用于激光加速质子参数表征的带电粒子活化测谱技术

基于传统带电粒子活化分析技术,发展了一种用于激光加速质子参数表征的带电粒子活化测谱方法.激光加速质子轰击不同厚度铜薄膜组成的诊断滤片堆栈,使铜片活化,通过测量各铜片活度及活性区的大小,获得加速质子的空间积分能谱、角分布等参数.详细讨论了活化测谱的滤片堆栈诊断排布、符合测量及解谱方法,并对该方法的可靠性进行了自洽检验;在XG-III皮秒激光装置上开展了带电粒子活化测谱实验,利用该诊断方法,得到了加速质子的角分布、空间积分能谱等参数,实验获得的质子最高截止能量18 MeV,激光能量到质子(4 MeV)的转换效率为1.07%.

Charged paricle activation analysis for characterizing parameters of laser-accelerated protons

The protons accelerated by ultra-high intensity laser have been extensively studied. The most commonly used detectors for measuring laser-driven proton are Tomspon parabola ion energy analyser (TP) and filtered nuclear track detectors, such as radiochromic films (RCF). The TP uses a parallel magneto-electric field to distinguish ions. This conventional technique can precisely identify the species and energy spectra of ions. However, the strong electromagnetic field produced by the laser-plasma interaction has an effect on TP, which results in no spatial resolution of TP. The RCF can give the spatial integration spectrum of proton, but it is easy to be saturated and cannot be reused anymore. In this paper, we present a method based on the traditional charged particle activation analysis and the gamma-gamma coincidence measurement to measure the spectrum of protons accelerated by ultra intense lasers. In this method, a copper plate stack is placed in the proton emission direction. Colliding with MeV proton converts ⁶³Cu in the copper plates into radionuclide ⁶³Zn whose decay can be easily observed and measured. Proton spectrum is then recovered from ⁶³Zn decay counts from layers in the copper stack. The layout of diagnostics and the method to solve proton spectrum are discussed in detail and a self-consistent test is given. This spectrum analysis method is used in a laser-driven proton acceleration experiment carried out on XG-Ⅲ laser facility. The results show that protons up to 18 MeV are obtained, and the spatial integrated spectrum and a laser-proton conversion efficiency of 1.07% are achieved. In conclusion, our method has some advantages as a laser-driven ion diagnostic tool. It has no saturation problem and is not affected by strong electromagnetic fields. The basic principle of charged particle activation analysis is based on nuclear reaction, and can be extended to the measuring of other charged particle beams besides protons, such as deuterons, helium ions produced by ultra-high intensity laser.