Zygmanski Piotr, Shrestha Suman, Briovio Davide, Karellas Andrew, Sajo Erno
Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02115.
Department of Radiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655.
Med Phys. 2016 Jan;43(1):16. doi: 10.1118/1.4935532.
The authors experimentally investigate the effect of direct energy conversion of x-rays via selfpowered Auger- and photocurrent, potentially suitable to practical radiation detection and dosimetry in medical applications. Experimental results are compared to computational predictions. The detector the authors consider is a thin-film multilayer device, composed of alternating disparate electrically conductive and insulating layers. This paper focuses on the experiments while a companion paper introduces the fundamental concepts of high-energy current (HEC) detectors.
The energy of ionizing radiation is directly converted to detector signal via electric current induced by high-energy secondary electrons generated in the detector material by the incident primary radiation. The HEC electrons also ionize the dielectric and the resultant charge carriers are selfcollected due to the contact potential of the disparate electrodes. Thus, an electric current is induced in the conductors in two different ways without the need for externally applied bias voltage or amplification. Thus, generated signal in turn is digitized by a data acquisition system. To determine the fundamental properties of the HEC detector and to demonstrate its feasibility for medical applications, the authors used a planar geometry composed of multilayer microstructures. Various detectors with up to seven conducting layers with different combinations of materials (250 μm Al, 35 μm Cu, 100 μm Pb) and air gaps (100 μm) were exposed to nearly plane-parallel 60-120 kVp x-ray beams. For the experimental design and verification, the authors performed coupled electron-photon radiation transport computations. The detector signal was measured using a commercial data acquisition system with 24 bits dynamic range, 0.4 fC sensitivity, and 0.9 ms sampling time.
Measured signals for the prototype detector varied depending on the number of layers, material type, and incident photon energy, and it was in the range of 30-150 nA/cm(2) for unit air kerma (1 Gy), which is viable for practical applications. The experiments had an excellent agreement with the computations. Within the examined range of 60-120 kVp, the energy dependence of the HEC (normalized to the x-ray tube output) was relatively small.
Based on the experimental results for 100 ms sampling time, it would be possible to measure the time dependence of x-ray beams for x-ray tube current of 0.1 mA or higher. Significant advantages of the HEC device are that generation of its signal does not require external power supply, it can be made in any size and shape, including flexible curvilinear forms, and it is inexpensive. It remains to be determined, which of the potential applications in medical dosimetry (both in vivo and external), or radiation protection would benefit from such selfpowered detectors.
作者通过自供电俄歇电流和光电流对X射线进行直接能量转换的实验研究，这可能适用于医学应用中的实际辐射检测和剂量测定。将实验结果与计算预测进行比较。作者所考虑的探测器是一种薄膜多层器件，由交替的不同导电层和绝缘层组成。本文重点关注实验，而一篇配套论文介绍了高能电流（HEC）探测器的基本概念。
电离辐射的能量通过入射初级辐射在探测器材料中产生的高能二次电子感应的电流直接转换为探测器信号。HEC电子也使电介质电离，并且由于不同电极的接触电位，产生的电荷载流子被自收集。因此，无需外部施加偏置电压或放大，就能以两种不同方式在导体中感应出电流。这样，产生的信号进而由数据采集系统数字化。为了确定HEC探测器的基本特性并证明其在医学应用中的可行性，作者使用了由多层微结构组成的平面几何结构。将各种具有多达七个导电层、不同材料组合（250μm铝、35μm铜、100μm铅）和气隙（100μm）的探测器暴露于近平面平行的60 - 120 kVp X射线束下。为了进行实验设计和验证，作者进行了电子 - 光子耦合辐射传输计算。使用具有24位动态范围、0.4 fC灵敏度和0.9 ms采样时间的商用数据采集系统测量探测器信号。
原型探测器的测量信号因层数、材料类型和入射光子能量而异，对于单位空气比释动能（1 Gy），其范围为30 - 150 nA/cm²，这在实际应用中是可行的。实验与计算结果具有很好的一致性。在60 - 120 kVp的检查范围内，HEC（归一化到X射线管输出）的能量依赖性相对较小。
基于100 ms采样时间的实验结果，对于0.1 mA或更高的X射线管电流，有可能测量X射线束的时间依赖性。HEC器件的显著优点是其信号产生不需要外部电源，可以制成任何尺寸和形状，包括柔性曲线形式，并且成本低廉。在医学剂量测定（体内和体外）或辐射防护中的哪些潜在应用将受益于这种自供电探测器，仍有待确定。
