碳化硅与硅探测器辐射探测性能比较

碳化硅(SiC)材料因其禁带宽度大、晶体原子离位能高等物理特性,而被视为制作耐高温和抗辐射器件极具潜力的宽带隙半导体材料。本文采用Geant4模拟得到了30μm厚的SiC和Si材料对不同能量的电子、质子、α粒子以及X射线的响应,并对SiC和Si探测器器件的I-V特性和能谱测量结果进行了比较。仿真及试验结果证明,SiC粒子阻挡本领及X射线探测效率与Si探测器相当,SiC与Si探测器对带电粒子的能谱分辨率也没有明显差别。     

Simulation study of the dose and energy responses of FNTD personal neutron dosimetry

The objective of this work was to use the Geant4 toolkit to perform simulation studies on the personal dose response of fluorescent nuclear track detectors(FNTDs).The entire structure of the FNTD response can be designed,and the detector's energy and dose responses can be optimized in a broad energy range(0.01 eV-20 MeV). In general, the detectors used ~6LiF and CH_2 converters that have high energy and high dose response at neutron energies lower than 10 eV and greater than 1 MeV, respectively. The method of least squares was used to optimize the dose response of H*(10) and the energy response corresponding to Rtotal. The values of the optimized response of H*(10) lie between 0.8 and 1.4, corresponding to the energy ranges 0.01 eV-70 keV and 4-14 MeV, respectively. This occupies nearly eight out of the nine orders of the total energy range. Even though the optimized response of Rtotal is constrained between 0.89 and 1.1 in the energy range of 0.01 eV-20 MeV, it is suitable for obtaining the broad neutron spectrum of fluence with good accuracy.     

Ionization and excitation collision processes of electrons in liquid water

This paper reports on Monte Carlo simulations of electrons in liquid water using a set of electron collision cross-sections constructed with data published recently. The track history of electrons having initial energy ranged from 1 keV to 10 keV is investigated looking at the ionization and excitation processes. The results show that the ratio of the ionization and excitation events per track history is unique independent of the initial electron energy above a couple of 100 eV and these inelastic processes occur with low energy electrons frequently below 100 eV. In particular, the excitation processes are dominated by the electrons below 50 eV. Flight distance distributions between the inelastic collisions are also discussed.     

Total cross sections for ionizing processes induced by proton impact on molecules of biological interest: A classical trajectory Monte Carlo approach

In the current work, we present a study of ionizing interactions between protons and molecular targets of biological interest like water vapour and DNA bases. Total cross sections for single and multiple ionizing processes are calculated in the independent electron model and compared to existing theoretical and experimental results for impact energies ranging from 10 keV/amu to 10 MeV/amu. The theoretical approach combines some characteristics of the classical trajectory Monte Carlo method with the classical over-barrier framework. In this “mixed” approach, all the particles are described in a classical way by assuming that the target electrons are involved in the collision only when their binding energy is greater than the maximum of the potential energy of the system projectile-target. We test our theoretical approach on the water molecule and the obtained results are compared to a large set of data and a reasonable agreement is generally observed specially for impact energies greater than 100 keV, except for the double ionization process for which large discrepancies are reported. Considering the DNA bases, the obtained results are given without any comparison since the literature is till now very poor in terms of cross section measurements.     

基于Geant4-DNA工具包的单能电子微剂量学模拟研究

应用Geant4－DNA工具包分析和评估了不同物理过程影响因素对低能电子在液态水中的影响,为建立放射治疗与辐射防护所关心的微剂量学基本数据库提供理论支撑。在最新版本的Geant4中,Geant4-DNA工具包中共有7个物理模型可模拟电子在液态水中的输运。根据不同模型的特点,本文选择其中5个适用的模型来模拟单能入射电子(0.1～20 keV)在液态水中的输运过程;比较各模型记录的径迹结构具体信息,包括相互作用过程总次数、电离和激发次数及相应沉积能量占比等;分析Geant4-DNA选项模型、抽样位点小球半径及相互作用过程等因素对线能均值的影响。模拟结果显示:“option0”与“option2” 之间、“option4”与“option5”之间的模拟结果基本吻合;由于各个模型相互作用截面的不同,“option2”、“option4”和“option6”的径迹信息及线能均值均存在差异;模型对线能均值的影响随着抽样位点小球半径的增大而减小。本工作通过设置不同输运控制条件较全面地比较了Geant4-DNA工具包中的不同选项模型,对用户根据需要选择合适的模型模拟单能电子有帮助和指导作用,为完善电子在液态水中的微剂量学数据库并用于评估电离辐射在微观尺度的生物学效应提供依据。     

Secondary electron emission from a thin carbon foil induced by equi-velocity protons and electrons of 0.5-3.5 MeV/u

The statistical distributions of the number of simultaneously forward-emitted secondary electrons (SEs) from a thin carbon foil have been measured with H⁰ and H⁺ projectiles of 0.5-3.5 MeV in coincidence with the foil-transmitted protons, irrespective of projectile incident charge state. From these results, the probabilities of the simultaneous emission of n electrons and the SE yields by loss electrons have been evaluated. At energies of 2.5-3.5 MeV, the obtained emission probabilities and the SE yields induced by loss electrons are larger than the corresponding ones by protons of the same velocity. Furthermore, the material parameter is found to be almost constant and is slightly larger than that for the protons. On the other hand, the forward SE yields by loss electrons is decreasing to zero with decreasing the incident energy. This behavior is consistent with the range-energy relation of electrons in carbon.     

高能~(56)Fe离子入射屏蔽材料的次级粒子模拟分析

空间辐射环境下的高能重离子入射屏蔽材料会产生大量次级粒子,为研究屏蔽材料产生的次级粒子对太空舱内辐射环境的影响,本文使用基于蒙特卡罗方法的Geant4软件模拟空间高能~(56)Fe离子入射铝、碳、聚乙烯、水4种屏蔽材料,分析透射屏蔽体的初级粒子及由屏蔽材料产生的次级电子、次级中子、次级质子和次级γ的能谱以及水吸收体中的能量沉积和深度剂量分布。分析产生的次级重粒子类型和能量,比较4种屏蔽材料对高能Fe离子的屏蔽性能。结果表明,聚乙烯材料对高能重离子的屏蔽性能最好,但同时产生的次级重粒子的能量最大,约为铝材料产生次级重粒子能量的4倍。屏蔽体产生所有次级粒子中,次级质子和原子序数为22-26的次级重粒子贡献最大。     

γ全吸收型探测装置中子束流监视器的Geant4模拟

利用Geant4模拟了γ全吸收型探测装置中用于中子束流监视的锂玻璃探测器的相对探测效率,给出了锂玻璃探测器在10keV～1MeV区间的中子探测效率,并对影响探测效率的相关因素进行了分析,讨论了弹性散射引起的时间滞后对TOF(time-of-flight)测量的影响和不同厚度锂玻璃探测器的n-γ分辨效果。在未来使用锂玻璃探测器作为keV中子源监视器时,这项工作将为更好地理解相关中子飞行时间谱和能谱提供依据。     

Proton elastic scattering and proton induced γ-ray emission cross-sections on Na from 2 to 5 MeV

Differential cross-sections for proton elastic scattering on sodium and for γ-ray emission from the reactions ²³Na(p,p′γ)²³Na (E_γ = 440 keV and E_γ = 1636 keV) and ²³Na(p,α′γ)²⁰Ne (E_γ = 1634 keV) were measured for proton energies from 2.2 to 5.2 MeV using a 63 μg/cm² NaBr target evaporated on a self-supporting thin C film.The γ-rays were detected by a 38% relative efficiency Ge detector placed at an angle of 135° with respect to the beam direction, while the backscattered protons were collected by a Si surface barrier detector placed at a scattering angle of 150°. Absolute differential cross-sections were obtained with an overall uncertainty estimated to be better than ±6.0% for elastic scattering and ±12% for γ-ray emission, at all the beam energies.To provide a convincing test of the overall validity of the measured elastic scattering cross-section, thick target benchmark experiments at several proton energies are presented.     

Charge evolution and energy loss associated with electron transmission through a macroscopic single glass capillary

Charge (time) evolution and the angular dependence of incident electrons in the range 300-1030 eV through a single macroscopic glass capillary was studied. Charge measurements were done at a sample tilt angle of ψ = 2? for observation angles θ = 0? and 0.5? (both ψ and θ were measured with respect to the incident beam direction) at incident energies of 520.7 and 824.5 eV using a parallel-plate spectrometer. After equilibrium of transmission, electrons had lower average centroid (mean) energies than the respective primary beam values. Centroid energies of transmitted electrons at the centroid of the angular distribution (where the observation angle θ is nearly equal to tilt angle of the sample ψ) were found to decrease exponentially with increasing sample tilt angles for all the measured electron energies. This energy loss is attributed to inelastic scattering of electrons with the inner wall of the sample close to the capillary entrance. Furthermore, the centroid energies of the transmitted electron angular distributions at 520.7 eV were found to lose energy for angular positions away from the capillary axis (angular centroid position) for all tilt angles, indicating a higher degree of inelastic scattering at the edges of the angular distributions.     

Electronic stopping power of hydrogen in KCl at the stopping maximum and at very low energies

The electronic energy loss of hydrogen ions in KCl was investigated in a wide energy range. Thin films of KCl were evaporated on an Au/Si substrate. Rutherford Backscattering Spectrometry (RBS) was performed with protons and deuterons at energies from 30 to 400 keV/nucleon. At lower energies experiments were performed by Time-Of-Flight Low energy ion scattering (TOF-LEIS) again with proton and deuteron projectiles. Experimental results are compared to calculated/tabulated values for the electronic energy loss. Whereas at energies beyond the stopping maximum very good agreement is found, at lower ion energies discrepancies between experiment and calculations increase. At very low ion velocities the extrapolated stopping cross section ε predicts vanishing electronic energy loss at energies below 100 eV/nucleon.     

Measurements of the W values in argon,nitrogen, and methane for 0.93 to 5.3 MeV alpha particles

The W values in argon, nitrogen and methane have been measured for alpha particles with energies from 0.93 MeV to 5.3 MeV by gridded ionization chambers. The energies of alpha particles emitted from ²¹⁰Po (5.3 MeV) were reduced by passage through Al foils and Mylar films of various thicknesses. The mean energies of these “reduced” alpha particles were determined using a silicon detector calibrated with five natural alpha sources. The W values for the gases and energies studied are obtained by assuming a W value of 26.31 eV in pure argon for 5.3 MeV alpha particles. The W value in argon is found to be slightly dependent on the energy of alpha particles. In nitrogen and methane, the W values for an alpha energy of 0.93 MeV are about 5 and 8% larger, respectively, than for an alpha energy of 5.3 MeV.     

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.     

Differential cross sections for ejection of electrons from helium by protons

A compilation is presented of the results of several authors on experimental cross sections for ejection of electrons from helium gas by protons of 5 keV to 5 MeV energy. The data are given as a function of the angle and energy of ejection. Angles from 10° to 160° are surveyed and electron energies range from 1 to 8600 eV. The doubly differential cross sections are integrated in various ways to obtain cross sections differential only in energy or angle, total electron production cross sections, and mean energies of ejection.     

X-ray production cross-sections measurements for high-energy alpha particle beams: New dedicated set-up and first results with aluminum

The “IPNAS” laboratory, in collaboration with the “Centre Européen d’Archéométrie” is partly focused on material analysis by means of IBA techniques: PIXE, PIGE and RBS. A new transport beam line has been developed at our CGR-520 MeV cyclotron to analyze Cultural Heritage objects using these techniques. This facility allows us to produce proton and alpha particle beams with energies up to 20 MeV. A vacuum chamber dedicated to X-ray production and Non-Rutherford cross-section measurements has been recently constructed. After determination of the chamber’s geometry for X-ray detection using thin foils of several elements (11 ? Z ? 82) and 3 MeV proton beams, the measurement of the X-ray production cross-sections in the 6-12 MeV energy range has started using alpha particle beams on light element targets. These experiments contribute to the filling a serious lack of experimental values for alpha particles of this particular energy range in databases. The recent decision to focus our work on the alpha particle interaction with light elements was taken because of the high interest of the low Z elements in the field of archaeometry.     

Experimental bremsstrahlung yields for MeV proton bombardment of beryllium and carbon

Experimental bremsstrahlung yields for 2, 3 and 4 MeV protons on thin beryllium and carbon targets have been measured. The yields have been corrected for detector efficiency, self-absorption in the target and fitted to 9th order polynomials over the X-ray energy range 1-10 keV for easy comparison with theoretical calculations.     

Stopping cross-sections of liquid ethanol for swift He ions

Energy loss spectra of 4.0 MeV helium ions after penetrating through liquid phase ethanol have been measured at scattering angles from 10 to 50 mrad. Experimental data were fitted with Monte Carlo simulations using a GEANT4 toolkit, and stopping cross-sections were deduced in an energy range from 1.9 to 4 MeV. Present results are in fairly good agreement with previous experimental data and SRIM2008 values. By applying the Bragg’s additivity rule, the mean excitation energy was deduced as I = 62.0 ± 1.8 eV for liquid ethanol. The present I-value is compared with other experimental and calculated results.     

A new mini gas ionization chamber for IBA applications

Novel prototypes of high resolution gas ionization chambers (GIC) were designed with increased compactness and simplicity of the setup. They have no Frisch-grid and a simple anode wire. Under certain operating conditions these mini detectors have an energy resolution comparable with that of state-of-the-art GICs of much higher complexity. They can be operated both under vacuum and atmospheric pressure. First measurements were made with protons in the energy range of 0.3-1.0 MeV. For protons at 0.3 MeV an energy resolution of about 12 keV was achieved. With a 72 MeV ¹²⁹Xe beam a relative resolution of 1.4% was obtained. Due to their versatility and reduced size the detectors can easily be applied in the field of ion beam analysis (IBA) and accelerator mass spectrometry (AMS). Since they are almost completely insensitive to radiation damage they are especially suited for use in high fluence applications such as scanning transmission ion microscopy (STIM). A comparison of the radiation hardness of the mini GIC with a Si PIN diode was therefore performed. The GIC showed no peak shift or change in energy resolution at all after collecting 10¹⁵ protons per cm² while the performance of the Si detector clearly started to degrade at 10¹² particles per cm².     

质子在氮化镓中产生位移损伤的Geant4模拟

材料受到辐照时产生的位移损伤会导致其微观结构发生变化,从而使其某些使用性能退化,影响其使用效率,减短其使用寿命。利用Geant4模拟了质子在氮化镓中的输运过程,计算了1、10、100、500 MeV能量质子入射氮化镓材料产生的初级撞出原子的种类、能量信息及离位原子数。获得了10 MeV质子产生的位移缺陷分布;计算了4种能量质子入射氮化镓材料产生的非电离能量损失(NIEL);研究了质子产生位移损伤过程的影响要素。研究发现,入射质子能量对其在材料中产生的初级撞出原子的种类、能量、离位原子数等信息有着非常大的影响;单位厚度所沉积NIEL随着入射质子能量的增大而减小;10 MeV质子入射氮化镓所产生的离位原子数随入射深度的增加而增加,但在超出其射程范围以外有一巨大回落;能量并不是影响质子与氮化镓靶材料相互作用的唯一因素。