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首页> 外文学位 >Automated variance reduction technique for three-dimensional Monte Carlo coupled electron-photon-positron simulation using deterministic importance functions.
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Automated variance reduction technique for three-dimensional Monte Carlo coupled electron-photon-positron simulation using deterministic importance functions.

机译：使用确定性重要性函数进行三维蒙特卡罗耦合电子-光子-正电子模拟的自动方差减少技术。

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摘要

Three-dimensional Monte Carlo coupled electron-photon-positron transport calculations are often performed to determine characteristics such as energy or charge deposition in a wide range of systems exposed to radiation field such as electronic circuitry in a space-environment, tissues exposed to radiotherapy linear accelerator beams, or radiation detectors. Modeling these systems constitute a challenging problem for the available computational methods and resources because they can involve; (i) very large attenuation, (ii) large number of secondary particles due to the electron-photon-positron cascade, and (iii) large and highly forward-peaked scattering.; This work presents a new automated variance reduction technique, referred to as ADEIS (Angular adjoint-Driven Electron-photon-positron Importance Sampling), that takes advantage of the capability of deterministic methods to rapidly provide approximate information about the complete phase-space in order to automatically evaluate variance reduction parameters. More specifically, this work focuses on the use of discrete ordinates importance functions to evaluate angular transport and collision biasing parameters, and use them through a modified implementation of the weight-window technique. The application of this new method to complex Monte Carlo simulations has resulted in speedups as high as five orders of magnitude.; Due to numerical difficulties in obtaining physical importance functions devoid of numerical artifacts, a limited form of smoothing was implemented to complement a scheme for automatic discretization parameters selection. This scheme improves the robustness, efficiency and statistical reliability of the methodology by optimizing the accuracy of the importance functions with respect to the additional computational cost from generating and using these functions.; It was shown that it is essential to bias different species of particles with their specific importance functions. In the case of electrons and positrons, even though the physical scattering and energy-loss models are similar, the importance of positrons can be many orders of magnitudes larger than electron importance. More specifically, not explicitly biasing the positrons with their own set of importance functions results in an undersampling of the annihilation photons and, consequently, introduces a bias in the photon energy spectra.; It was also shown that the implementation of the weight-window technique within the condensed-history algorithm of a Monte Carlo code requires that the biasing be performed at the end of each major energy step. Applying the weight-window earlier into the step, i.e., before the last substep, will result in a biased electron energy spectrum. This bias is a consequence of systematic errors introduced in the energy-loss prediction due to an inappropriate application of the weight-window technique where the actual path-length differs from the pre-determined path-length used for evaluating the energy-loss straggling distribution.
机译：经常进行三维蒙特卡洛耦合电子-光子-正电子输运计算来确定特性，例如在暴露于辐射场的各种系统（例如空间环境中的电子电路），暴露于放射线疗法的组织中的能量或电荷沉积加速器光束或辐射探测器。对这些系统进行建模对于可用的计算方法和资源而言是一个具有挑战性的问题，因为它们可能涉及到； （i）非常大的衰减，（ii）由于电子-光子-正电子级联而产生的大量次级粒子，以及（iii）大且高度前向散射。这项工作提出了一种新的自动方差减少技术，称为ADEIS（角伴生驱动电子-光子-正电子重要采样），该技术利用确定性方法的功能快速按顺序提供有关完整相空间的近似信息。自动评估方差减少参数。更具体地说，这项工作着重于使用离散纵坐标重要度函数来评估角传输和碰撞偏置参数，并通过修改权重窗技术来使用它们。这种新方法在复杂的蒙特卡洛模拟中的应用导致加速高达五个数量级。由于在获取没有数字伪像的物理重要性函数时存在数值困难，因此实施了一种有限形式的平滑处理，以补充自动离散化参数选择的方案。这种方案通过相对于生成和使用这些函数的额外计算成本来优化重要函数的准确性，从而提高了方法的鲁棒性，效率和统计可靠性。结果表明，用不同的重要性函数来偏置不同种类的粒子是至关重要的。对于电子和正电子，即使物理散射和能量损失模型相似，但正电子的重要性可能比电子重要性大许多数量级。更具体地，不用其自己的重要函数集来对正电子进行显式偏置会导致the没光子的欠采样，从而在光子能谱中引入偏置。还显示出在蒙特卡洛代码的压缩历史算法内权重窗口技术的实现要求在每个主要能量步骤的末尾执行偏置。在步骤的较早阶段（即在最后一个子步骤之前）应用权重窗口将导致电子能谱出现偏差。该偏差是由于不正确应用权重窗口技术而在能量损耗预测中引入系统误差的结果，其中实际路径长度与用于评估能量损耗散布分布的预定路径长度不同。

著录项

  • 作者

    Dionne, Benoit.;

  • 作者单位

    University of Florida.;

  • 授予单位 University of Florida.;
  • 学科 Engineering Nuclear.
  • 学位 Ph.D.
  • 年度 2007
  • 页码 185 p.
  • 总页数 185
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 原子能技术;
  • 关键词

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