源效率函数研究

在层析γ扫描分析方法的效率矩阵求解过程中,建立准确尺寸的HPGe晶体计算模型对所求的探测器效率矩阵准确性有很大影响。在使用原始的实验探测器晶体尺寸建立的模型进行探测效率计算时,发现计算与实验所得的探测效率最大误差达19%。调整探测器计算模型的死层厚度可使计算结果准确度得到很大改善。研究发现,死层厚度与各测量点相对误差平均值存在线性关系,在此基础上提出一种可快速确定最优死层厚度的修正方法。使用调整后探测器晶体尺寸,计算出不同位点源位置不同射线能量下的探测效率,拟合出探测效率与点源位置及射线能量的函数关系,用于快速求得探测效率矩阵。     

Evaluation of K X-ray escape and crosstalk in CdTe detectors andmulti-channel detectors

The simple structure of CdTe semiconductor detectors facilitates their downsizing, and their possible application to radiographic sensors has been studied. The escape of K X-rays from these detectors increases with reduction of their dimensions and affects the measurements of X- and gamma-ray spectra. K X-rays also produce crosstalk in multi-channel detectors with adjacent channels. Therefore, K X-rays which escape from the detector elements degrade both the precision of energy spectra and spatial resolution. The ratios of escape peak integrated counts to total photon counts for various sizes of CdTe single detectors were calculated for gamma rays using the Monte Carlo method. Also, escape and crosstalk ratios were simulated for the CdTe multi-channel detectors. The theoretical results were tested experimentally for 59.54-keV gamma rays from a ²⁴¹Am radioactive source. Results showed that escape ratios for single detectors were strongly dependent on element size and thickness. The escape and crosstalk ratios increased with closer channel pitch. Our calculated results showed a good agreement with the experimental data. The calculations made it clear that K X-rays which escaped to neighboring channels induced crosstalk more frequently at smaller channel pitch in multi-channel detectors. A radiation shielding grid which blocked incident photons between the boundary channels was also tested by experiment and by calculation. It was effective in reducing the probability of escape and crosstalk     

A Monte Carlo Model for Gamma-Ray Klein-Nishina Scattering Probabilities to Finite Detectors

This report introduces the Monte Carlo simulating processes for gamma-ray Klein-Nishina scattering probabilities to finite detectors and a few modifications for producing photon mode and gamma production data from the neutron collision. Those approaches have been used in MCNP code. The calculation results are given according to the primary continuous photons, the line photons and the Compton photons, respectively. The results are compared with that of MCNP code. It indicates that the approach is valid and efficient for deep-penetration problems.     

Modeling and simulation of uniformity effects in Cd/sub 1-x/Zn/sub x/Te gamma-ray spectrometers

Monte Carlo simulations of pulse height spectra for Cd/sub 1-x/Zn/sub x/Te detectors are used to investigate the effect of variations in alloy composition and carrier drift lengths on energy resolution. The results, which are based on a simple phenomenological model, show that these nonuniformities can have significant detrimental effects on spectrometer performance. For the case of Bridgman-grown material, the orientation of the growth axis relative to the detector axis is shown to be an important consideration, especially for crystals which come from the heel end of a boule, where the composition gradient due to zinc segregation is large. Other effects which we have simulated include growth striations, zinc segregation at grain boundaries, and trapping by inclusions and grain boundaries; each of these effects is detrimental to energy resolution. We conclude that material nonuniformity is a major obstacle to achieving statistically limited energy resolution in cadmium zinc telluride detectors.     

Monte Carlo simulation of spectrum changes in a photon beam due to a brass compensator

Monte Carlo simulations were used to study the changes in the incident spectrum when a poly-energetic photon beam passes through a static brass compensator. The simulated photon beam spectrum was evaluated by comparing it against the incident spectra. We also discriminated the changes in the transmitted spectrum produced by each of the microscopic processes. (i.e. Rayleigh scattering, photoelectric effect, Compton scattering, and pair production). The results show that the relevant process in the energy range considered is the Compton Effect, as expected for composite materials of intermediate atomic number and energy range considered.     

Application of the MTSVD unfolding method for reconstruction of primary X-ray spectra using semiconductor detectors

A thorough knowledge of the primary spectrum is very important to perform a quality control (QC) of X-ray tubes. In previous works, a methodology to assess primary spectrum using a Compton spectrometer and applying the Monte Carlo (MC) method has been analyzed. By means of a Monte Carlo model, a response matrix can be built, relating the Pulse Height Distribution (PHD) recorded in the detector to the primary X-ray spectrum. Subsequently, an unfolding method based on the application of a Modified Truncated Singular Value Decomposition (MTSVD) is applied to the response matrix to assess the primary spectrum. Germanium (Ge), Silicon (Si) and Cadmium-Telluride (CdTe) detectors are considered in this work. The main goal of the paper is to determine whether the MTSVD unfolding method is adequate to provide an acceptable reproduction of spectra for these detectors.