Green D G
Centre for Theoretical Atomic, Molecular and Optical Physics, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, United Kingdom.
Phys Rev Lett. 2017 Nov 17;119(20):203403. doi: 10.1103/PhysRevLett.119.203403. Epub 2017 Nov 16.
Positron cooling and annihilation in room temperature noble gases is simulated using accurate scattering and annihilation cross sections calculated with many-body theory, enabling the first simultaneous probing of the energy dependence of the scattering and annihilation cross sections. A strikingly small fraction of positrons is shown to survive to thermalization: ∼0.1 in He, ∼0 in Ne, ∼0.15 in Ar, ∼0.05 in Kr, and ∼0.01 in Xe. For Xe, the time-varying annihilation rate Z[over ¯]{eff}(τ) is shown to be highly sensitive to the depletion of the momentum distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements. Overall, the use of the accurate atomic data gives Z[over ¯]{eff}(τ) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalization and/or the presence of impurities.
利用多体理论计算出的精确散射和湮灭截面，对室温惰性气体中的正电子冷却和湮灭进行了模拟，从而首次同时探究了散射和湮灭截面的能量依赖性。结果表明，只有极小一部分正电子能够存活到热化阶段：氦气中约为0.1，氖气中约为0，氩气中约为0.15，氪气中约为0.05，氙气中约为0.01。对于氙气，随时间变化的有效湮灭率(Z[\overline{]}{eff}(τ))对由于湮灭导致的动量分布耗尽高度敏感，最终解释了气室测量和基于陷阱的测量之间长期存在的差异。总体而言，使用精确的原子数据得到的(Z[\overline{]}{eff}(τ))与除氖气之外的所有惰性气体的实验结果都非常吻合，氖气实验被认为是由于对存活到热化阶段的正电子分数了解不完整和/或存在杂质而受到影响。
