光子能量沉积计算的一种新方法

光子沉积能广泛地应用于放射医疗和辐射防护领域。 MCNP程序中*F8功能统计的是进出网格的光子与电子能量差。 *F8只能采用真实网格, 计算效率较低。 由于光子的能量沉积都是通过次级电子来完成的, 对光子能量沉积的计算可转换为次级电子能量沉积的计算。 文中据此给出了一种新的光子能量沉积统计方法, 该方法只统计次级电子能量沉积, 且可以采用虚拟网格计数。 新方法若采用真实网格, 计算精度与效率与*F8完全一样; 若采用虚拟网格, 新方法计算精度略低, 但几何建模简单, 计算效率较高。 Energy deposition distribution is very important to study radiotherapy or radioprotection. The *F8 method of MCNP program counts the energy loss of photon and electron together. Only real grid is allowed to *F8, so its computation efficiency is low. This paper gives a new calculation method for energy deposition of photon. Because the energy deposition of photon is accomplished by secondary electron, only electron is counted in the new method. The real collision of the electron is counted by the new method, whereas *F8 only counts the information of particle across the interface. Moreover, virtual grid is allowed in the new method. With real grid, the new method has the same precision and efficiency as *F8. If virtual grid is adopted by the new method, although the precision is slightly down, the efficiency is greatly increased.     

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron‐hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through interactions between the electron and hole with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron‐hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near‐term future for both fundamental research and the development of working devices which exploit MEG.     

The temperature of the track

The energy deposition of swift charged particles penetrating solids is accompanied by such processes as particle (electron, atom, ion, molecular ion, photon …) emission and/or a change of the solid along the particle track. The energy, velocity and mass distribution of such secondary particles obtained from thin solids (such as carbon, polyhydrocarbon, isolators and conductors) penetrated by projectiles (e.g. Ar 1.8 MeV) was measured quantitatively. A number of direct and indirect production mechanisms contribute to the internal source of electrons and secondary ions. The analysis of the ejected radicals give information on the emission processes, the temperature, the charge and the time scale of energy deposition near the surface of the solid which, eventually, become responsible for the track formation.     

提高光子能量沉积计算效率的几种技巧

MCNP程序提供了计算光子沉积能的两种方法, 分别是F6和*F8。 当次级电子射程与网格尺度比较接近时, *F8的精度比F6高, 但计算效率比F6低很多。 分析比较了几种提高计算效率的技巧, 分别是: 网格大于10倍电子射程用F6; F6与*F8的联合使用; 改变电子能量子步数; 关掉δ电子; 次级电子自适应截断等。 数值模拟表明, F6与*F8的联合使用以及次级电子自适应截断这两种技巧对精度与效率兼顾得很好。 F6 and *F8 are two methods to calculate energy deposition of photon by MCNP program. If the dimension of grid is almost the same long as the range of secondary electron, the precision of *F8 is higher than that of F6, but the efficiency of F6 is greatly higher than that of *F8. This paper presents five techniques to increase the efficiency,namely: F6 is use to the model with grid dimension bigger than 10 ranges of secondary electron; combination of F6 and *F8; change the substeps of electron; turn off knock on electron; self adaption cutoff secondary electron. The two techniques, combination of F6 and *F8,self adaption cutoff secondary electron, can provide a good precision and efficiency.     

基于NASIC程序的纳米金辐射增敏效应模拟研究

纳米金粒子（GNP）应用为放疗辐射增敏剂是目前国际上的一个研究热点。使用自主研发的纳剂量生物物理蒙特卡罗程序(NASIC),模拟研究了光子照射下细胞环境中GNP的物理增敏效应和生物增敏效应。通过建立单个GNP位于细胞核中心以及多个GNP在细胞内四种理想分布的GNP-细胞模型,分析光子能量、GNP粒子尺寸和分布对能量沉积、DNA辐射损伤和细胞存活的影响。结果表明,GNP附近约2 m的范围内具有能量沉积的增强效应,这主要是因为GNP内光电效应作用数目的显著增加。不同条件下细胞核内能量沉积、DSB数目和细胞存活分数增强效应的变化规律基本一致,但增强因子呈递减趋势,三种评价指标增强因子的最大值分别为1.55,1.32 和1.14。光子能量为40 keV、GNP直径为100 nm并分布在细胞核表面时,相比其他参数组合具有较高的物理和生物辐射增敏效应。     

Phase space evolution of pairs created in strong electric fields

We study the process of energy conversion from overcritical electric field into electron–positron–photon plasma. We solve numerically Vlasov–Boltzmann equations for pairs and photons assuming the system to be homogeneous and anisotropic. All the 2-particle QED interactions between pairs and photons are described by collision terms. We evidence several epochs of this energy conversion, each of them associated to a specific physical process. Firstly pair creation occurs, secondly back reaction results in plasma oscillations. Thirdly photons are produced by electron–positron annihilation. Finally particle interactions lead to completely equilibrated thermal electron–positron–photon plasma.     

Elastic and inelastic processes in the interaction of 1–10 eV ions with solids: ion transport through surface layers

We review the escape depth of secondary ions (or neutrals) desorbing from solid surfaces under the impact of electrons, photons or ions. We survey ion (or neutral) transport through many materials, but most are wide band gap insulators such as rare-gas solids and molecular solids. We address the issue of low-energy (<10 eV) ion—solid interactions, and review experimental and theoretical studies that provide insight into the physical mechanisms of these interactions, such as elastic scattering, charge transfer and ion—molecule reactions. Although it is usually assumed that most of the secondary ions stem from the top surface layer, we show that this is not necessarily the case: In certain instances, 1–10 eV ions are able to transmit solid films which are several monolayers thick. The transport of low-energy ions through materials has very broad implications. We point out the importance of these results for electron or photon stimulated desorption (ESD/PSD), secondary ion mass spectrometry (SIMS), and ion-sputtering of surfaces, and discuss their relevance to other fields, such as ion beam deposition (IBD), low-energy ion implantation, and electrochemistry.     

Characteristics of secondary electron emission from few layer graphene on silicon (111) surface

As a typical two-dimensional (2D) coating material, graphene has been utilized to effectively reduce secondary electron emission from the surface. Nevertheless, the microscopic mechanism and the dominant factor of secondary electron emission suppression remain controversial. Since traditional models rely on the data of experimental bulk properties which are scarcely appropriate to the 2D coating situation, this paper presents the first-principles-based numerical calculations of the electron interaction and emission process for monolayer and multilayer graphene on silicon (111) substrate. By using the anisotropic energy loss for the coating graphene, the electron transport process can be described more realistically. The real physical electron interactions, including the elastic scattering of electron—nucleus, inelastic scattering of the electron—extranuclear electron, and electron—phonon effect, are considered and calculated by using the Monte Carlo method. The energy level transition theory-based first-principles method and the full Penn algorithm are used to calculate the energy loss function during the inelastic scattering. Variations of the energy loss function and interface electron density differences for 1 to 4 layer graphene coating GoSi are calculated, and their inner electron distributions and secondary electron emissions are analyzed. Simulation results demonstrate that the dominant factor of the inhibiting of secondary electron yield (SEY) of GoSi is to induce the deeper electrons in the internal scattering process. In contrast, a low surface potential barrier due to the positive deviation of electron density difference at monolayer GoSi interface in turn weakens the suppression of secondary electron emission of the graphene layer. Only when the graphene layer number is 3, does the contribution of surface work function to the secondary electron emission suppression appear to be slightly positive.     

基于切伦科夫辐射的核素治疗剂量测量可行性分析

针对现有核素治疗中内照射剂量测量缺乏简单、高效方法的问题,基于内照射剂量与切伦科夫辐射之间的关系,提出一种基于切伦科夫辐射的核素治疗内照射剂量测量的新方法。利用蒙特卡罗计算程序Geant4,模拟放射性核素131I在水体模型和甲状腺模型中产生切伦科夫辐射与剂量沉积的分布情况,并定量分析切伦科夫光子数与剂量之间的关系。计算结果表明:在水体模的半径方向上切伦科夫光子数与剂量之间有着相同的变化趋势,且两者有着相同的二维分布规律;核素131I在介质中产生的切伦科夫光子数与剂量两者之间存在一定的线性关系,且这种线性关系与核素的分布情况无关。研究结果证实,将这种放射性核素在介质中产生的切伦科夫辐射应用于内照射剂量学具有非常大的研究潜力和价值。     

Evaluation of the dosimetric characteristics of a radiophotoluminescent glass dosimeter for high-energy photon and electron beams in the field of radiotherapy

We aimed to evaluate the suitability of a glass dosimeter (GD) for high-energy photon and electron beams in experimental and clinical use, especially for radiation therapy. We examined the expanded dosimetric characteristics of GDs including dose linearity up to 500 Gy, uniformity among GD lots and for individual GDs, the angular dependence, and energy dependence of 4 therapeutic x-ray qualities. In addition, we measured the dosimetric features (dose linearity, uniformity, angular dependence, and energy dependence) of the GD for electron beams of 10 different electron energy qualities. All measurements with the exception of dose linearity for photon beam were performed in a water phantom. For high-energy photon beams, dose linearity has a linear relationship for a dose ranging from 1 to 500 Gy with the coefficient of determination; R² of 0.998. The uniformity of each GD of dose measurements was within ±0.5% for four GD lots and within ±1.2% for 80 GDs. In terms of the effects of photon beam angle, lower absorbed doses of within 1.0% were observed between 60° and 105° than at 90°. The GD energy dependence of 4 photon beam energy qualities was within ±2.0%. On the other hand, the result of the dose linearity for high-energy electron beams showed well fitted regression line with the coefficient of determination; R² of 0.999 between 6 and 20 MeV. The uniformity of GDs exposed to the nominal electron energies 6, 9, 12, 16, and 20 MeV was ±1.2%. In terms of the angular dependence to electron beams, absorbed doses were within 2.0% between 60° and 105° than at 90°. In evaluation of the energy dependence of the GD at nominal electron energies between 5 and 20 MeV, we obtained responses between 1.1% and 3.5% lower than that for a cobalt-60 beam. Our results show that GDs can be used as a detector for determining doses when a high-energy photon beam is used, and that it also has considerable potential for dose measurement of high-energy electron beam.     

Bremsstrahlung spectrum of nonrelativistic electrons scattered in metallic target: Theory and experiment

This work is devoted to theoretical and experimental investigation of the bremsstrahlung (BrS) spectrum of electrons with an energy of 10–30 eV scattered in a copper plate. Modeling of the photon yield from a target has been carried out taking into account the polarization BrS channel, elastic and inelastic electron scattering by medium atoms, photon absorption in the target material, and coherent effects when electrons interact with a polycrystalline structure of metal. An FEI Quanta 200 scanning electron microscope with a supplement for X-ray microanalysis was used. Good agreement between experimental and theoretical results has been obtained.     

极端紫外波段碱卤化物光阴极材料量子效率计算

为了满足极端紫外波段微通道位敏阳极光子计数探测器研究的需要,研究了碱卤化物光阴极材料的量子效率.由于光阴极材料的光电发射电流主要是由次级电子形成的,给出碱卤化物光阴极材料次级电子发射的理论模型,推导出次级电子产出的计算公式,针对光子能量30—250 eV范围内,计算并分析了光阴极材料厚度和光入射角对次级电子产出的影响.分析结果显示,光阴极材料厚度大于100 nm并且掠入射角大于临界角,是获得高次级电子产出的最佳条件.最后,应用推导的公式分析20种碱卤化物在能量30—250 eV范围内次级电子产出的光谱响应 关键词： 极端紫外 碱卤化物 光阴极 次级电子     

Cavity‐Photon‐Induced High‐Order Transitions between Ground States of Quantum Dots

It is shown that quantum electromagnetic transitions to high orders are essential to describe the time‐dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high‐order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para‐ and diamagnetic electron–photon interactions are treated with step‐wise exact numerical diagonalization and truncation of appropriate many‐body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.     

光子与相对论麦克斯韦分布电子散射的能谱角度谱研究

光子与相对论麦克斯韦分布电子散射的描述及能谱角度谱计算非常复杂且费时.本文提出了一种光子与相对论麦克斯韦速度分布电子散射的蒙特卡罗(MC)模拟方法,该方法能够细致模拟高温等离子体中任意能量光子与任意温度电子的Compton和逆Compton散射问题.对于散射后光子的能谱和角度谱参数,可以根据电子温度抽样若干不同状态的电子,分别模拟其与光子发生散射,可以得到各次散射后的光子能量和偏转角度,取统计平均后的结果即可获得该光子与该温度电子散射的能谱和角度谱分布.根据该方法编写了光子与相对论电子散射MC模拟程序,开展了高温全电离等离子体中光子与相对论电子散射的能谱角度谱计算和分析,分析结果显示:热运动电子将展宽出射光子能谱,且低能光子与高温电子散射后的蓝移现象明显;出射光子的角度谱很复杂,其决定于入射光子能量、出射光子能量及电子温度.基于该方法计算并以数表形式给出的光子-相对论电子散射能谱角度谱数据,可以供辐射输运数值模拟程序使用.     

放疗绝对剂量的数学算法模型

本文提出一种通过物理模型计算放疗过程中每一个组织深度处绝对剂量的算法,它可代替蒙特卡罗仿真的部分工作且耗费时间更少.这个算法是基于对照射野内X射线产生电子的能量注量的积分运算,并考虑了射线的能谱及二次散射线,得到了后向散射对表面剂量的贡献比例,同时得到前向散射、后向散射及原射线剂量贡献的关系.比较了二次光子和二次电子的三维能谱,得出该能谱是粒子注量关于粒子能量和粒子运动方向的函数.为了得到每一深度处的光子注量,计算了有连续能谱的X射线的期望质量衰减系数.上述算法计算得到的绝对剂量与蒙特卡罗方式仿真的结果趋势一致,两者的差异在于算法未考虑高于二次的散射线.最后将算法应用到非均匀模体剂量计算,能准确反映其中剂量分布特点且具有较小的误差.     

Berechnung von Elektronen-Photonen-Kaskaden nach der Monte-Carlo-Methode

Using the Monte-Carlo-Method linear electron-photon showers in lead have been calculated for primary electron and photon energies of 100, 200 and 380 MeV. We find that, beyond the maximum, the slope of the shower curves depends on the primary energy as well as on the cut-off energy of the electron and photon spectra. This behaviour is in agreement with recent experimental data.     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Monte Carlo simulation of electron beam air plasma characteristics

A high-energy electron beam generator is used to generate a plasma in atmosphere. Based on a Monte Carlo toolkit named GEANT4, a model including complete physics processes is established to simulate the passage of the electron beam in air. Based on the model, the characteristics of the electron beam air plasma are calculated. The energy distribution of beam electrons (BEs) indicates that high-energy electrons almost reside in the centre region of the beam, but low-energy electrons always live in the fringe area. The energy deposition is calculated in two cases, i.e., with and without secondary electrons (SEs). Analysis indicates that the energy deposition of SEs accounts for a large part of the total energy deposition. The results of the energy spectrum show that the electrons in the inlet layer of the low-pressure chamber (LPC) are monoenergetic, but the energy spectrum of the electrons in the outlet layer is not pure. The SEs are largely generated at the outlet of the LPC. Moreover, both the energy distribution of BEs and the magnitude of the density of SEs are closely related to the pressure of LPC. Thus, a conclusion is drawn that a low magnitude of LPC pressure is helpful for reducing the energy loss in the LPC and also useful for greatly increasing the secondary electron density in dense air.     

Investigation of radiological properties of some shielding materials on charged and uncharged radiation interaction for neutron generator

Radiation interaction parameters such as total stopping power, projected range (longitudinal and lateral) straggling, mass attenuation coefficient, effective atomic number (Z_eff) and electron density (N_eff) of some shielding materials were investigated for photon and heavy charged particle interactions. The ranges, stragglings and mass attenuation coefficients were calculated for the high-density polyethylene(HDPE), borated polyethylene (BPE), brick (common silica), concrete (regular), wood, water, stainless steel (304), aluminum (alloy 6061-O), lead and bismuth using SRIM Monte Carlo software and WinXCom program. In addition, effective atomic numbers (Z_eff) and electron densities (N_eff) of HDPE, BPE, brick (common silica), concrete (regular), wood, water, stainless steel (304) and aluminum (alloy 6061-O) were calculated in the energy region 10?keV–100?MeV using mass stopping powers and mass attenuation coefficients. Two different methods namely direct and interpolation procedures were used to calculate Z_eff for comparison and significant differences were determined between the methods. Variations of the ranges, longitudinal and lateral stragglings of water, concrete and stainless steel (304) were compared with each other in the continuous kinetic energy region and discussed with respect to their Z_effs. Moreover, energy absorption buildup factors (EABF) and exposure buildup factors (EBF) of the materials were determined for gamma rays as well and were compared with each other for different photon energies and different mfps in the photon energy region 0.015–15?MeV.     

Neutron-photon studies of condensed matter

A theory of simultaneous photon absorption and inelastic neutron scattering is developed by treating the photon and neutron-matter interactions perturbatively. The leading-order mixing between the interactions shows that the neutron scattering cross-section is proportional to the dynamic structure factor (or Van Hove function) evaluated at an energy that is enhanced by the photon energy. The photon induced modification of the scattering vector is negligible. Thus, the proposed technique affords the possibility of measuring the dynamic structure factor at large energies and modest wavevectors which is a domain that is usually difficult to access because of kinematic constraints. The theory is developed in detail for some models of nuclear and magnetic systems. The results show that, in most cases, the experiments are likely to demand the use of very high intensity light sources. A particularly promising application appears to be in the study of electron plasmas since, using readily available pulsed lasers, the neutron cross-section is comparable with that for pure magnetic scattering.