New halide scintillators for gamma ray detection (2013)

Scintillators are used for the detection of ionizing radiation. Despite decades of intensive search and numerous compounds discovered, there is still a need for materials with improved properties. Recently, several new scintillators with excellent light yield, energy resolution, and proportionality have been discovered. Among them are Eu2+ doped SrI2 and CsBa2I5 first reported in Refs. [1, 2], and LaBr3:Ce improved by various co-dopants discovered in this thesis work. These materials were studied with the goal to understand the mechanism of the scintillation processes, and in order to improve the scintillation properties. The results of these studies are presented in this thesis. Table I summarizes the characteristics of the studied scintillators. Table I. Overview of inorganic scintillators for X-ray and ?-ray detection and their characteristics: density (?), effective atomic number Zeff, light yield, energy resolution (R), principal decay (?), and emission wavelength (?). Scintillator Co-dopant ? (g/cm3) Zeff Light yield (ph/MeV) R (% at 662 keV) ? (ns) ? (nm) LaBr3:5%Ce3+ - 5.3 46.9 76000 2.7 15 370 Li+ 78000 2.7 16 370 Na+ 73000 2.7 17 370 Mg2+ 73000 3.0 15 370 Ca2+ 71000 2.9 18+long 370 Sr2+ 78000 2.05** 18+long 380 Ba2+ 69000 3.7 17+long 370 Scintillator Eu2+ (%) SrI2 0 4.6[1] 50.3 43000 5.5 450 500-650 0.5 61000 3.7 700* 430 0.86 74000 3.3 800* 430 2 90000 2.8 1000* 430 5 85000 3.1 1200* 430 CsBa2I5 0 4.8[2] 54.1 22000 9.6 150 400-600 0.5 62000 5.1 700* 430 5 80000 2.3** 1200* 430 *for 1-3 mm thick samples at 295 K **measured with a super bialkali R6231-100 PMT In chapter 4, we for the first time demonstrate that several scintillation properties of LaBr3:5%Ce3+ can be significantly improved by Ca2+, Sr2+, or Ba2+ co-doping. Energy resolution of LaBr3:5%Ce,Sr shows record low values of 2% at 662 keV and 6.5% at 59.5 keV. The proportionality of the 8 keV- 1.33 MeV ?-ray response approaches the ideal response much closer as compared to standard LaBr3:5%Ce. The origin of these improvements is a better proportionality of the ~0.5-30 keV electron response. Ca2+, Sr2+, or Ba2+ co-doping creates charge carrier trapping centers in LaBr3:Ce. This results in multiple thermoluminescence glow peaks at temperatures below 350 K, ?s-long decay time components at 295 K, and decrease of the light yield at temperatures below 295 K. Li+, Na+, and Mg2+ co-dopants neither improve proportionality, nor create charge carrier traps. Chapter 5 concerns optical spectroscopy and decay time studies of LaBr3:5%Ce,Sr. Three different Ce3+ lattice sites are revealed. Ce3+ on an unperturbed site has the same optical properties as Ce3+ in standard LaBr3:Ce. The 4f?5d1 excitation and 5d1?4f emission bands of Ce3+ on the perturbed sites are red-shifted. The Ce3+ emission decay time in LaBr3:Ce,Sr is longer than in standard LaBr3:Ce. This is ascribed to a longer radiative lifetime of the excited state of Ce3+ on the perturbed sites and to self-absorption of Ce3+ emission. Lowering of the 5d1 level is attributed to a larger crystal field interaction on the perturbed sites. Two types of the crystal lattice point defects on the perturbed sites were proposed to explain the obtained results: the Br- vacancy and an interstitial site (0,0,z) occupation. A presence of Ce4+ ions as charge compensation centers for Sr2+ on La3+ sites was not confirmed. The studies presented in chapters 4 and 5 are a first attempt to explain the effects of co-dopants on the scintillation properties of LaBr3:Ce. Several issues require further investigation. First, we expect that by varying the co-doping concentration or by using different type of co-dopants the proportionality can be further engineered towards the ideal one. Second, the origin of the proportionality improvement remains unclear. Chapters 6 to 9 deal with Eu2+ doped scintillators. In chapters 6 and 7, the scintillation properties and self-absorption of SrI2:Eu are treated, and in chapter 9 those of CsBa2I5:Eu. Both scintillators possess very high light yield, excellent energy resolution and proportionality, but moderate density and Zeff. The decay time is strongly affected by self-absorption of Eu2+ emission. For example, the decay time constant lengthens from 400 ns for a 1 mm thick SrI2:5%Eu crystal at 78 K to 8 ?s for a 7.5 mm thick crystal at 600 K. The self-absorption red-shifts and narrows the Eu2+ emission peak. The light yield and energy resolution are strongly affected by temperature, sample size, and Eu concentration as well. To explain the obtained results, the self-absorption model was developed and applied to the experimental data. The self-absorption probability a and quantum efficiency ? of Eu2+ emission were derived from the model. a varies from 0.05 to 0.98 depending on temperature (78-600K), Eu concentration (0.5-5%), and sample thickness (1-7mm). ? was found to be 0.95 ± 0.05 for temperatures up to 600 K. The radiative lifetime of the Eu2+ 4f65d excited state is 350-400 ns in both SrI2:Eu and CsBa2I5:Eu. Worsening of light yield and energy resolution with increase of temperature and sample size can be attributed to self-absorption of Eu emission in combination with ?<1. Losses due to electron-hole recombination processes prior to excitation of Eu2+ and to poor optical crystal quality and light collection efficiency cannot be excluded. The analysis of energy resolution of SrI2:Eu in chapter 8 shows that it is also affected by the proportionality of the SrI2:Eu response. That proportionality varies with temperature, and it is optimal at 295 K. Chapters 6, 7, and 9 present a simple model of self-absorption that explains the obtained results fairly well. Nevertheless, several aspects require further research. First, undoped SrI2 and CsBa2I5 compounds both emit broad bands at 360-400 nm and 500-600 nm. The origin of these bands and the mechanisms of energy transfer from the host lattice to Eu2+ remain unclear. Second, for the application of large sized SrI2:Eu and CsBa2I5:Eu crystals one needs to reduce the effects of self-absorption. The search for methods other than the decrease of the sample size, Eu concentration, or temperature remains an important task. [1] N. J. Cherepy, G. Hull, A. D. Drobshoff, et al., Appl Phys Lett 92 (2008) 083508. [2] E. D. Bourret-Courchesne, G. Bizarri, R. Borade, et al., Nucl Instr Meth A 612 (2009) 138.