Recently discovered scintillators for gamma ray spectroscopy - single-crystal SrI<sub>2</sub>(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<sub>2</sub>(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<sub>2</sub>(Eu), Gd-based transparent ceramics, and Bi-loaded
plastic scintillators can advance the state-of-the art in ionizing radiation detection systems.
Scintillator nonproportionality is a key mechanism by which the energy resolution is degraded in gamma spectroscopy. Herein, we survey the results obtained for a number of inorganic scintillating materials, including alkali and multivalent halides, fluorides, simple oxides, silicates, and a tungstate. The results are interpreted in the context of a model that accounts for carrier attraction by the Onsager mechanism and exciton-exciton annihilation by the Birks model. We then utilize the theory of Landau fluctuations in combination with the fitted experimental nonproportionality curves to deduce the predicted value of the resolution degradation.
Transparent ceramics combine the scintillation performance of single crystals with the ruggedness and processability of glass. We have developed a versatile, scaleable fabrication method, wherein nanoparticle feedstock is consolidated at temperatures well below melting to form inch-scale phase-pure transparent ceramics with optical scatter of α <0.1 cm<sup>-1</sup>.
We have fabricated Cerium-doped Gadolinium Garnets with light yields of ~50,000 Ph/MeV and energy resolution of <5% at 662 keV. We have also developed methods to form sheets of the high-Z ceramic scintillator, Europium-doped Lutetium Oxide Bixbyite, producing ~75,000 Ph/MeV for radiographic imaging applications.
We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of
SrI<sub>2</sub>(Eu)-based gamma ray spectrometers. SrI<sub>2</sub>(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield
proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI<sub>2</sub> and EuI<sub>2</sub>
indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy.
We have demonstrated energy resolution with SrI<sub>2</sub>(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals,
while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful
in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed
and re-emitted in large and/or highly Eu<sup>2+</sup> -doped crystals. The light yield proportionality of SrI<sub>2</sub>(Eu) is found to be
superior to that of other known scintillator materials, such as LaBr<sub>3</sub>(Ce) and NaI(Tl).
The characteristics and performance for two of the latest coplanar grid CdZnTe detectors, which use the third-generation coplanar grid design, will be discussed. These detectors, with dimensions of 1.5x1.5x0.9 cm<sup>3</sup> and 1.5x1.5x0.95 cm<sup>3</sup>, were fabricated by Baltic Scientific Instruments, Ltd., using crystals from Yinnel Tech, Inc. The high electron mobility-lifetime product measured for these crystals will lead to improved charge collection efficiency and better energy resolution. The spectroscopic performance obtained from the detectors, employing various methods such as depth sensing, radial sensing, and relative gain compensation, will be reported. Results from these measurements will give us insight into the material properties as well as the charge induction uniformity of the detector.