Image plate multi-scan response to fusion protons in the range of 1–14 MeV

Image plates (IPs) are a quickly recoverable and reusable radiation detector often used to measure proton and x-ray fluence in laser-driven experiments. Recently, IPs have been used in a proton radiography detector stack on the OMEGA laser, a diagnostic historically implemented with CR-39, or radiochromic film. The IPs used in this and other diagnostics detect charged particles, neutrons, and x-rays indiscriminately. IPs detect radiation using a photo-stimulated luminescence (PSL) material, often phosphor, in which electrons are excited to metastable states by ionizing radiation. Protons at MeV energies deposit energy deeper into the IP compared with x rays below ~20 keV due to the Bragg peak present for protons. This property is exploited to discriminate between radiation types. Doses of mono-energetic protons between 1.7 and 14 MeV are applied to IPs using the MIT linear electrostatic ion accelerator. This paper presents the results from consecutive scans of IPs irradiated with different proton energies. The PSL ratios between subsequent scans are shown to depend on proton energy, with higher energy protons having lower PSL ratios for each scan. This finding is separate from the known energy dependence in the absolute sensitivity of IPs. The results can be compared to complimentary work on x rays, showing a difference between protons and x rays, forging a path to discriminate between proton and x-ray fluence in mixed radiation environments.

成像板（Image Plates，IPs）是一类可快速回收、重复使用的辐射探测器，常应用于激光驱动实验中以测量质子与X射线注量。近年来，IPs被应用于OMEGA激光器的质子射线照相探测器堆叠系统中；该诊断装置此前通常采用CR-39或辐射变色薄膜（radiochromic film）作为核心探测介质。本研究及其他同类诊断装置所使用的IPs，可无差别地探测带电粒子、中子与X射线。IPs依托光激发发光（Photo-Stimulated Luminescence，PSL）材料（通常为磷光体）实现辐射探测：电离辐射可将该材料中的电子激发至亚稳态能级。相较于能量低于约20 keV的X射线，兆电子伏特（MeV）能级的质子会在IPs内部沉积更深的能量，这一现象源于质子特有的布拉格峰（Bragg Peak）效应。科研人员可利用这一特性实现不同辐射类型的甄别。本研究借助麻省理工学院（MIT）的线性静电离子加速器，对IPs辐照了能量介于1.7 MeV至14 MeV之间的单能质子。本文报道了对经不同质子能量辐照的IPs进行连续扫描所得到的实验结果。实验结果表明，相邻两次扫描的PSL比值与质子能量相关，每次扫描过程中，质子能量越高，其对应的PSL比值越低。该发现与IPs绝对灵敏度的已知能量依赖特性相互独立。本研究结果可与X射线领域的相关互补研究进行对照，二者清晰展现了质子与X射线间的差异，为混合辐射环境下甄别质子与X射线注量开辟了可行路径。