Collaborative Research: ARI-MA: Realizing high performance inorganic scintillators at low cost

1348361 (Williams), 1348139 （Burger）和1348341 （Biswas）。全球核探测架构（GNDA）旨在探测世界范围内非法和不受管制的核材料和放射性材料。广泛部署需要快速准确的检测技术，其灵敏度和分辨率接近当前最先进的闪烁体，但成本要低得多。这种方法的基本前提是已经有无机闪烁体，其光产率和比例接近在~ 5ev带隙范围内可以达到的极限；这个小组已经开发了一个预测模型，它描述了为什么有一个材料参数的最佳点来定义当前性能最高的闪烁体组。这用于缩小该项目中理论、晶体生长和评估搜索的范围，以便他们可以更深入地研究三元和四元成分，同时更深入地研究影响成本和性能的参数。一些最好的新闪烁体宿主实际上是由丰富的（廉价的）元素组成的。目前它们成本高的原因不是材料，而是生长大单晶的产率低。该项目将寻求四种研究途径来解决或回避晶体生长问题，在实现经济探测器的过程中，从结构、成分和物理参数的甜蜜点材料中识别出维克森林、菲斯克、LBNL等人自2010年左右开发的设计规则。降低成本的方法之一是研究如何提高难以大尺寸生长的小晶体块的分段索引匹配组件的比例。这种与波长移动相结合的分段探测器还提供了处理自吸收（光子扩散）的灵活性，这限制了目前SrI2:Eu探测器的尺寸。该小组将测量和模拟分段闪烁体中块的外围如何导致非比例性，以及可以做些什么来改进它。一种超低成本但高风险的可能性是使用化学制造商批量供应的预掺杂SrI2:Eu珠进行指数匹配的颗粒闪烁体。从SrI2:Eu这样的优秀闪烁体开始，通过对分段闪烁体及其建模的经验指导，并将热和化学处理应用于珠状材料，该小组将研究限制颗粒探测器分辨率的物理原因，以及如何提高性能超越先前的指数匹配颗粒闪烁体。第三种降低成本的方法是通过对现有优秀的闪烁体进行硬化和增韧来提高晶体生长的产率，同时注意硬化措施不会降低比例和轻产率。第四个目标是理论和实验研究新的三元和四元晶体，这些晶体具有立方体结构和其他有利于大晶体生长、慢电子热化、差热电子迁移率、好热化电子迁移率和低俄歇速率，从而获得最佳比例和轻产率。三个大学团队在以下方面提供了互补的专业知识和设备：(1)晶体生长和表征；(2)闪烁的超快激光探针和相关的输运、俘获和非线性淬火的数值模拟；(3)候选晶体、缺陷和掺杂剂的电子结构计算。在预测物理模型的指导下，运用这三所大学互补的实验和理论技术阵列来解决在物理参数方面处理的成本问题，以及在伽马和中子探测器中决定比例和光产率的问题。结果，积极或消极，将告知整个领域的模型和方法所采用的。更广泛的影响将是，通过使改进的、负担得起的核监测部署成为可能，加强国家和全球安全；增加在材料技术方面受过培训的美国大学毕业生的数量，这些材料技术是广泛建立和部署此类系统所必需的；并建立一个由三所大学组成的桥梁系统，在东南部三个州的参与大学之间建立本科生、硕士和博士课程的互动和机会，为代表性不足和经济条件差的学生提供进入核探测工作队伍的途径。

1348361 (Williams), 1348139 (Burger), and 1348341 (Biswas). The Global Nuclear Detection Architecture (GNDA) is intended to detect illicit and unregulated nuclear and radiological materials worldwide. Wide deployment calls for quick and accurate detection technology with sensitivity and resolution close to current state-of-the-art scintillators but at much lower cost. The basic premise of this approach is that there are already inorganic scintillators with light yield and proportionality close to the limits that can be achieved in their ~5 eV band gap range; and that this group haswe developed a predictive model that describes why there is a sweet spot of material parameters defining the current group of highest performance scintillators. This is used to narrow the scope of theoretical, crystal-growth, and evaluative searches in this project so that they can be targeted farther into ternary and quaternary compositions and at the same time deeper into parameters that affect cost and performance. Some of the best new scintillator hosts are in fact composed of abundant (cheap) elements. The reason for their current high cost is not materials, but low yield of growing large single crystals. This project will pursue four research avenues to solve or side-step the crystal growth problems on the way to realizing an economical detector from materials in the sweet spot of structures, compositions, and physical parameters identified from the design rules developed by Wake Forest, Fisk, LBNL, and others since about 2010. One of the approaches to reduce cost examines how to improve the proportionality of segmented index-matched assemblies of smaller crystal blocks that are difficult to grow in large sizes. Such segmented detectors combined with wavelength shifting also provide flexibility to deal with self-absorption (photon diffusion) that currently limits the size of SrI2:Eu detectors. The group will measure and model how the peripheries of blocks in segmented scintillators contribute to nonproportionality, and what can be done to improve it. One ultra-lowcost but high-risk possibility involves index-matched granular scintillators using pre-doped SrI2:Eu beads supplied in bulk by chemical manufacturers. Starting with an excellent scintillator like SrI2:Eu, guided by experience with the segmented scintillators and their modeling, and applying thermal and chemical processing to the beaded material, the group will investigate the physics of what limits the resolution of granular detectors and how to advance performance beyond prior index-matched granular scintillators. A third approach to cost reduction will raise the yield of crystal growth by hardening and toughening existing excellent scintillators while taking care that the hardening measures do not degrade proportionality and light yield. A fourth targets theoretical and experimental searches toward new ternary and quaternary crystals with cubic structures and other properties favoring large crystal growth as well as slow electron thermalization, poor hot electron mobility, good thermalized electron mobility, and low Auger rates leading to best proportionality and light yield. Three university teams bring to this quest their complementary expertise and facilities in (1) crystal growth and characterization, (2) ultrafast laser probes of scintillation and associated numerical modeling of transport, trapping, and nonlinear quenching, and (3) electronic structure calculations on candidate crystals, defects, and dopants. The intellectual merit lies in applying this complementary three university array of experimental and theoretical techniques guided by predictive physical models to attack the cost problem treated in terms of physical parameters alongside those determining proportionality and light yield in both gamma and neutron detectors. The results, positive or negative, will inform the whole field on the models and methods employed. The broader impacts will be to increase national and global security by making possible improved, affordable deployment of nuclear monitoring; increasing the pool of U.S. university graduates trained in the materials technologies necessary to build and deploy such systems widely; and instituting a 3- university bridge system of interacting undergraduate, M.S., and Ph.D. programs and opportunities among the participating universities in three southeastern states to provide a path into the nuclear detection workforce for under-represented and economically disadvantaged students.