Inorganic scintillators for radiation detection [electronic resource] : high pressure and cryogenic temperature studies

Inorganic scintillators are typically single-crystalline halide and oxide compounds that convert ionizing radiation into visible photons. Due to this property, scintillators are used in homeland security, medical imaging, high-energy physics, and petroleum engineering applications. While the required set of characteristics vary by application, the ideal scintillator possesses a high light yield (greater than 50,000 photons/MeV), a proportional light yield response, a high energy resolution (less than 5%), and a short decay time (less than several hundred nanoseconds). The preference for such properties has motivated an ongoing search for new materials as well as a growing interest in scintillator optimization. The approach to scintillator discovery and optimization began with observation and trial-and-error, and now includes extrapolation and combinatorial chemistry. However, further progress will require methods that are more time- and cost-efficient. This dissertation introduces a novel, high-throughput approach for the identification of new and proportional scintillators through complementary efforts in computation and experimentation. In the first phase of study, a comparison of computed scintillator band structures revealed that compounds with effective mass ratios close to unity exhibit greater light yield proportionality. To verify this correlation, isostatic pressure was selected as a physical parameter to probe scintillator structure, and consequently, the correlation between the effective mass ratio and proportionality. A unique, 1 GPa isostatic press with the ability to accommodate large, bulk samples was designed and constructed. The press was successfully used to characterize the light yield, emission spectra, and decay time of three widely-used scintillators, CsI(Tl), NaI(Tl), and LaBr3(Ce). It was discovered that the observed evolution of light output and decay time with pressure was directly related to shifts in their emission wavelengths. The pressure-dependency of proportionality was also determined for LaBr3(Ce) and found to agree with predictions. The promising results of this investigation strongly suggest that the effective mass ratio is an important predictive parameter in the search for new and proportional scintillators. Preliminary scintillation and x-ray diffraction studies on uniaxially-deformed CsI(Tl) also explored the influence of defects in scintillation performance. The remainder of the dissertation addresses the temperature-dependent behavior of two well-known scintillators, SrI2(Eu) and Lu2SiO5(Ce), from 4.3 to 300 K. To date, SrI2(Eu) has been reported as the brightest scintillator. It has an excellent proportional response, but a poor decay time of 1 [microsecond]. The studies described in this work revealed the radiative lifetime of europium in SrI2 to be much shorter, 0.37 [microsecond]. In fact, the previously reported 1 [microsecond] decay time is a result of emission-reabsorption between the europium ions, which are typically present in the high dopant concentration of 5 atomic%. In contrast to SrI2(Eu), Lu2SiO5(Ce) is favored for its short decay time of 40 ns, but has a mediocre light yield, energy resolution, and proportionality. The presented studies on Lu2SiO5(Ce) reveal the evidence of a defect, most likely an oxygen vacancy, localized to the cerium dopant. The presence of this defect is reflected in a poor energy resolution, degraded proportionality response, and a shorter decay time. Following these temperature studies, recommendations are presented for performance optimization.

Keywords: Materials Science and Engineering.
Advisor: Cui Yi
Feigelson Robert S.
McIntyre Paul Cameron advisor. Thesis advisor