Abstract-Transparent polycrystalline ceramic scintillators based on the garnet structure and incorporating gadolinium for high stopping power are being developed for use in gamma spectrometers. Optimization of energy resolution for gamma spectroscopy involves refining the material composition for high stopping and high light yield, developing ceramics fabrication methodology for material homogeneity, as well as selecting the size and geometry of the scintillator to match the photodetector characteristics and readout electronics. We have demonstrated energy resolution of 4% at 662 keV for 0.05 cm 3 GYGAG(Ce) ceramics with photodiode readout, and 4.9% resolution at 662 keV for 18 cm 3 GYGAG(Ce) ceramics and PMT readout. Comparative gamma spectra acquired with GYGAG(Ce) and NaI(Tl) depict the higher resolution of GYGAG(Ce) for radioisotope identification applications. Light yield nonproportionality of garnets fabricated following different methods reveal that the fundamental shapes of the light yield dependence on energy are not intrinsic to the crystal structure, but may instead depend on trap state distributions. With exposure to 9 MeV Brehmsstrahlung radiation, we also find that GYGAG(Ce) ceramics exhibit excellent radiation hardness.

Abstract-High energy resolution gamma-ray detectors that can be formed into relatively large sizes while operating at room temperature offer many advantages for national security applications. We are working toward that goal through the development of SrI 2 (Eu) scintillator detectors, which have already achieved <3.0% energy resolution at 662 keV with volumes >10 cm 3 . In this study, we have tested pure, undoped SrI 2 to gain a better understanding of the scintillation properties and spectroscopic performance achievable without activation. An undoped crystal grown from 99.999% pure SrI 2 pellets was tested for its spectroscopic performance, its light yield, and uniformity of scintillation light collection as a function of gammaray interaction position relative to the crystal growth direction. We measured an energy resolution of 5.3% at 662 keV, and the light collection non-uniformity varied by only 0.72% over the length of the crystal. Measurements of both a 3% Eu-doped and the undoped SrI 2 detector were carried out in the SLYNCI facility to determine their light yield non-proportionality, which will be reported. The surprisingly good scintillation properties of the pure SrI 2 crystal suggests that with high-purity feedstock, further reduction of the Eu concentration can be made to grow larger crystals while not adversely impacting the spectroscopic performance.

Thallium bromide (TlBr) is a compound semiconductor with a band gap of 2.68 eV making it ideal for room temperature radiation detection. The high atomic numbers, 81 and 35, and the high density of 7.56 g/cm 3 give it excellent gamma-ray stopping power. TlBr is a cubic material that melts congruently at a relatively low temperature (∼480 • C). These properties make it relatively easy to grow good quality crystals with high yield. As a result of improvements in the purification of TlBr mobility-lifetime product of electrons, µτ e , is now on the order of 10 −2 cm 2 /V, which is similar to that of CZT. High µτ e enables the fabrication of thicker detectors with good charge collection and energy resolution. The properties of TlBr make it ideal for use in room temperature gamma radiation operation [1]. Single carrier devices such as small pixel arrays [2] and Frisch collar devices [3] which were developed for CZT can also been applied to TlBr. For example, better than 1% FWHM at 662 keV has been obtained for single pixel events with small (e.g., 3 × 3 pixels, 1-mm pitch, 5-mm thick) arrays.

Recently discovered scintillators for gamma ray spectroscopy, single crystal SrI 2 (Eu), GYGAG(Ce) transparent ceramic and Bismuth-loaded plastics, offer resolution and fabrication advantages compared to commercial scintillators, such as NaI(Tl) and standard PVT plastic. Energy resolution at 662 keV of 2.7% is obtained with SrI 2 (Eu), while 4.5% is obtained with GYGAG(Ce). A new transparent ceramic scintillator for radiographic imaging systems, GLO(Eu) offers high light yield of 70,000 Photons/MeV, high stopping, and low radiation damage. Implementation of single crystal SrI 2 (Eu), Gd-based transparent ceramics, and Biloaded plastic scintillators can advance the state-of-the art in ionizing radiation detection systems.

Abstract-Transparent polycrystalline ceramic scintillators based on the garnet structure and incorporating gadolinium for high stopping power are being developed for use in gamma spectrometers. Optimization of energy resolution for gamma spectroscopy involves refining the material composition for high stopping and high light yield, developing ceramics fabrication methodology for material homogeneity, as well as selecting the size and geometry of the scintillator to match the photodetector characteristics and readout electronics. We have demonstrated energy resolution of 4% at 662 keV for 0.05 cm 3 GYGAG(Ce) ceramics with photodiode readout, and 4.9% resolution at 662 keV for 18 cm 3 GYGAG(Ce) ceramics and PMT readout. Comparative gamma spectra acquired with GYGAG(Ce) and NaI(Tl) depict the higher resolution of GYGAG(Ce) for radioisotope identification applications. Light yield nonproportionality of garnets fabricated following different methods reveal that the fundamental shapes of the light yield dependence on energy are not intrinsic to the crystal structure, but may instead depend on trap state distributions. With exposure to 9 MeV Brehmsstrahlung radiation, we also find that GYGAG(Ce) ceramics exhibit excellent radiation hardness.