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1.
为计算医用加速器中束流经过270°偏转磁铁系统电子损失所造成的辐射剂量问题, 将束流传输相应的计算公式和蒙特卡罗抽样方法相结合, 在一阶近似条件下计算了电子在偏转系统中的输运过程, 分析了不同初始条件对电子输运和电子损失的影响;模拟结果表明能散是产生电子损失的主要因素之一. 计算得到了损失电子所处位置、能量和飞行方向等信息, 把计算得到的信息作为蒙特卡罗程序的输入源, 进一步计算出束流损失所产生的辐射剂量分布, 从而能更完善地设计医用加速器照射头的屏蔽. 文中给出在电子束初始半径为1mm、散角为5mrad、能散为10%条件下电子损失率为13.5%, 损失电子主要是向加速器照射头部上方辐射出去. 相似文献
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束流在270°偏转磁铁系统输运过程中的损失计算 总被引:1,自引:0,他引:1
为计算医用加速器中束流经过270°偏转磁铁系统电子损失所造成的辐射剂量问题,将束流传输相应的计算公式和蒙特卡罗抽样方法相结合,在一阶近似条件下计算了电子在偏转系统中的输运过程,分析了不同初始条件对电子输运和电子损失的影响;模拟结果表明能散是产生电子损失的主要因素之一.计算得到了损失电子所处位置、能量和飞行方向等信息,把计算得到的信息作为蒙特卡罗程序的输入源,进一步计算出束流损失所产生的辐射剂量分布,从而能更完善地设计医用加速器照射头的屏蔽.文中给出在电子束初始半径为1mm、散角为5mrad、能散为10%条件下电子损失率为13.5%,损失电子主要是向加速器照射头部上方辐射出去. 相似文献
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采用蒙特卡罗方法模拟了电子辅助化学气相淀积(EACVD)金刚石薄膜中的氢分解过程,建立了电子碰撞使氢分解的模型,给出了电子能量分布及氢原子数的空间分布,讨论了电偏压和气压对氢分解的影响。这些结果对EACVD制造金刚石薄膜的研究有重要意义。 相似文献
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采用粒子模拟,得到了阳极杆箍缩二极管阳极钨针上电子的空间分布和入射角分布,分析二极管工作状态得到了电子的能量分布.在此基础上建立阳极杆箍缩二极管的蒙特卡罗模型,模拟得到了阳极杆箍缩二极管的辐射能谱和X射线的平均能量,并与实验结果进行了比较.结果表明:09006炮光子平均能量为0.441 MeV,计算该能谱射线经过不同厚度铅衰减片后的剂量衰减情况,与叠片法PIN探测器所测的实验数据基本一致.
关键词:
阳极杆箍缩二极管
粒子模拟
蒙特卡罗方法
X射线能谱 相似文献
8.
蒙特卡罗与热工水力的耦合计算是目前反应堆数值模拟的重要研究方向,在蒙特卡罗方法连续能量点截面的基础上结合热工程序的温度反馈,反应堆中子计算的准确性得到大幅提高。为了提高计算精度,堆芯模型分辨率也需进一步提高,相比于组件均匀化模型,pin-by-pin的建模方式能够获得更好的结果。利用蒙特卡罗程序JMCT与子通道程序COBRA-EN实现了蒙特卡罗-热工的内耦合,内耦合方式通过内存进行数据传递,其计算效率及安全性均优于外耦合方法。随后利用NURISP项目迷你堆的pin-by-pin模型对耦合程序进行验证。计算结果与同类耦合程序相似,验证了程序的准确性。同时,对耦合过程的收敛性问题进行了初步分析。 相似文献
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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。 相似文献
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基于经验赝势法得到的能带结构数据,采用分段多项式拟合获得ZnS能带结构的解析表达式 ,建立解析能带模型.使用建立的模型计算得到各能谷的态密度和总的散射速率,并与文献 的计算结果进行了对比,验证该解析能带模型既具有非抛物型多能谷能带模型运算速度快、 使用方便的优势,又具有与采用全导带模型相近的计算精度.进一步利用该模型进行蒙特卡罗 模拟,得到第一导带和第二导带中电子数随电场强度的变化、不同电场中能量分布函数以及 包含与不包含碰撞离化情况下电子能量随时间变化的曲线.讨论在外加电场下,电子在导带 内各个能谷间和
关键词:
蒙特卡罗模拟
解析能带模型
多项式拟合
碰撞离化 相似文献
11.
M. U. Bug E. Gargioni S. Guatelli S. Incerti H. Rabus R. Schulte A. B. Rosenfeld 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2010,60(1):85-92
The increasing use of MRI-guided radiation therapy evokes the necessity to investigate the potential impact of a magnetic
field on the biological effectiveness of therapeutic radiation beams. While it is known that a magnetic field, applied during
irradiation, can improve the macroscopic absorbed dose distribution of electrons in the tumor region, effects on the microscopic
distribution of energy depositions and ionizations have not yet been investigated. An effect on the number of ionizations
in a DNA segment, which is related to initial DNA damage in form of complex strand breaks, could be beneficial in radiation
therapy.
In this work we studied the effects of a magnetic field on the pattern of ionizations at nanometric level by means of Monte
Carlo simulations using the Geant4-DNA toolkit. The track structure of low-energy electrons in the presence of a uniform static
magnetic field of strength up to 14 T was calculated for a simplified DNA segment model in form of a water cylinder. In the
case that no magnetic field is applied, nanodosimetric results obtained with Geant4-DNA were compared with those from the
PTB track structure code.
The obtained results suggest that any potential enhancement of complexity of DNA strand breaks induced by irradiation in a
magnetic field is not related to modifications of the low-energy secondary electrons track structure. 相似文献
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Thin films of the short single strand of DNA, GCAT, in which guanine (G) or adenine (A) have been removed, were bombarded under vacuum by 4 to 15 eV electrons. The fragments corresponding to base release and strand breaks (SB) were analyzed by high performance liquid chromatography and their yields compared with those obtained from unmodified GCAT. From such a comparison, it is shown that, using GCAT as a model system, (1) most SB result from electron capture by DNA bases followed by electron transfer to the phosphate group and (2) the initial capture probability depends on the coherence of the electron wave within the tetramer. 相似文献
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Collisions of 0-4 eV electrons with thin DNA films are shown to produce single strand breaks. The yield is sharply structured as a function of electron energy and indicates the involvement of pi* shape resonances in the bond breaking process. The cross sections are comparable in magnitude to those observed in other compounds in the gas phase in which pi* electrons are transferred through the molecule to break a remote bond. The results therefore support aspects of a theoretical study by Barrios et al. [J. Phys. B 106, 7991 (2002)]] indicating that such a mechanism could produce strand breaks in DNA. 相似文献
14.
We propose a framework to calculate the intermolecular multiple elastic scattering of low-energy electrons from helical macromolecules and indicate how it affects the resonant capture cross section. Using a model of DNA, an appreciable enhancement of the elastic and resonant capture cross sections is predicted at incident energies below 15 eV. These results may qualitatively explain the observed prominence of low-energy resonances in strand breaking of plasmid DNA. 相似文献
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We propose a mechanism of DNA single strand breaks
induced by low-energy electrons. Density functional theory calculations have
been performed on a neutral, hydrogenated, and/or negatively charged
nucleotide of cytosine in the gas phase to identify barriers for the
phosphate-sugar O–C bond cleavage. Attachment of the first excess electron
induces intermolecular proton transfer to cytosine. The resulting neutral
radical of hydrogenated cytosine binds another excess electron, and the
excess charge is localized primarily on the C6 atom. A barrier encountered
for proton transfer from the C2’ atom of the adjacent sugar unit to the C6
atom of cytosine is 3.6 and 5.0 kcal/mol, based on the MPW1K and B3LYP
electronic energies corrected for zero-point vibrations, respectively. The
proton transfer is followed by a barrier-free sugar-phosphate C–O bond
cleavage. The proton transfer is impossible for the neutral nucleotide, as
there is no local minimum for the product. In the case of anionic and
hydrogenated nucleotides the same barrier determined at the B3LYP level is
as large as 29.3 and 22.4 kcal/mol respectively. This illustrates that the
consecutive hydrogenation and electron attachment make the nucleotide of
cytosine susceptible to a strand break. The rate of the C–O bond cleavage in
the anion of hydrogenated nucleotide of cytosine is estimated to be ca.
1010 s-1. The proposed mechanism proceeds through bound anionic
states, not through metastable states with finite lifetimes and discrete
energy positions with respect to the neutral target. The results suggest
that
at least for DNA without hydration
even very low-energy electrons may cleave the DNA backbone. 相似文献
17.
We use dibutyl phosphate to simulate the behavior of the phosphate group in DNA towards the attack of low energy electrons. We find that the compound undergoes effective dissociative electron attachment within a low energy resonant feature at 1 eV and a further resonance peaking at 8 eV. The dissociative electron attachment (DEA) reactions are associated with the direct cleavage of the C-O and the P-O bond but also the excision of the PO-, PO3-, H2PO3- units. For the phosphate group coupled in the DNA network these reactions represent single strand breaks. We hence propose that the most direct mechanism of single strand breaks occurring in DNA at subexcitation energies (< 4 eV) is due to DEA directly to the phosphate group. 相似文献
18.
E. Surdutovich O. I. Obolensky E. Scifoni I. Pshenichnov I. Mishustin A. V. Solov’yov W. Greiner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,51(1):63-71
This work is the first stage in the development of an inclusive approach to calculation of the DNA damage caused by irradiation
of biological tissue by ion/proton beams. The project starts with an analysis of ionization caused by the projectiles and
the characteristics of secondary electrons produced in tissue-like media. We consider interactions with the medium on a microscopic
level and this allows us to obtain the energy spectrum and abundance of secondary electrons as functions of the projectile’s
kinetic energy. The physical information obtained in this analysis is related to biological processes responsible for the
DNA damage induced by the projectile. In particular, we consider double strand breaks of DNA caused by secondary electrons
and free radicals, and local heating in the ion’s track. The heating may enhance the biological effectiveness of electron/free
radical nteractions with the DNA and may even be considered as an independent mechanism of DNA damage. Numerical estimates
are performed for the case of carbon-ion beams. The obtained dose-depth curves are compared with results of the MCHIT model
based on the GEANT4 toolkit. 相似文献
19.
Solid films of DNA with and without the chemotherapeutic agent cisplatin bonded to guanine were bombarded with electrons of 1, 10, 100, and 60,000 eV causing single and double strand breaks. In the presence of cisplatin this damage was increased by factors varying from 1.3 to 4.4 owing to an increase in bond dissociation triggered by the formation of transient anions. This mechanism may lie at the basis of the efficiency of concomitant cisplatin-radiation therapy. 相似文献
20.
D'Souza JS Dharmadhikari JA Dharmadhikari AK Rao BJ Mathur D 《Physical review letters》2011,106(11):118101
Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in aqueous media exposed to strong fields generated using ultrashort laser pulses (820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities of 1-12 TW?cm(-2). The strong fields generate, in situ, electrons and radicals that induce transformation of supercoiled DNA into relaxed DNA, the extent of which is quantified. Introduction of electron and radical scavengers inhibits DNA damage; results indicate that OH radicals are the primary (but not sole) cause of DNA damage. 相似文献