“预聚束器-斩波器”结构对慢正电子束流束团化的影响模拟

Simulating the effect of “Prebuncher-Chopper” on pulsing slow positron beam

慢正电子湮没寿命谱测量是一种灵敏度高且可以无损探测材料微观缺陷的重要分析方法,束流束团化系统是实现该技术的核心部件,主要用来产生满足寿命测量时间分辨的束团并提供正电子产生的时间信号。本文选用Parmela程序,基于“预聚束器-斩波器”束团化系统,模拟计算了不同束流参数对150 ps(半高宽)束团化结果的影响并与“斩波器-预聚束器”束团化系统的计算结果做了对比,结果表明:提高束流能量、降低束流能散以及缩小束斑尺寸有利于提高束团化效率。数据显示:当束流能量≥500 eV、能散≤5 eV、束斑≤12 mm,“预聚束-斩波器”束团化系统的正电子利用率≥20%、聚束效率≥85%;且在相同束流参数下,“预聚束器-斩波器”束团化系统的正电子利用率要优于“斩波器-预聚束器”。

[Background]Slow positron annihilation lifetime spectrum measurement is an important analytical method for high sensitivity and non-destructive detection of material micro-defects.Pulsed positron system is the core component of this technology.[Purpose]This study aims to obtain periodic positron pulses with high time resolution for lifetime measurement and to provide a signal marking the time of positron generation.[Methods]In this paper,the Parmela program was applied to simulate and calculate the effect of different beam parameters on positrons bunching based on the"Prebuncher-Chopper"pulsed system.Effect of different beam parameters on 150 ps(half-width)bunching was simulated and compared with calculation results of"Prebuncher-Chopper"bunching system.[Results]The results show that increasing beam energy,reducing beam energy spread and reducing beam spot size are beneficial to improve pulse efficiency.Moreover,when the positron beam energy is greater than 500 eV,the energy spread is less than 5 eV and the beam spot is less than 12 mm,the positron utilization ratio is more than 20%and the bunching efficiency is more than 85%using"Prebuncher-Chopper"pulsed system.[Conclusion]In contrast to the"Chopper-Prebuncher"system,"Prebuncher-Chopper"pulsed system can provide a higher positron utilization rate under the same beam parameters.Meanwhile,the time resolution(≤150 ps)and the bunching efficiency(≤85%)meet the measurement requirement.