Improvement of the sensitivity of germanium detectors for safety and security applications

伽马辐射的探测在安全和安保领域有着广泛的应用。伽马辐射可以独特地用于识别各种同位素，并提供存在的同位素数量的定量测量。探测仪可在离被调查物品很近或几米远的地方使用。目前锗探测器的局限性之一是它们对伽马辐射探测的峰本底响应较差。当伽马射线与锗相互作用时，主要机制是通过康普顿散射。这就产生了一个连续的背景，在这个背景上，光峰位于能谱中。光峰（可以识别伽马射线）的计数通常占光谱总数的20-30%。光峰强度取决于锗晶体的尺寸（和成本）。问题是，如果高能量伽马射线的背景康普顿连续体隐藏了低能量的光峰，一些同位素可能会被遗漏（或难以识别和量化）。特殊核材料241Am就是一个例子。这可以通过60 keV的伽马射线来识别，但这通常被康普顿连续高能量伽马射线所掩盖，例如来自137Cs源的662 keV伽马射线。这两种同位素都可能存在于放射性废物中。在这个项目中，利物浦大学将与堪培拉（一个探测器和仪器供应商）合作，通过增加锗探测器对背景响应的峰值来提高其灵敏度。所使用的方法将是研究探测器对不同伽马射线能量和探测器体积内不同相互作用位置的电荷收集响应。当低能量伽马射线被探测器（通常在表面附近）吸收时的响应与由高能量伽马射线的康普顿散射引起的整个探测器的相互作用之间预计会存在差异。该项目将研究如何利用这些信息通过消除康普顿背景有效地提高探测器的响应。将研究许多不同尺寸的探测器，以检查效果如何随晶体尺寸而变化。同样标称尺寸的探测器也将被研究，以检查由晶体性质引起的变化，如杂质浓度。所开发的算法将使用数字电子设备实施，以便用户可以选择一种选项，允许探测器对正在调查的情况做出更好的响应（例如，在137Cs存在的情况下，241Am）。堪培拉希望这一发展能够改善其探测器和仪器在许多不同安全和安保应用中的反应。

The detection of gamma radiation is widely used in safety and security applications. Gamma radiation can uniquely be used to identify a wide range of isotopes and to provide a quantitative measurement of the amount of the isotope present. The detectors can be used either close to or a number of metres away from the item(s) under investigation. One of the current limitations of germanium detectors is their poor peak to background response to the detection of the gamma radiation. When a gamma ray interacts with the germanium the principal mechanism is through Compton scattering. This results in a continuous background on which the photopeak sits in the energy spectrum. The number of counts in the photopeak (that allows the gamma ray to be identified) is typically 20-30% of the total in the spectrum. The photopeak intensity depends on the size (and cost) of the germanium crystal.The problem is that some isotopes can be missed (or be difficult to identify and quantify) if the background Compton continuum from higher energy gamma rays hides lower energy photopeaks. One example is the Special Nuclear Material 241Am. This can be identified through a 60 keV gamma ray, but this is often hidden by the Compton continuum of higher energy gamma rays such as the 662 keV gamma ray from a 137Cs source. Both of these isotopes are potentially present in radioactive waste.In this project the University of Liverpool will work with Canberra (a detector and instrument supplier) to improve the sensitivity of a germanium detector by increasing its peak to background response. The method used will be to investigate the charge collection response of the detector to different gamma-ray energies and different interaction positions within the detector volume. It is expected that there will be a difference between the response when a low energy gamma ray is absorbed by the detector (usually near the surface) and interactions throughout the detector arising from Compton scattering of higher energy gamma rays. The project will investigate how this information can be used to effectively improve the response of the detector by eliminating Compton background.A number of different detectors sizes will be investigated to check how the effect varies with the crystal dimensions. Detectors of the same nominal size will also be investigated to check for variations arising from the crystal properties such as the impurity concentration. The algorithms developed will be implemented using digital electronics so that the user can select an option that allows a better response of the detector for the situation under investigations (e.g. 241Am in the presence of 137Cs).Canberra expect this development to lead to an improved response of their detectors and instruments in a number of different safety and security applications.