High-Z X-ray detectors for security imaging

该项目的主要目的是向DSTL霍尔斯特德堡转让高Z半导体X射线探测器的开发和应用方面的专业知识，该探测器是由PPARC资助的萨里大学研究开发出来的。由塞林博士领导的萨里小组是该国领先的高Z半导体探测器开发中心之一，在宽带隙探测器材料的表征以及X射线和伽马射线探测器原型的开发方面拥有特别的专业知识。这项工作主要集中在用于常温硬X射线成像的CdZnTe/CdTe器件。PPARC对这项工作的支持主要来自两个来源：(1)PPARC案例研究(2006/2009年)“用作硬X射线成像探测器的High-Z材料和像素化电子设备的特性”，CCLRC卢瑟福·阿普尔顿实验室(Paul Sell)是合作伙伴；(2)PIPSS奖(2000-2003年)“基于罗克韦尔CMOS读数的镉锌Te像素探测器的特性”，与莱斯特大学空间中心合作。该项目开发了用于未来空间科学应用的第一批原型镉锌碲成像探测器之一。在这项建议中，我们解决了DSTL对能量范围为100keV/1 MeVkeV的X射线成像的特殊要求。在现有的各种X射线探测器中，半导体器件在能量分辨率、空间分辨率和紧凑性方面比基于闪烁体的探测器具有几个优点。然而，作为电荷粒子探测的首选材料，硅的X射线探测效率极低。因此，人们对用作高能X射线探测器的高Z化合物半导体材料有相当大的兴趣，如CdZnTe。对于DSTL所要求的特殊的安全成像应用，即使是CdZnTe也不够敏感，不能满足~200keV以上的光子能量。为此，我们建议开发由非常高Z的材料HgI2和TlBr2制成的辐射探测器，它结合了优异的光子探测效率和良好的光谱性能。基于我们在镉锌Te探测器方面的专业知识，萨里将开发和表征一套原型HgI2和TlBr探测器，这些探测器将被转移到DSTL用于其特定应用。虽然HgI2和TlBr2的光谱性能已经在实验室中得到了验证，但这些相对较新的探测器材料尚未转移到实际应用领域。这些材料由于电荷输运不佳(特别是空穴迁移率低)而存在潜在的问题，这可能会降低大有源体探测器的光谱性能。因此，必须采用一系列仪器技术来最大限度地减少这些有害影响，包括使用Frisch栅格电子结构和使用数字脉冲处理算法。这将需要探测器物理的结合，以实现对设备性能的基本了解，数字数据采集和脉冲处理技术的发展，以及在真实光子束中的探测器测试。通过两个中心之间的密切合作，从这些系列测试中获得的数据将用于优化探测器的性能，以满足DSTL基于特定安全的应用。该项目的最终目标是通过在国防部/DSTL内部直接开发，或通过某种形式的合资公司或其他许可安排，将这一新的探测器技术商业化。

The primary aim of this project is to transfer expertise and knowledge to DSTL Fort Halstead in the development and application of high-Z semiconductor X-ray detectors, which has been developed out of PPARC-funded research at the University of Surrey. The Surrey group, led by Dr Sellin, is one of the country's leading centres for high-Z semiconductor detector development, with particular expertise in the characterisation of wide bandgap detector materials, and the development of prototype X-ray and gamma ray detectors. This work has been focussed on CdZnTe/CdTe devices, for applications in ambient temperature hard X-ray imaging. The PPARC support for this work has come through two main sources; (1) PPARC CASE Studentship (2006 / 2009) 'Characterisation of High-Z materials and pixellated electronics for use as Hard X-ray imaging detectors', with CCLRC Rutherford Appleton Laboratory (Paul Seller) as the collaborating partner; (2) PIPSS award (2000-2003) 'Characterisation of CdZnTe pixel detectors based on Rockwell CMOS Readout', jointly with Leicester University Space Centre. This project developed one of the first prototype CdZnTe pixellated imaging detectors for future space science applications In this proposal we address a specific requirement from DSTL for X-ray imaging over the energy range 100 keV / 1 MeV keV. Of the various X-ray detectors available, semiconductor devices provide several advantages over scintillator-based detectors in terms of energy resolution, spatial resolution, and compactness. However silicon, which is the material of choice for charge particle detection, has an extremely low X-ray detection efficiency. As a result there has been considerable interest in high-Z compound semiconductor materials for use as high energy X-ray detectors, such as CdZnTe. For the particular security imaging application that DSTL require even CdZnTe is not sensitive enough for photon energies above ~200 keV. For this reason we propose to develop radiation detectors made from the very high-Z materials HgI2 and TlBr which combine an excellent photon detection efficiency with good spectroscopic performance. Based on our expertise with CdZnTe detectors, Surrey will develop and characterise a set of prototype HgI2 and TlBr detectors, which will be transferred to DSTL for use in their specific application. Although the spectroscopic performance of both HgI2 and TlBr has been proven in the laboratory these relatively new detector materials have not yet been transferred into the real application areas. There are potential problems due to poor charge transport (especially low hole mobility) in these materials which can degrade the spectroscopic performance in large active volume detectors. Consequently a range of instrumentation techniques must be adopted to minimise these deleterious effects, including the use of a Frisch grid electron structure and the use of digital pulse processing algorithms. This will require a combination of detector physics to achieve a fundamental understanding of the performance of the devices, the development of digital data acquisition and pulse processing techniques, and detector tests in realistic photon beams. By close co-operation between the two centres the data obtained from these series of tests will be used to optimise the performance of the detectors for DSTL's specific security-based applications. The final aim of the project is to commercialise this new detector technology through either direct exploitation within the MoD/DSTL or through some form of joint venture company or other licensing arrangement.