PSI LEM束线的Geant4模拟：薄箔中的能量损失和μ素形成以及未慢化μ子对$μ$ SR谱仪的影响
Geant4 simulation of the PSI LEM beam line: energy loss and muonium formation in thin foils and the impact of unmoderated muons on the $μ$SR spectrometer
摘要：
PSI低能$μ$ SR谱仪是一种专门用于测量μ子自旋旋转和弛豫的仪器。μ子束参数的知识，如空间，动能和到达时间分布在样品位置是分析$μ$ SR谱的重要成分。在这里，我们提出了测量的能量损失在薄碳箔的μ子启动探测器推导出的飞行时间测量。μ子启动探测器的薄碳箔（10 nm厚）中的μ 鎓形成也会影响可测量的衰变不对称性，因此需要考虑。在Geant 4蒙特卡罗模拟中实现了起始探测器中μ子的形成和能量损失，其相关性随着μ子注入能量（$<10$ keV）的降低而增加，以再现测量的飞行时间谱。只有当引入一小部分污染$μ$ SR谱仪单能μ子束的所谓“未慢化”μ子时，模拟和测量的飞行时间和束斑才一致。此外，一个特殊形状的“鼻子”样品板的光束尺寸和相关的上游-下游不对称性的灵敏度进行了研究，为各种光束线设置，这是相关的热μ素发射到真空中的介孔二氧化硅在低温下的研究。
英文摘要：
The PSI low-energy $μ$SR spectrometer is an instrument dedicated to muon spin rotation and relaxation measurements. Knowledge of the muon beam parameters such as spatial, kinetic energy and arrival-time distributions at the sample position are important ingredients to analyze the $μ$SR spectra. We present here the measured energy losses in the thin carbon foil of the muon start detector deduced from time-of-flight measurements. Muonium formation in the thin carbon foil (10 nm thickness) of the muon start detector also affect the measurable decay asymmetry and therefore need to be accounted for. Muonium formation and energy losses in the start detector, whose relevance increase with decreasing muon implantation energy ($<10$ keV), have been implemented in Geant4 Monte Carlo simulation to reproduce the measured time-of-flight spectra. Simulated and measured time-of-flight and beam spot agrees only if a small fraction of so called "unmoderated" muons which contaminate the mono-energetic muon beam of the $μ$SR spectrometer is introduced. Moreover the sensitivity of the beam size and related upstream-downstream asymmetry for a specially shaped "nose" sample plate has been studied for various beam line settings, which is of relevance for the study of thermal muonium emission into vacuum from mesoporous silica at cryogenic temperatures.
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