AI 中文总结
本研究通过蒙特卡洛模拟和扩散模型，比较钨与固态氖慢化体在不同能量下的效率，发现固态氖在低能下效率高一个数量级，多箔钨在MeV级能量下更优，并提出结合减速腔与低温慢化体以提升直线加速器源的端到端效率。
AI 中文摘要
慢正电子束可实现多种应用，从表面敏感的材料研究到正电子素物理，但进展受到源强度和亮度的限制。在基于直线加速器的源中，单能慢正电子是通过对高能电子入射高Z靶产生的宽发散快正电子分布进行慢化而产生的。这一过程本质上效率低下。传统的基于直线加速器的设计将慢化体放置在靶附近。钨慢化体在高功率快正电子加热时会变得效率降低，导致与缺陷相关的损失。低温慢化体，如固态氖，在暴露于高功率快正电子时会熔化。遵循O'Rourke等人的方法，我们使用蒙特卡洛模拟结合扩散模型，研究慢化体几何形状、材料以及入射快正电子能量如何影响慢正电子产生。我们比较了单钨箔和多箔配置与固态氖慢化体在反射和透射几何中的表现。我们发现，对于低于300 keV的快正电子能量，固态氖由于具有更大的扩散长度，其效率比钨高一个数量级，而在MeV级能量下，多箔钨由于散射最大化而具有更高的效率。我们还发现，在模拟中加入靶代理可以通过允许一部分初始反射的正电子返回慢化体，将低能效率提高多达四倍。我们得出结论，在直线加速器源上实现端到端效率的显著提升，可能需要将减速腔与解耦的低温慢化体相结合。
英文摘要
Slow positron beams enable diverse applications, from surface-sensitive materials studies to positronium physics, but progress is limited by source intensity and brightness. In linac-based sources, mono-energetic, slow positrons are produced by moderating the broad, divergent distribution of fast positrons produced by high-energy electrons incident on a high-Z target. This process is intrinsically inefficient. Conventional linac-based designs place the moderator close to the target. Tungsten moderators become less efficient when heated by high-power, fast positrons, leading to defect-related losses. Cryogenic moderators, like solid neon, melt when exposed to high-power, fast positrons. Following the method of O'Rourke et al., we use Monte Carlo simulations combined with a diffusion model to investigate how moderator geometry, material, and incident fast positron energy affect slow positron production. We compare single tungsten foil and multi-foil configurations with solid neon moderators in reflection and transmission geometries. We find that for fast positron energies below 300 keV, solid neon offers order-of-magnitude higher efficiency than tungsten due to its larger diffusion length, whereas at MeV-scale energies multi-foil tungsten has higher efficiency due to the maximization of scattering. We also find that including a target proxy in simulation increases low-energy efficiency up to fourfold by allowing a fraction of initially reflected positrons to return to the moderator. We conclude that substantial gains in start-to-end efficiency at linac sources will likely require combining a decelerating cavity with a decoupled cryogenic moderator.
Comments7 pages, 4 figures, Low Energy Electron Positron Physics Workshop at Jefferson Lab
