Advanced Compton-geometry gamma radiation imaging for radionuclide measurement in soils and geomaterials

受地震破坏的日本福岛第一核电站不断释放放射性物质，对水、沿海环境、农业用地、动物和人类健康构成风险，这引起了国际关注。这一事件与一代人之前的切尔诺贝利灾难一起，突出了能够探测、测量和监测我们环境中的辐射的重要性。这不是一项容易的挑战——放射性量通常很低（相对于受控的医疗或工业环境），或者在土壤、沉积物和水中高度分散。我们周围也有相当大的背景辐射，不仅来自人类核技术的遗产，而且来自天然矿物、气体（例如二氧化碳）。氡（某些地区的一个主要问题）、宇宙和太阳能。另一方面，这种放射性被地球和环境科学家广泛用于测定岩石的年代，监测沉积物运动和地貌变化，或植物和动物的生长速度和生活史。如果我们要测量环境放射性，不仅是为了帮助事故释放后的清理和恢复，也是为了监测场地，防止泄漏，支持核设施的安全运行和退役（以及支持这一系列的科学研究需求），那么我们需要不断改进传感器，使其能够检测和量化辐射源，以更高的分辨率，更低的检测阈值和更短的测量时间。当前这一代传感器基于机械准直器，这是一种类似于数码相机中的“像素化”图像传感器的技术，在这种技术中，到达表面任何一点的辐射被用来建立辐射源的二维图像。利物浦大学的核物理学家最近开发了一种探测伽马辐射的新方法，称为康普顿几何成像。在这种方法中，两个传感器一个放在另一个的前面，测量是基于它们之间的辐射散射。这项技术是强大的，因为辐射源的位置是通过数学重建许多散射事件的起源来确定的，而不是通过入射辐射在准直器表面上的物理位置。这种“电子”准直可以比机械准直具有更高的精度和灵敏度来解决源的位置，具有能够在3D中定位源的优势，并且产生更小，更轻的探测器设备，可以节省测量时间。目前，世界上只有另外两个研究小组在研究这项技术。本提案的目的是了解如何将这种强大的新技术优化用于环境伽马放射性测量。到目前为止，研究主要集中在工业和医疗应用的康普顿相机原型的开发上，这对前面描述的环境条件提出了非常不同的挑战。通过与环境科学和工程领域的学术和行业最终用户密切合作，结合世界领先的设备开发专业知识，这项技术概念验证提案旨在制定设计标准，优化系统规格，以及康普顿相机的第一个原型，我们打算为下一代环境放射性传感器设定基准。想象一下，能够定位地下的放射性物质，并监测它如何随着水流或沉积物运动的变化而移动。或者使用便携式设备实时观察植物和动物如何从受污染的土壤中吸收放射性物质并将其转移到食物链中。星际迷航科学？也许现在是这样，但环境康普顿相机是这个研究项目的长期目标，它使我们朝着这个愿景迈出了重要的一步。

The continued releases of radioactive material from the earthquake-damaged Fukushima Dai-ichi nuclear power station in Japan, with the risks to water, coastal environments, agricultural land, animals and human health have drawn international concern. The incident, together with the Chernobyl disaster a generation earlier, has highlighted the importance of being able to detect, measure and monitor radiation in our environment. This is no easy challenge - the amounts of radioactivity are often low (relative to controlled medical or industrial settings) or highly dispersed through soils, sediments and water. There is also a considerable background radiation all around us, not only from the legacy of human nuclear technology but from natural minerals, gases (eg. radon, a major problem in some regions), cosmic and solar sources. On the other hand, this radioactivity is used widely by earth and environmental scientists to date rocks, monitor sediment movement and geomorphological changes, or the growth rates and life histories of plants and animals. If we are to measure environmental radioactivity, not just to help clean-up and recovery after an accidental release but also to monitor sites, prevent releases and support the safe operation and decommissioning of nuclear facilities (as well as support that range of scientific research needs), then we need continuous improvement of sensors which can detect and quantify radiation sources to higher resolution, lower detection thresholds and shorter measurement times. The current generation of sensors is based on mechanical collimators, a technology similar to the 'pixellated' image sensors in digital cameras, in which the radiation arriving at any point on the surface is used to build up a 2D image of the radiation source. Nuclear physicists at the University of Liverpool have recently developed a new approach for detection of gamma radiation called Compton-geometry imaging. In this approach, two sensors are placed one in front of the other and the measurement is based on the scattering of radiation between them. The technique is powerful because the position of the radiation source is located by mathematically reconstructing the origin of many scattering events, rather than by the physical position of the incident radiation on the collimator surface. This 'electronic' collimation can resolve the position of the source with much greater accuracy and sensitivity than mechanical collimation, has the advantage of being able to locate the source in 3D, and yields smaller, lighter detector equipment with potential savings in measurement time. Currently, only two other research groups in the world are working with this technology.The objective of this proposal is to understand how this powerful new technology can be optimised for environmental gamma radioactivity measurements. Research so far has focused on the development of prototype Compton cameras for industrial and medical applications, which present very different challenges to the environmental conditions described earlier. By combining a world leading expertise in device development in close collaboration with academic and industry end-users in environmental science and engineering, this Technology Proof-of-Concept proposal aims to develop design criteria, optimised system specifications, and a first prototype for a Compton camera which we intend will set a benchmark for the next generation of environmental radioactivity sensors. Imagine being able to locate a radioactive substance beneath the ground and monitor how it moves with changes in water flow or sediment movement. Or to watch, using a portable device, in real-time how plants and animals take up radioactive materials from contaminated soils and move them into the food chain. Star Trek science? Perhaps for now, but the environmental Compton camera that is the long-term goal of this research project moves us a significant step closer towards that vision.