基于2.5 MeV质子加速器BNCT装置的中子慢化材料性能模拟研究

基于加速器中子源的硼中子俘获治疗(Boron Neutron Capture Therapy, BNCT)是新一代的放射治疗方法,束流整形体(Beam Shaping Assembly, BSA)作为硼中子俘获治疗装置的重要组成部分,其作用是将中子源中的快中子束流慢化至超热中子能区(0.5 eV~10 keV),并尽可能减少快中子、热中子以及$\gamma $射线的成分,使其满足BNCT用于治疗的中子束要求。本工作基于蒙特卡罗软件包Geant4(Geometry and Tracking),以2.5 MeV,10 mA质子流强的7Li(p, n)7Be中子源为对象,研究分析了AlF₃ 、Fluental、Al₂O₃、Al作为慢化体材料时,不同的厚度对束流出口处的超热中子注量率、超热中子注量与热中子注量比值、快中子成分、$ \gamma $成分所产生的影响。计算表明,当选用厚度为25 cm的AlF₃作为慢化体材料时,经过整形慢化后的超热中子束的束流参数,均满足国际原子能机构(International Atomic Energy Agency, IAEA)的中子束流参数推荐值。     

BNCT中子源用RFQ加速器

 分析了加速器作为硼中子俘获治疗(BNCT)中子源的优势,提出以射频四极场（RFQ）加速器作为BNCT用中子源的首选机型。对该RFQ的参数进行了选择,利用质子轰击锂靶近阈反应产生的前冲中子束能散低、散角小的优势,设定RFQ最终能量为1.9 MeV。采用“匹配均温”设计方法进行了此强流质子RFQ的束流动力学设计,并对设计方案进行了传输模拟,在入口归一化均方根发射度为0.25 mm·mrad、流强为100 mA时,束流传输效率为99.3％。选择合适厚度的锂靶,经过整形即可得到满足BNCT治疗需要的中子束。     

硼中子俘获治疗的蒙特卡罗方法模拟

用通用蒙特卡罗程序MCNP模拟了粒子在人脑中的输运过程. 吸收剂量率主要来自以下四个反应:¹⁰B(n,α)⁷Li,¹⁴N(n,p)¹⁴C,¹H(n,γ)²D,快中子弹性散射反应.对肿瘤区的贡献主要来自硼中子吸收反应.结果表明,超热中子比热中子适合于深肿瘤的治疗,而热中子对浅肿瘤的治疗有优越性,比如皮肤癌.同确定论方法的结果相比,蒙特卡罗方法不失为一种模拟中子俘获治疗的好工具.     

用于硼中子俘获治疗的FFAG加速器Lattice设计

研究设计了一种用于硼中子俘获治疗(BNCT)的紧凑型固定磁场交变梯度(FFAG)加速器。首先采用FFAG加速器磁铁的线性简化模型,计算确定了加速器的基本结构参数;再用WIN.AGILE程序进行了磁聚焦结构的设计和优化,得到了Beta函数、色散函数、包络函数及工作点等;采用MAD程序对设计结果进行了复算,并对两种程序的计算结果作了进一步的对比分析;最后采用ZGOUBI程序进行非线性磁场下粒子追踪。FFAG加速器的超周期为6,能量为11 MeV,场指数k为1.9,周长为11.1795 m,结构紧凑。分析结果表明,优化设计的FFAG加速器达到了预期的设计目标。     

基于3.5 MeV射频四极质子加速器硼中子俘获治疗装置的束流整形体设计

在硼中子俘获治疗(BNCT)装置中,束流整形体(BSA)的作用是将中子源产生的快中子束流慢化至超热中子能区(0.5 eVE10 keV),并尽可能减弱快中子、热中子和γ射线的成分,同时保证中子的方向性,其设计与优化是BNCT装置设计工作的核心内容之一.本文采用3.5 MeV,10 mA的质子束轰击锂靶,由核反应~7Li(p,n)~7Be产生的中子为源项,针对BSA的慢化体材料和结构、γ屏蔽层和热中子吸收层的厚度等参数进行蒙特卡罗模拟设计与优化.研究发现,采用Fluental和LiF两种慢化材料间隔2 cm层状堆叠的三明治BSA构型,在保证快中子剂量成分(D_f/φ_(epi)),γ剂量成分(D_γ/φ_(epi))和热中子比例φ_(th)/φ_(epi)满足IAEA-TECDOC-1223报告推荐要求的同时,在BSA出口处超热中子注量率优于单独使用Fluental和单独使用LiF的BSA设计.BSA出口处修正的Synder人头几何模型中的剂量分布计算结果显示:上述三明治构型的深度剂量分布与单独使用Fluental材料构型的结果基本相当,优于单独使用LiF构型,表明Fluental和LiF层状堆叠的三明治BSA构型是一种可行的BSA结构.     

蒙特卡洛方法计算材料表面激发参数

电子非弹性散射平均自由程（IMFP）是用表面电子能谱进行表面化学定量分析时极为重要的一个参数,它可以用测量的弹性峰电子能谱分析以及蒙特卡洛模拟来确定.为了更加精确地确定电子非弹性散射平均自由程,必须对弹性峰电子能谱中的表面激发效应进行修正,通常使用介电响应理论方法计算得到的表面激发参数.然而,通过理论计算得到的表面激发参数不能包含电子在材料内部输运过程中弹性散射的影响,进而影响所测的电子非弹性散射平均自由程的准确度.在这个工作中,我们采用蒙特卡洛方法来确定包含弹性散射效应时的表面激发参数.所得到的表面激发参数在不同能量、角度情况下,尤其是在弹性散射效应显著的60°以上的大角度入射、出射情况下,都与实验测量值符合得非常好.基于这些新确定的表面激发参数,可以在弹性峰电子能谱测量中获得更为准确的电子非弹性散射平均自由程数据.     

硼中子俘获治疗中的含硼-10药物分布及浓度在体测量方法研究进展

硼中子俘获治疗(boron neutron capture therapy,BNCT)是一种结合含硼-10靶向药物和重离子肿瘤治疗的二元精确放射治疗方法,但经过近70年的发展,BNCT仍然未能真正进入临床应用.含硼-10药物在体内的浓度分布测量方法不能满足临床需求,影响治疗的效果和安全性,是目前BNCT亟待解决的核心问题之一.本文对目前含硼-10药物浓度分布测量方法进行综述,包括已经用于临床的有创估算方法及在研的单光子发射断层成像方法、正电子发射断层扫描方法及核磁共振方法等,分析各种方案的优势与局限性.并根据硼-10元素旋磁比低及磁共振横向弛豫时间极短的特点,从理论上简要分析了基于超短回波时间磁共振成像的硼-10体内分布定量测量方法的可行性.     

基于蒙特卡罗方法的α粒子细胞S值计算

在放射性免疫治疗以及硼中子俘获治疗（BNCT）等放射性治疗过程中,粒子通过与人体组织材料相互作用产生次级电子将能量传递给人体组织,放射性核素在细胞尺度分布的不均匀性将严重影响靶区剂量分布。为深入了解放射性核素在细胞中不同位置分布对靶区剂量影响,采用基于历史凝聚算法的Monte Carlo 工具包Geant4编写了细胞S值计算程序。计算了2种细胞尺寸,12种粒子能量,3种源分布方式的细胞S值,与医学内照射剂量（MIRD）委员会解析算法的计算结果进行对比,发现两者差异在1%以内。证明了Geant4在m尺度下细胞剂量计算的可行性,并对BNCT治疗过程中产生的粒子(1.47 MeV与1.78 MeV)的细胞S值进行计算。     

利用蒙特卡洛方法对钨酸铅长晶体光收集均匀性的研究

在电磁量能器中,晶体闪烁光收集沿长度方向的均匀性对能量分辨率有相当的影响.利用蒙特卡洛(MC)方法对钨酸铅(PbWO4或PWO)长晶体的光传导与光收集过程进行了模拟计算,着重讨论了晶体吸收长度,折射率等因素对光收集的影响.     

BNCT优化网格设计及相关算法研究

用MCNP蒙特卡罗程序模拟了硼中子俘获治疗(BNCT)3种国际基准网格模型, 并与 修正的Snyder椭球模型进行了比较. 在此基础上, 给出了一种保质量守恒、内存量少、易于产生输入文件的4种基本材料成分的BNCT网格模型. 计算结果表明, 在4mm网格下, 新模型可以达到基准模型的精度; 根据解析模型剂量随深度的变化规律, 研究构造了多网格组合模型, 在重要区域计算精度不损失的条件下, 计算时间大大缩短. 最后研究给出了一个既保证精度、又在可接受的时间内完成剂量计算的模型、样本数和相应的算法, 它基本上满足临床BNCT的要求.     

α粒子对人体癌细胞损伤过程的蒙特卡罗模拟及分析

在重离子束治癌以及硼中子俘获治疗(BNCT)中，需要了解载能离子注入人体组织及人体细胞中引起的相关生物效应．为了更好地了解单个离子穿过单个细胞引起的生物学效应(即单粒子效应)，理解在细胞或亚细胞水平上的微观剂量分布，文章以BNCT为例，采用蒙特卡罗程序模拟α粒子在人体细胞中的输运过程，通过研究粒子的射程分布、径迹结构、能量沉积及靶损伤情况等，分析注入粒子对人体细胞的损伤效应．     

Influence of thyroid volume reduction on absorbed dose in 131I therapy studied by using Geant4 Monte Carlo simulation

A simulation study has been performed to quantify the effect of volume reduction on the thyroid absorbed dose per decay and to investigate the variation of energy deposition per decay due to β- and γ-activity of 131I with volume/mass of thyroid, for water, ICRP- and ICRU-soft tissue taken as thyroid material. A Monte Carlo model of the thyroid, in the Geant4 radiation transport simulation toolkit was constructed to compute the β- and γ-absorbed dose in the simulated thyroid phantom for various values of its volume. The effect of the size and shape of the thyroid on energy deposition per decay has also been studied by using spherical, ellipsoidal and cylindrical models for the thyroid and varying its volume in 1-25 cm3 range. The relative differences of Geant4 results for different models with each other and MCNP results lie well below 1.870%. The maximum relative difference among the Geant4 estimated results for water with ICRP and ICRU soft tissues is not more than 0.225%. S-values for ellipsoidal, spherical and cylindrical thyroid models were estimated and the relative difference with published results lies within 3.095%. The absorbed fraction values for beta particles show a good agreement with published values within 2.105% deviation. The Geant4 based simulation results of absorbed fractions for gammas again show a good agreement with the corresponding MCNP and EGS4 results （±6.667%） but have 29.032% higher values than that of MIRD calculated values. Consistent with previous studies, the reduction of the thyroid volume is found to have a substantial effect on the absorbed dose. Geant4 simulations confirm dose dependence on the volume/mass of thyroid in agreement with MCNP and EGS4 computed values but are substantially different from MIRD8 data. Therefore, inclusion of size/mass dependence is indicated for 131I radiotherapy of the thyroid.     

Influence of thyroid volume reduction on absorbed dose in ~(131)I therapy studied by using Geant4 Monte Carlo simulation

A simulation study has been performed to quantify the effect of volume reduction on the thyroid absorbed dose per decay and to investigate the variation of energy deposition per decay due to β- and γ-activity of ~(131)I with volume/mass of thyroid,for water,ICRP- and ICRU-soft tissue taken as thyroid material.A Monte Carlo model of the thyroid,in the Geant4 radiation transport simulation toolkit was constructed to compute the β- and γ-absorbed dose in the simulated thyroid phantom for various values of its volume.The effect of the size and shape of the thyroid on energy deposition per decay has also been studied by using spherical,ellipsoidal and cylindrical models for the thyroid and varying its volume in 1-25 cm~3 range.The relative differences of Geant4 results for different models with each other and MCNP results he well below 1.870%.The maximum relative difference among the Geant4estimated results for water with ICRP and ICRU soft tissues is not more than 0.225%.S-values for ellipsoidal,spherical and cylindrical thyroid models were estimated and the relative difference with published results lies within3.095%.The absorbed fraction values for beta particles show a good agreement with published values within 2.105%deviation.The Geant4 based simulation results of absorbed fractions for gammas again show a good agreement with the corresponding MCNP and EGS4 results(±6.667%) but have 29.032%higher values than that of MIRD calculated values.Consistent with previous studies,the reduction of the thyroid volume is found to have a substantial effect on the absorbed dose.Geant4 simulations confirm dose dependence on the volume/mass of thyroid in agreement with MCNP and EGS4 computed values but are substantially diiferent from MIRD8 data.Therefore,inclusion of size/mass dependence is indicated for ~(131)I radiotherapy of the thyroid.     

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。     

241Am-Be中子参考辐射的蒙特卡罗模拟研究

研究用于校准场所中子剂量监测仪表的241Am-Be中子参考辐射场计量特性。采用蒙特卡罗方法模拟了空气自由中子参考辐射(FRNR),GB/T 14055规定的最小尺寸中子参考辐射(SRNR)和实际中子参考辐射(ARNR)中不同检验点处中子周围剂量当量率、散射中子占比和能谱分布特征。研究结果表明,空气对FRNR中的剂量率和能谱分布影响小,近似为理想中子参考辐射;采用5%含硼聚乙烯作屏蔽的最小尺寸SRNR可减少热中子,降低散射中子占比,影锥法不适用于小尺寸中子参考辐射中对散射中子的修正;ARNR中的散射中子更少、占比更低,影锥法所得散射中子占比与理论值基本一致。     

Monte Carlo simulation of the property of a scintillation bar in the multi-neutron correlation spectrometer

To perform a kinematically complete measurement of the dissociation reaction for neutron-rich nuclei, a multi-neutron correlation spectrometer is proposed at Peking University. A Monte Carlo simulation code based on GEANT4 is developed for a single scintillation bar which processes not only the energy deposition but also the light propagation in the scintillator and the light collection and conversion to signal at the end of the bar in a realistic way. The simulating method is described in detail in this paper, and the timing and position resolutions and detector efficiency are studied based on the simulation and compared with the experimental results. A new method of crosstalk rejection has been demonstrated to be important for the design of the whole spectrometer.     

Monte Carlo simulation of the property of a scintillation bar in the multi-neutron correlation spectrometer

To perform a kinematically complete measurement of the dissociation reaction for neutron-rich nuclei, a multi-neutron correlation spectrometer is proposed at Peking University.A Monte Carlo simulation code based on GEANT4 is developed for a single scintillation bar which processes not only the energy deposition but also the light propagation in the scintillator and the light collection and conversion to signal at the end of the bar in a realistic way. The simulating method is described in detail in this paper, and the timing and position resolutions and detector efficiency are studied based on the simulation and compared with the experimental results.A new method of crosstalk rejection has been demonstrated to be important for the design of the whole spectrometer.     

血液中Na活化放射性测量的蒙特卡罗模拟

针对超临界事故中人体受到中子辐照后感生的24Na活度测量,采用MCNP软件建立蒙特卡罗模拟模型,分别模拟不同类型NaI探测器对24Na衰变的两条射线全能峰的探测效率和塑料闪烁体探测器对24Na衰变的射线总计数的探测器效率。模拟结果表明：井型NaI探测器与圆柱型相比,24Na衰变的1.38 MeV和2.76 MeV 射线全能峰探测效率分别提高了4.30倍和4.11倍;塑料闪烁体探测器对24Na衰变的射线的探测效率是NaI探测器对24Na 射线的探测效率的1.72倍;同时粗略计算了探测器计数与人体所受中子辐照剂量之间的关系。     

血液中Na活化放射性测量的蒙特卡罗模拟

针对超临界事故中人体受到中子辐照后感生的24Na活度测量,采用MCNP软件建立蒙特卡罗模拟模型,分别模拟不同类型NaI探测器对24Na衰变的两条射线全能峰的探测效率和塑料闪烁体探测器对24Na衰变的射线总计数的探测器效率。模拟结果表明:井型NaI探测器与圆柱型相比,24Na衰变的1.38 MeV和2.76 MeV 射线全能峰探测效率分别提高了4.30倍和4.11倍;塑料闪烁体探测器对24Na衰变的射线的探测效率是NaI探测器对24Na 射线的探测效率的1.72倍;同时粗略计算了探测器计数与人体所受中子辐照剂量之间的关系。