[论文解读] Po-production in lead: A benchmark between Geant4, FLUKA and MCNPX

Alfredo Ferrari, D. Kiselev+3 more|arXiv (Cornell University)|Jun 10, 2018

Nuclear Physics and Applications被引用 4

一句话总结

本基准研究使用Geant4、FLUKA和MCNPX，结合INCL4.6/ABLA07模型，评估了575 MeV质子轰击铅时钋同素体的生成。结果表明，此前在默认MCNPX中被低估的α粒子和氚核反应的精确处理，可使模拟结果与实验数据高度吻合，凸显了核反应模型在标准第一代核内弦模型之外对钋活度预测的敏感性。

ABSTRACT

On the last SATIF a comparison between the measured activities of the polonium isotopes Po-208, Po-209 and Po-210 and the simulated results using MCNPX2.7.0 was presented. The lead samples were cut from the SINQ spallation target at the Paul Scherrer Institut (PSI) and irradiated in 2000/2001 by 575 MeV protons. The Po-isotopes were separated using radiochemical methods by the group of D. Schumann at PSI and measured. Choosing the default model in MCNPX, Bertini-Dresner, the prediction underestimated the measured activities by up to several orders of magnitude. Therefore the Liège intranuclear-cascade model (INCL4.6) coupled to the de-excitation model ABLA07 were implemented into MCNPX2.7.0 and very good agreement was found to the measurement. The reason for the disagreement was traced back to a suppression of alpha reactions on the lead isotopes leading to Po and neglecting the triton capture on Pb-208, which leads to Pb-210 and decays into Po-210 with a much longer life time (22.3 years) than the decay of Po-210 itself (138 days). The prediction of the Po-isotope activities turns out to be a sensitive test for models and codes as it requires the accurate treatment of reaction channels not only with neutrons, protons and pions but also with alphas and tritons, which are not considered in intra-nuclear cascade models of the first generation. Therefore it was decided to perform a benchmark by comparing the results obtained with MCNPX2.7.0 using INCL4.6/ABLA07 to the predictions of FLUKA and Geant4. Since the model of the SINQ spallation source requires an elaborate geometry a toy model was setup. The toy model has a simplified geometry preserving the main features of the original geometry. The results for the activities of the three Po-isotopes and Pb-210 as well as the energy spectra for alphas and tritons obtained with the three particle transport Monte Carlo codes are presented.

研究动机与目标

• 评估Geant4、FLUKA和MCNPX在模拟铅中钋同素体生成（散裂辐照条件下）的预测准确性。
• 识别MCNPX默认模型（特别是Bertini-Dresner核内弦模型）的缺陷，该模型显著低估了Po-208、Po-209和Po-210的活度。
• 利用SINQ散裂靶辐照实验数据，验证MCNPX中INCL4.6/ABLA07模型的改进实现。
• 确立钋同素体生成作为核反应模型的敏感基准，尤其针对涉及α粒子和氚核的反应。

提出的方法

• 开发了一种简化的玩具几何模型，以在计算效率的前提下保留SINQ散裂靶复杂几何结构的关键特征。
• 使用Geant4、FLUKA和MCNPX2.7.0进行蒙特卡罗粒子输运模拟，核反应级联与退激发过程采用INCL4.6/ABLA07模型。
• 对比了三种代码在钋同素体活度及α粒子和氚核能量谱方面的模拟结果。
• 以辐照铅样品中Po-208、Po-209、Po-210和Pb-210的放射化学分离与测量实验数据作为基准。
• 重点识别反应通道处理中的差异，特别是α粒子发射和Pb-208对氚核的俘获。
• 进行敏感性分析，以分离特定反应通道（如208Pb(t,α)210Po和208Pb(α,2n)210Po）对最终钋同素体产额的影响。

实验结果

研究问题

• RQ1 为何MCNPX默认模型（Bertini-Dresner）在预测铅中钋同素体活度时失败，导致结果低了几个数量级？
• RQ2 Geant4、FLUKA和MCNPX在INCL4.6/ABLA07模型下对铅中Po-208、Po-209和Po-210生成的预测，与实验数据相比如何？
• RQ3 α粒子和氚核诱导的反应在多大程度上贡献于钋同素体的生成？为何它们对精确模拟至关重要？
• RQ4 钋同素体生成能否作为评估核输运代码模拟散裂反应性能的敏感基准？
• RQ5 在原始MCNPX模拟中，哪些特定反应通道（尤其是涉及氚核和α粒子等轻离子的通道）导致了结果差异？

主要发现

• MCNPX默认模型（Bertini-Dresner）因铅同素体上α反应被抑制，导致钋同素体活度被低估了几个数量级。
• 在MCNPX2.7.0中实现INCL4.6/ABLA07模型后，模拟结果与实验数据高度一致，证实了其更高的准确性。
• Pb-208对氚核的俘获（生成Pb-210，其22.3年半衰期衰变为Po-210）被识别为Po-210活度的关键贡献者，此前在第一代模型中被忽略。
• 三种代码（Geant4、FLUKA、MCNPX）预测的α粒子和氚核能量谱高度一致，验证了其对轻离子发射处理的可靠性。
• 本研究证实，钋同素体生成是核反应模型的高敏感性基准，尤其对涉及轻离子（如氚核和α粒子）的反应而言。
• 该基准表明，精确模拟散裂过程需要对超出质子、中子和π介子之外的非标准反应通道进行仔细建模。