蒙地卡羅法MCNP程式對多層屏蔽
之光子曝露增建因數研究

謝楊威¹　林威廷²

¹慈濟技術學院放射醫學科學研究所理學碩士

²慈濟技術學院放射醫學科學研究所助理教授

摘要

屏蔽設施為多層屏蔽時，由於增建因數的變化相當複雜，若點核仁法採用單層屏蔽增建因數，容易得到低估或過度高估的結果。本研究以簡單及容易使用為考量，選取平面單向射源平板雙層屏蔽的Kalos、Modified Kalos經驗公式，以及點射源球形雙層及多層屏蔽的Lin & Jiang經驗公式，分別和使用蒙地卡羅法MCNP5程式計算點均向射源（0.1、0.3、0.6、1、3、6、10 MeV），在三種不同材質（水、鐵及鉛）的球形多層屏蔽曝露增建因數的計算值進行比較，以驗證各經驗公式的適用性程度。在雙層屏蔽，Lin & Jiang經驗公式在入射光子能量為0.1至10 MeV時，其整體適用性和改善效果較為顯著，而Kalos、Modified Kalos經驗公式在入射光子能量為0.1 MeV時，且低原子序材質後面銜接高原子序材質（水/鉛、鐵/鉛）時，容易產生過度高估的結果。當點均向射源球形雙層屏蔽擴展至多層，Lin & Jiang經驗公式在入射光子能量為0.3至10 MeV時，其曝露增建因數計算值和MCNP5程式計算結果較為吻合；若入射光子能量為0.1 MeV，Lin & Jiang經驗公式和直接採用ANSI/ANS-6.4.3-1991曝露增建因數的結果作比較，確實有明顯的改善效果，但和MCNP5程式計算值比較，Lin & Jiang經驗公式對鐵/鉛和鉛/鐵等多層屏蔽材質組合仍會發生曝露增建因數過度高估之現象。

關鍵字：經驗公式、蒙地卡羅法、MCNP、曝露增建因數

Yang-Wei Hsieh ¹　Uei-Tyng Lin ²

¹ Master of Science, Institute of Radiological Science, Tzu-Chi College of Technology

² Assistant Professor, Institute of Radiological Science, Tzu-Chi College of Technology

Abstract

Concerning multi-layered shields, the variation in buildup factor is complicated.  If buildup factors for single-layered spherical shields are adopted in point-kernel codes, underestimate or overestimate results are likely to be obtained. Simple and easy to use are the considerations of this study. Kalos and Modified Kalos empirical formulas of double-layered slabs at plane monodirectional source, and Lin & Jiang empirical formula of double-and multi-layered spherical shields consisting of three kinds of different materials (water, iron, and lead) at point isotropic source (0.1, 0.3, 0.6, 1, 3, 6, and 10 MeV) are selected to be compared with the theoretical calculated value of MCNP5, in order to verify the applicability of each empirical formula. In double-layered shield, the overall applicability and improvement of Lin & Jiang empirical formula are more significant when the energy of incident photon is 0.1 to 10 MeV.  As for Kalos and Modified Kalos empirical formulas, it is likely to cause overestimation result when the energy of incident photon is 0.1 MeV and low atomic number material is linked with high atomic number material (water/lead, iron/lead) behind. When double-layered spherical shields at point isotropic source expands to multi-layered and the energy of incident photon is 0.3 to 10 MeV, the exposure buildup factors calculated values of Lin & Jiang empirical formula are more similar to the calculated results of MCNP5. If the energy of incident photon is 0.1 MeV, significant improvement can be found when comparing the results of Lin & Jiang empirical formula and those directly adopt ANSI/ANS-6.4.3-1991 exposure buildup factors.  However, when comparing with MCNP5 calculated values, there is still exposure buildup factor overestimation in Lin & Jiang empirical formula for multi-layered shields material combinations, such as iron/lead and lead/iron.

Keywords：empirical formula, Monte Carlo method, MCNP, exposure buildup factor.