Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA.
University of Massachusetts Lowell, Lowell, MA, USA.
Med Phys. 2021 Apr;48(4):1921-1930. doi: 10.1002/mp.14717. Epub 2021 Feb 16.
This is a computational study to develop a rugged self-powered Radioisotope Identification Device (RIID). The principle of operation relies on the High Energy Current (HEC) concept (Zygmanski and Sajo, Med Phys. 43 4-15, 2016) with measurement of fast electron currents between low-Z and high-Z thin-film electrodes separated by nanoporous aerogel films in a multilayer detector structure whose prototypes were previously investigated (Brivio, Albert, Freund, Gagne, Sajo and Zygmanski, Med Phys, 46 4233-4240, 2019), (Brivio, Albert, Gagne, Freund, Sajo and Zygmanski, J Phys D Appl Phys, 53 265303, 2020). Here, we present an optimal detector design that accounts for a wide energy range (keV-MeV) of x-ray-emitting radioisotopes that are of interest to national security and radiation therapy.
We studied numerous multilayer detector geometries with N = 1..24 basic detector elements composed of 3 electrodes: N x (Al-aerogel-Ta-aerogel-Al). The thicknesses of electrodes and their total number were varied depending on the incident x-ray spectra and its ability to penetrate and interact with the different layers, producing fast electrons. We used radiation transport simulations to find a balanced geometry that accounts for all energies from 10 keV to 6 MeV in a single design with relatively few detector elements (N = 24). In the balanced design, the electrodes have increasing thickness as a function of depth in the detector, ranging from 0.5 μm-Ta and 10 μm-Al at the entrance to 10 mm-Ta and 2.5 mm-Al at the exit. Aerogel thickness was fixed at 50 μm. Electron currents forming RIID signals were acquired from all Ta electrodes. A model function M(x, E ) representing the detector yield as a function of the cumulative Ta thickness (x) for 70 monoenergetic incident beams (E) was derived. We also investigated the detector response to selected radioactive isotopes (Pd-103, I-125, Pu-239, U-235, Ir-192, Cs-137, Co-60). Additional studies were performed with Bremsstrahlung spectra produced by electron beams in kVp tubes and in MV Linacs used in radiology and radiation therapy departments. We investigated different algorithms for radioisotope identification that would work for unknown unshielded as well as shielded sources.
Characteristic features of response functions for monoenergetic beams and radioisotopes were determined and used to develop two inverse algorithms of radioisotope identification. Using these algorithms, we were able to identify the unshielded and shielded sources, quantify the minimum, mean and maximum effective energies of the shielded spectra, and estimate the amount of Compton background in the spectrum.
A multilayer sensor based on fast electron current was optimized and studied in its abilities as RIID. A balanced design permits the identification of radioisotopes with of a wide range of keV-MeV energies. The device is low cost, rugged, self-powered and can withstand very high dose rates, allowing deployment in difficult conditions, including radiation incidents. The algorithm we developed for radioisotope identification and spectral unfolding is robust and it is an important component in practical applications.
本研究旨在开发一种坚固耐用的自给式放射性同位素识别装置（RIID）。其工作原理依赖于高能电流（HEC）概念（Zygmanski 和 Sajo，Med Phys. 43 4-15，2016），通过测量在纳米多孔气凝胶薄膜分离的低 Z 和高 Z 薄膜电极之间的快速电子电流来实现，该多层探测器结构的原型先前已进行了研究（Brivio、Albert、Freund、Gagne、Sajo 和 Zygmanski，Med Phys，46 4233-4240，2019），（Brivio、Albert、Gagne、Freund、Sajo 和 Zygmanski，J Phys D Appl Phys，53 265303，2020）。在这里，我们提出了一种优化的探测器设计，该设计考虑了宽能区（keV-MeV）的 X 射线发射放射性同位素，这些同位素对国家安全和放射治疗感兴趣。
我们研究了许多具有 N = 1..24 个基本探测器元件的多层探测器几何形状，每个元件由 3 个电极组成：N x（Al-aerogel-Ta-aerogel-Al）。电极的厚度和总数根据入射 X 射线光谱及其穿透和与不同层相互作用产生快速电子的能力而变化。我们使用辐射传输模拟来找到一个平衡的几何形状，该形状可以在单个设计中考虑从 10 keV 到 6 MeV 的所有能量，并且相对较少的探测器元件（N = 24）。在平衡设计中，电极的厚度随探测器深度的增加而增加，从入口处的 0.5 μm-Ta 和 10 μm-Al 到出口处的 10 mm-Ta 和 2.5 mm-Al。气凝胶厚度固定为 50 μm。从所有 Ta 电极采集形成 RIID 信号的电子电流。推导了一个模型函数 M（x，E），代表探测器的产率作为累积 Ta 厚度（x）的函数，用于 70 个单能入射光束（E）。我们还研究了探测器对选定放射性同位素（Pd-103、I-125、Pu-239、U-235、Ir-192、Cs-137、Co-60）的响应。在用于放射学和放射治疗部门的 kVp 管和 MV 直线加速器中的电子束产生的韧致辐射光谱中进行了额外的研究。我们研究了用于识别未知未屏蔽和屏蔽源的放射性同位素的不同算法。
确定了单能束和放射性同位素响应函数的特征，并用于开发两种放射性同位素识别的逆算法。使用这些算法，我们能够识别未屏蔽和屏蔽的源，量化屏蔽光谱的最小、平均和最大有效能量，并估计光谱中的康普顿背景量。
优化了基于快速电子电流的多层传感器，并研究了其作为 RIID 的能力。平衡设计允许识别具有宽 keV-MeV 能量范围的放射性同位素。该设备成本低、坚固耐用、自给自足，能够承受非常高的剂量率，允许在包括辐射事件在内的困难条件下部署。我们开发的用于放射性同位素识别和光谱展开的算法是稳健的，是实际应用中的重要组成部分。
