Selection and Optimization of Radiation Detector Materials

Threat reduction and nonproliferation activities urgently require improved radiation detectors. As such, it is vital that we move beyond the largely empirical approach of detector material discovery and optimization. We propose to integrate atomic scale computer simulation and experimental material science, to discover and optimize candidate scintillator detector material compositions. When appropriately coupled, these techniques will create a physics-based feedback loop, which will lead to an approach through which it is possible to optimize the energy resolution of candidate scintillators. Furthermore, this approach is independent of material type (system). Although single crystals are used here to determine scintillator properties, improvements in the understanding and control of defects can be incorporated into other material forms (e.g. nanophosphors or polycrystalline scintillators).While nonproliferation and security activities are beneficiaries of the proposed work, other activities will also directly benefit, such as high resolution radiography for passive evaluation of nuclear power installations. Furthermore, an active industrial market interested in detector development exists for applications such as oil well logging and medical imaging.The general requirements for detector materials are that they are dense (stopping power), bright (conversion of incident radiation energy to light output) and fast (quickly convert the incident energy to light output). While many current detector materials offer some of these properties (e.g. Tl doped NaI is bright and fast but not dense) there are families of compositions that offer improvements, in particular, rare earth oxides (which are much more dense) and halides (which are brighter).The majority of solid state systems for radiation detection require that the incident energy excites an electron that is initially associated with an activator ion embedded in a host lattice. Subsequently, the electron returns to the ground state and light is emitted (that can be detected electronically to produce a signal). This scintillation process depends crucially on the behaviour of the electron (and hole) and hence on the local environment of the activator ion in the crystal as well as the propensity for electrons or holes to become trapped at other defect sites in the lattice. Here three series of host materials and activators will be investigated, as a function of their constituent chemical species, using atomic scale computer simulation and experimental techniques and the results correlated with observed detector efficiency. By predicting defect behaviour, the atomic scale simulations will identify compositional regions of potential significance. Subsequently the experimental work, single crystal growth, luminescence, site selective excitation and Raman spectroscopy, will focus on the specific compositions and determine their properties. This provides a test of the simulation approach in addition to a verification of the efficacy of the materials as luminescence based radiation detectors. The combined approach will allow for a vital, defect property-based optimization, where historically improvements have been empirical.