衰减透射法测量高能X射线能谱

研究了基于衰减透射原理的高能X射线能谱测量与重建。利用蒙特卡罗方法对神龙一号直线感应加速器的X射线源穿过不同厚度铝时的衰减透射过程进行模拟实验。解谱方法采用迭代扰动法,对不同的初始能谱估计和测量噪声水平条件下的能谱重建进行计算分析。结果表明:实验测量不包含噪声时,选择合适的初始能谱可以获得比较准确的能谱重建结果;在实验测量数据包含5.0%高斯随机噪声情况下,X射线能谱重建计算结果仍比较稳定,与真实值偏差相对较小。     

基于蒙特卡罗方法的6 MV Truebeam剂量计算

使用蒙特卡罗方法快速准确地模拟6 MV Varian Truebeam医用电子直线加速器射野剂量特性,探究厂家提供相位空间源的可用性及模拟方法的准确性。以Varian公司提供的出束窗口位置处相位空间文件作为Beamnrc/EGSnrc输入源,模拟射野形成结构并计算10 cm10 cm射野下的均匀水体模中的剂量分布,将计算结果与相同条件下的实验测量数据进行比较。同一计算机模拟时,传统完整机头模拟方法需4~5 h,文中所述方法模拟时间可缩减至48 min,剂量计算结果与相同条件下所得实际测量数据可较好地吻合,两者的百分深度剂量差异低于3%,且建成区吻合良好;不同深度处80%等剂量线所包含的射野区域内百分离轴剂量比差异均低于3%,且半影区效果好。使用厂家提供相位空间文件作为EGSnrc入射源,能快速模拟治疗头,得到剂量计算结果与实际测量值的差异满足剂量计算精度要求。     

北京质子直线加速器(10MeV段)的束流横向运动匹配计算

本文描述北京质子直线加速器(10MeV段)的束流横向运动匹配计算. 在该加速器的运行调试中已直接利用本文计算结果. 当腔内透镜电流运行在计算值或其附过值时, 束流在腔里的传输效率可高达63.6%(只用一个聚束器时), 达到了较高水平. 实验已表明: 本文的计算对于该加速器的调试运行很有用处.     

BEPCⅡ直线加速器发射度测量

BEPCⅡ直线加速器采用改变四极磁铁励磁电流,测量获取的束流横截面荧光图像得到束流包络,进而通过对束流包络和励磁电流的拟合计算得到束流发射度和Twiss参数.在BEPCⅡ直线加速器的改进过程中,发现并解决了影响发射度测量结果准确性的多个问题,改进了束流横截面测量方法和发射度计算方法,提高了测量结果的准确性.根据BEPCⅡ直线加速器的具体情况,还编写了束流截面测量程序和发射度计算程序,提高了发射度的测量和计算速度,方便了发射度测量实验的使用.     

BEPCⅡ直线加速器发射度测量

BEPCⅡ直线加速器采用改变四极磁铁励磁电流, 测量获取的束流横截面荧光图像得到束流包络, 进而通过对束流包络和励磁电流的拟合计算得到束流发射度和Twiss参数. 在BEPCⅡ直线加速器的改进过程中, 发现并解决了影响发射度测量结果准确性的多个问题, 改进了束流横截面测量方法和发射度计算方法, 提高了测量结果的准确性. 根据BEPCⅡ直线加速器的具体情况, 还编写了束流截面测量程序和发射度计算程序, 提高了发射度的测量和计算速度, 方便了发射度测量实验的使用.     

针孔单光子发射计算机断层成像的空间分辨率研究

针孔单光子发射计算机断层(SPECT)成像的空间分辨率通常是根据Anger经验公式来进行估算,与实际测量存在较大偏差.本文通过对针孔成像的物理过程进行分析,提出了一个近似度更高的计算公式.利用精确的蒙特卡罗方法模拟针孔SPECT成像,采用OSEM(ordered subsets expectation maximization)算法对投影数据进行图像重建,并与模具实验进行比较,验证了理论公式的适用性.同时还讨论了体素尺寸、几何映射获取投影矩阵以及探测器尺寸与成像物体尺寸比值对断层图像空间分辨率的影响.实验结果显示,该理论公式所估算的空间分辨率比实验值平均偏小约10%,而Anger经验公式所估算的空间分辨率比实验值平均偏大约60%.因此,该理论公式能更好地估算针孔SPECT成像的空间分辨率,可为针孔SPECT系统的设计和使用提供有价值的参考.     

用于心磁图测量的基于高温超导量子干涉仪磁强计的有源补偿

我们建立了一套基于高温超导量子干涉仪(SQUID)的可靠的、经济型的单通道心磁图(MCG)系统,该系统在一个简易的磁屏蔽室(MSR)中运行.实验结果表明,对于消除心磁测量中的环境噪声,简易磁屏蔽室和有源补偿相结合是一个很合适的方案,这种方案对心磁图的普及有实际的帮助.同时,本文给出了一个有源补偿的简单模型,并将计算结果与实验数据进行了比较.     

中子产额活化测量法中γ γ符合探测效率的模拟计算

 为了估计聚变中子产额Cu活化测量法中加速器中子源刻度与实际测量中几何条件不一致对测量结果的影响，通过对MCNP进行二次开发，用蒙特卡罗方法计算了这两种情况下有角关联的γ γ符合探测效率，通过与刻度实验结果比较，验证了计算结果的正确性，得出了几何条件的差别引起探测效率的增加小于3%的结论。     

一种动-静态光散射颗粒折射率测量法

提出一种动静态光散射颗粒折射率测量方法.该方法分为三部分,一部分是用动态光散射技术测得颗粒样品的线宽分布函数,然后将此线宽分布连同一组折射率值代入静态光散射公式推算出不同折射率值下的散射光强空间分布;第二部分是用光电探测器测出实际的散射光空间分布;第三部分是将不同折射率下的推算值与实测值做均方偏差运算以做比较,取均方偏差最小时所对应的代入折射率值作为颗粒折射率的最终测量值.此方法对样品的分散度无要求,适用于微米到亚微米级的颗粒物.     

用蒙特卡罗方法计算光通过薄层随机分布粒子的散射

本文利用蒙特卡罗方法计算准直光束通过薄层随机分布粒子散射的透射和反射光强,并和输运理论的扩散近似结果做了比较。当散射接近各向同性时两者符合良好。当散射明显地成为各向异性时,蒙特卡罗方法的结果是合理的,而输运理论的扩散近似失效。     

Validation of Monte Carlo simulation of 6 MV photon beam produced by Varian Clinac 2100 linear accelerator using BEAMnrc code and DOSXYZnrc code

The Monte Carlo model for the photon-beam output from the Varian Clinac 2100 linear accelerator was validated to compare the calculated to measured PDD and beam dose profiles The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible. The Monte Carlo codes used in this work were the BEAMnrc code to simulate the photons beam and the DOSXYZnrc code to examinate the absorbed dose in the water phantom. We have calculated percentage depth dose (PDD) and beam profiles of the 6 MV photon beam for the 6 × 6 cm², 10 × 10 cm² and 15 × 15 cm² field sizes. We have used the gamma index technique for the quantitative evaluation to compare the measured and calculated distributions. Good agreement was found between calculated PDD and beam profile compared to measured data. The comparison was evaluated using the gamma index method and the criterions were 3% for dose difference and 3 mm for distance to agreement. The gamma index acceptance rate was more than 97% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. The Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc and DOSXYZnrc codes package.     

Determination of mass attenuation coefficient of low-Z dosimetric materials

The mass attenuation coefficients of some low-Z dosimetric materials with potential applications in dosimetry, medical and radiation protection have been investigated using the Monte Carlo simulation code Monte Carlo N-Particle (MCNP). Appreciable variations are noted for the mass attenuation coefficient by changing the photon energy. The MCNP-simulated parameters are compared with the experimental data wherever possible and theoretical values through the WinXcom program. The simulated results obtained by MCNP generally agree well with the experiment and WinXcom predictions for various low-Z dosimetric and tissue substitute materials. In addition, the mass attenuation coefficients around the k-edges for low-Z dosimetric materials estimated from the MCNP code agree very well with WinXcom prediction. Finally, the results indicate that this simulation process can be followed to determine the interaction parameters of gamma rays in such low-Z materials for which there are no satisfactory experimental values available.     

Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams

Monte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.     

中子管非中子产物性质

采用两只经过标定的ST401闪烁探测器,测量了脉冲中子管的中子产额,在其中一只探测器前端增加铅板屏蔽,1 cm的铅屏蔽使探测器输出减少了18.20%,在加速器中子源上进行的类似实验表明,0.5 cm的铅使探测器的输出减少了2.90%。对两个中子源上测量的情况进行了蒙特卡罗模拟,加速器实验与模拟符合较好,脉冲中子管实验差别较大。对实验和模拟的情况进行了分析,结果表明:中子管除产生中子外,还会产生数量较多的轫致辐射X射线,这些X射线对准确测量中子管中子产额将造成不良影响。     

倒易方法在计算直线加速器中放射性问题的应用

本文应用倒易方法计算了直线加速器中的放射性问题。实际计算表明,该方法的结果比其它方法都要好,它有利于加速M——C方法的计算,从而大大节省了计算机时间。     

Analysis of radiation environmental safety for China's Spallation Neutron Source(CSNS)

The China Spallation Neutron Source(CSNS)is going to be located in Dalang Town,Dongguan City in the Guangdong Province.In this paper we report the results of the parameters related with environment safety based on experiential calculations and Monte Carlo simulations.The main project of the accelerator is an under ground construction.On top there is a 0.5 m concrete and 5.0 m soil covering for shielding,which can reduce the dose out of the tunnel's top down to 0.2 μSv/h.For the residents on the boundary of the CSNS,the dose produced by skyshine,which is caused by the penetrated radiation leaking from the top of the accelerator,is no more than 0.68 μSv/a.When CSNS is operating normally,the maximal annual effective dose due to the emission of gas from the tunnel is 2.40×10-3 mSv/a to the public adult,and 2.29×10-3 mSv/a to a child,both values are two orders of magnitude less than the limiting value for control and management.CSNS may give rise to an activation of the soil and groundwater in the nearest tunnels,where the main productions are 3H,7Be,22Na,54Mn,etc.But the specific activity is less than the exempt specific activity in the national standard GB13376-92.So it is safe to say that the environmental impact caused by the activation of soil and groundwater is insignificant.To sum up,for CSNS,as a powerful neutron source device,driven by a high-energy high-current proton accelerator,a lot of potential factors affecting the environment exist.However,as long as effective shieldings for protection are adopted and strict rules are drafted,the environmental impact can be kept under control within the limits of the national standard.     

用全带Monte Carlo方法模拟纤锌矿相GaN和ZnO材料的电子输运特性

报告了用全带MonteCarlo方法模拟纤锌矿相GaN和ZnO材料电子输运特性的结果.模拟所用的能带结构数据是用经验赝势法算得的.通过模拟得到了两种材料的平均漂移速度和平均能量与电场强度关系曲线,求得了电子迁移率.并且将两种材料的结果以及文献报道的GaAs的模拟结果进行了比较 关键词： 蒙特卡罗模拟 GaN ZnO 输运特性 能带结构     

基于蒙特卡罗方法的XHA600D医用电子直线加速器的入射电子束参数研究

使用蒙特卡罗方法研究入射电子束参数对XHA600D医用电子直线加速器产生的剂量分布的影响,并确定优化的入射电子束参数。根据厂商提供的XHA600D加速器治疗头的几何、材料参数,使用蒙特卡罗程序EGSnrc对不同的入射电子束参数进行模拟并记录其在水模体中产生的剂量分布,将模拟结果与测量结果进行比较。模拟的入射电子束参数包括平均能量、径向强度分布、角度展宽和能量展宽;实验测量数据包括4 cm×4 cm、10 cm×10 cm、30 cm×30 cm射野条件下的百分深度剂量与离轴剂量。结果表明当入射电子束的平均能量为6 MeV、径向强度的半高宽(Full Width at Half Maximum, FWHM)为0.25 cm、角度展宽为0.15°时,模拟结果和测量结果吻合非常好。这些参数可以作为建立适用于XHA600D加速器的TPS(Treatment Planning System)剂量计算模型的基础参数。     

Experimental and Monte Carlo absolute characterization of a medical electron beam using a magnetic spectrometer

The knowledge of the absolute energy distributions of the particles emitted from a clinical accelerator is important for the evaluation of Monte Carlo simulations developed for treatment planning. In this paper, an original approach is presented which allows to measure the absolute energy distribution of the electron beam delivered by a Varian 21Ex medical accelerator. The electron beam was characterized at the isocenter with calibrated image plates covering the exit window of a magnetic spectrometer. The characteristics of the electron beam emitted from an effective source have been inferred from the measurements using the Geant4 Monte Carlo code. The contribution of direct electrons to the absolute depth-dose curve in a water phantom is estimated.