4～400 MeV宇宙线质子对近月空间轨道γ能谱中湮灭辐射的贡献

为进一步明确月球表面湮灭辐射的主要来源和影响机理，构建了轨道γ能谱仪中湮灭辐射特征峰定量分析模型，采用GEANT4模拟不同能量质子轰击月岩诱发γ射线微分能谱，并以“嫦娥一号”高能粒子探测器(Chang’e-1 High-energy Particle Detector,CE1-HPD)数据作为输入项，计算了宇宙射线中4～400 MeV质子在月表5种典型岩石中诱发湮灭辐射的特征峰信息。在对“嫦娥一号”γ能谱仪(Chang’e-1 Gamma-ray Spectrometer,CE1-GRS)获取的0.511 MeV特征峰进行本底扣除和天然放射性核素影响剥离后，与4～400 MeV质子诱发湮灭辐射结果进行比较。研究结果表明：级联簇射产生湮灭辐射的概率与入射质子能量呈正相关，在4～400 MeV能量范围内，典型月岩的成分差异对诱发湮灭辐射的影响不明显。虽然该能量段质子的注量率较高，但由于其能量较低，无法有效地通过级联簇射形成正电子，最终导致湮灭辐射贡献率较低，仅为(1.97±0.66)×10^-4。     

基于HI-13串列加速器的中子源设计

正利用中国原子能科学研究院的HI-13串列加速器,使用脉冲化的质子束,通过~7Li(p,n)~7Be反应,建立keV能区的中子源。HI-13串列加速器能提供的主频为6 MHz,脉冲宽度2ns,平均束流强度约1μA,稳定运行的最低端电压是3 MV。由于~7Li(p,n)~7Be反应阈值为1.881 MeV,在阈上附近截面最大,因此需使用降能片将入射质子能量降低至2 MeV以下。靶管的设计如图1所示,根据STRIM程序的计算结果,300μm厚度的Al片作为降能片能满足实验的要求。同时,为提高中子产额,采用厚Li靶(厚度1mm)的设计方案。     

100 MeV回旋加速器引出质子束的能量测试

正串列加速器升级工程100 MeV强流质子回旋加速器(CYCIAE-100)于2014年首次出束以后,目前已经进入到运行阶段。CYCIAE-100通过剥离引出的方式,引出的质子能量范围为75～100 MeV。2017年,工程部组织专家对CYCIAE-100的引出能量进行了现场测试。不同能量的质子在水中的布拉格峰(Bragg峰)位置是固定的,即通过对质子在水中不同深度的吸收剂量来确定布拉格峰位置,从而确定该能量质子束     

Monte Carlo模拟韧致辐射光子穿透闪烁体的能量和通量分布

在工业CT设计中,为了选择闪烁体的种类和几何尺寸,需要知道韧致辐射光子穿透闪烁体的能量和通量分布。根据Monte Carlo模拟光子输运过程的理论,模拟了2MeV、9MeV韧致辐射光子在NaI、CsI、CdWO4闪烁体内的输运过程,讨论了这类光子穿透闪烁体后光斑展宽问题和能量及通量的分布。研究表明,该方法可准确直观地定量分析韧致辐射光子在闪烁体内任意位置的能量和通量分布规律。     

用14MeV中子活化分析测定几种作物种子中粗蛋白质的含量

以14MeV中子轰击样品,产生~(14)N(N,2n)~(13)N核反应,测量β~ 衰变过程中放出的0.511MeV峰计数,即求得氮的含量。再用转换因子6.25,得到样品中粗蛋白质含量。 1.原理 快中子轰击样品后,由于样品中含有大量的碳、氢、氧元素,快中子在碳氢化合物中与氢核碰撞,产生的反冲质子与碳、氧产生~(13)C(p,n)~(13)N和~(16)O(p,α)~(13)N核反应,它们都产生~(13)N核素,干扰氮的测定。 我们采用草酸铵作为氮的标准样品,因为它具有与样品相同反冲质子引起的“本底”。     

低能质子的Bragg曲线测量

正本研究中,利用射程移位装置和剂量胶片相结合的测量方法,得到了22 MeV低能质子贯穿深度与吸收剂量之间的关系,再通过理论拟合得到了如图1所示的质子的Bragg曲线(虚线),并与理论计算得到的LET与吸收剂量之间的关系(实线)进行了对比。结果显示,当离子束照射注量为1.99×10~8 cm~(-2)时,相对剂量从1Gy到2.98Gy逐渐增大,然后达到最大剂量5Gy后迅速降低,测得的曲线表现出典型的Bragg曲线的特点。与理论计算的结果相比,Bragg峰区有明显的展宽,这可能是由于随     

北京正负电子对撞机(BEPCⅡ)束流注入区辐射场研究

北京正负电子对撞机(BEPCⅡ)正负电子束流注入阶段的束流损失,影响储存环束流注入区及防护区辐射场。本文利用束流损失监测系统(BLM系统)分析了束流注入阶段注入区束流损失的位置和束流损失率,并结合FLUKA软件模拟计算对撞实验模式下束流注入阶段注入区的辐射场。结果表明:注入阶段注入区及其下游束流损失明显;辐射场内粒子能谱情况是中子为宽能谱且各向同性,切割磁铁 (铁靶)出射蒸发谱峰窄,峰值约为0.9 MeV,直接发射谱不显著;真空管(铝靶)出射蒸发谱峰宽且直接发射谱显著,峰值分别为4 MeV和20 MeV;光子能谱峰窄且前向性明显,能量在5 MeV以下分布集中;中子与光子剂量率水平相当;注入阶段比非注入阶段剂量率高约2个数量级;对撞实验模式下,实验测量整个运行阶段储存环光子剂量率平均水平约为1 000 μSv/h,中子剂量率比光子剂量率低1个数量级。     

反冲质子望远镜效率计算中的相对论修正

反冲质子望远镜和伴随粒子法是绝对测量14 MeV单能快中子通量的两种独立的方法,在高压倍加器上利用T(d,n)~4He反应可以获得14 McV单能快中子。反冲质子望远镜是通过中子入射在聚乙烯辐射体上产生反冲质子,用记录反冲质子数来确定中子通量的,而伴随粒子法则是通过记录α粒子数来测定中子通量。随着14 MeV单能快中子的应用越     

快中子能谱在线测量的反冲质子望远镜响应模拟研究

快中子能谱是基于散裂中子源开展大气中子单粒子效应研究的关键输入参数,在线测量宽能区快中子能谱在近散裂靶位置面临飞行时间法不确定度大、中子通量高、本底干扰强等问题。设计了反冲质子望远镜(RPT)系统,利用Geant4模拟了20～200 MeV中子轰击不同厚度聚乙烯转换靶产生的反冲质子产额、角分布以及能谱,为优化探测系统设计提供了指导依据。通过模拟硅探测器与新型快响应CLLB闪烁体组成的二重符合RPT系统对入射中子的响应,分析了影响探测系统探测效率和能量分辨率的因素,确定了聚乙烯转换靶厚度为1 mm、符合质子探测器摆放角度为26.6°和探测器尺寸等重要参数,得到了RPT系统的中子响应函数矩阵,并计算了其探测效率达10-5,对高中子通量和复杂本底干扰环境下的快中子能谱在线测量具有指导意义和参考价值。     

能量到GeV的质子入射铅靶的散裂产物分布研究(英文)

由中高能质子入射靶引起的散裂碎片的分布关系到散裂靶经长期辐照后的放射性废物的累积。在量子分子动力学模型(QMD)中考虑剩余核的裂变过程(FISSION),利用QMD FISSION模型研究了322,660,759 MeV的质子入射铅靶的散裂产物的分布。计算结果很好地再现了实验测量值。     

Validation of recent Geant4 physics models for application in carbon ion therapy

Cancer treatment with energetic carbon ions has distinct advantages over proton or photon irradiation. In this paper we present a simulation model integrated into the Geant4 Monte Carlo toolkit (version 9.3) which enables the use of ICRU 73 stopping powers for ion transport calculations. For a few materials, revised ICRU 73 stopping power tables recently published by ICRU (P. Sigmund, A. Schinner, H. Paul, Errata and Addenda: ICRU Report 73 (Stopping of Ions Heavier than Helium), International Commission on Radiation Units and Measurements, 2009) were incorporated into Geant4, also covering media like water which are of importance in radiotherapeutical applications. We examine, with particular attention paid to the recent developments, the accuracy of current Geant4 models for simulating Bragg peak profiles of ¹²C ions incident on water and polyethylene targets. Simulated dose distributions are validated against experimental data available in the literature, where the focus is on beam energies relevant to ion therapy applications (90-400 MeV/u). A quantitative analysis is performed which addresses the precision of the Bragg peak position and proportional features of the dose distribution. It is shown that experimental peak positions can be reproduced within 0.2% of the particle range in the case of water, and within 0.9% in the case of polyethylene. The comparisons also demonstrate that the simulations accurately render the full width at half maximum (FWHM) of the measured Bragg peaks in water. For polyethylene slight deviations from experimental peak widths are partly attributed to systematic effects due to a simplified geometry model adopted in the simulation setup.     

Angular benchmark data for 6.13 MeV source photons of disc geometry penetrating lead,steel and concrete

The ¹⁶N radiation facility at the Universities Research Reactor provides a high-intensity source of 6.13 MeV photons of disc geometry. Measurements of angular photon flux emerging from lead, steel and concrete shields were made using a collimated scintillation detector. Pulse-height distributions were unfolded using the code radak to provide benchmark angular spectra suitable for calculational code evaluation. Scalar flux spectra were established by integration of angular fluxes and these values were compared with calculated values using the 1-D transport code pipe and the Monte Carlo code sam-ce.     

GATE simulation based feasibility studies of in-beam PET monitoring in 12C beam cancer therapy

In comparison with conventional radiotherapy techniques,12C beam therapy has its significant advantage in cancer treatment because the radiation dose are mostly concentrated near the Bragg peak region and damage to normal tissues along the beam path is thus greatly reduced.In-beam PET provides a way to monitor dose distribution inside human body since several kinds of positron-emitting nuclei are produced through the interaction between 12C beam and body matters.In this work,we study the quantitative relationship between the spatial location of the Bragg peak and the spatial distribution of positrons produced by positron-emitting nuclei.Monte Carlo package GATE is used to simulate the interactions between the incident 12C beam of different energies (337.5,270.0 and 195.0 MeV/u) and various target matters (water,muscle and spine bone).Several data post-processing operations are performed on the simulated positron-emitting nuclei distribution data to mimic the impacts of positron generation and finite spatial resolution of a typical PET imaging system.Simulation results are compared to published experimental data for verification.In all the simulation cases,we fred that 10C and 11C are two dominant positron-emitting nuclei,and there exists a significant correlation between the spatial distributions of deposited energy and positrons.Therefore,we conclude that it is possible to determine the location of Bragg peak with 1 mm accuracy using current PET imaging systems by detecting the falling edge of the positron distribution map in depth direction.     

Terrestrial Gamma-Radiation Field in Natural Environment

The energy and directional distributions of the photon flux in air l m above mono-energetic semi-infinite isotropic volume sources were calculated with Monte Carlo technique for 5 different γ-ray energies from 0.35 to 2.65 MeV. The energy and directional distributions of the flux from U, Th and K were obtained with use made of the results of the mono-energetic calculations. The results agreed well with previously measured data on an actual natural radiation field.     

Geant4不同物理模型对放疗质子束模拟的影响

Monte Carlo方法常用于质子束剂量模拟以优化质子束临床治疗,合适的物理模型选择关系到Monte Carlo模拟结果的可靠性及模拟效率。本工作选取9个可用于质子模拟的Geant4物理模型,计算比较了质子束在多种介质中的剂量及次级粒子产额。结果表明:标准电磁物理模型、低能电磁Penelope模型和Livermore模型虽在模拟质子整体剂量分布上可接受,但在微观上缺失重粒子生成。参数化驱动模型LHEP的模拟时间最短,但与QGSP相同,均不能产生复杂重离子。QGSP_BIC_EMY模型较其他模型明显低估Bragg峰-入口剂量比和绝对剂量。QGSP_BERT模型、QGSP_BIC模型和FTFP_BERT模型较适合放疗质子束模拟。     

Hydrogen Permeation through Incoloy-800

The poloidal distribution of the first wall 14 MeV neutron flux and the tritium breeding ratio in a Tokamak fusion reactor were calculated using Monte Carlo method. The poloidal distribution of the 14 MeV neutron flux in the first wall was found to be quite different from that of the primary incident flux. The tritium breeding ratio calculated by the Monte Carlo method became about 5% larger than the value obtained from S_N transport calculations.     

ST-451液体闪烁体对快中子的响应矩阵

本文在给出了较好的物理过程描述和实验结果后,应用蒙特卡罗方法,计算了快中子入射到ST-451液体闪烁体中产生的反冲质子响应矩阵。给出了0.25—30 MeV范围内(间隔0.25或0.5 MeV的响应矩阵和中子探测器效率。并且可以很方便地扩展到能区0.25—200 MeV范围。     

Unfolding of Ge-detector response by Monte Carlo simulation and applications in radiation protection

Response functions to monoenergetic gamma radiation of a portable high purity Ge-detector have been calculated by Monte Carlo simulation for 180 photon energies ranging from 20 keV to 1.5 MeV and used to unfold measured pulse height spectra. The dependence of response functions on scattering from housing and cryostat of the detector and on the incidence angle of the radiation is discussed. Measured pulse height spectra in a house exposed to a ⁶⁰Co source were unfolded and compared to photon fluence rate spectra obtained by a Monte Carlo simulation of the photon transport in this environment, finding a very good agreement.