The effects of 137Cs gamma irradiation on photoluminescence properties, such as spectra, light output, and lifetime, of several types of colloidal nanocrystals have been investigated. Irradiation-induced damage testing was performed on CdSe/ZnS, LaF3:Eu, LaF3:Ce, ZnO, and PbI2 nanocrystals synthesized on a Schlenk line using appropriate solvents and precursors. Optical degradation of the nanocrystals was evaluated based on the measured dependence of their photoluminescence intensity on the irradiation dose. Radiation hardness varies significantly between various nanocrystalline material systems.

In this report, phenomenon of acquisition of spontaneous electron conductivity (SEC) of porous CsBr is investigated and compared with that for porous CsI at the same experimental conditions.

Due to its high density and cubic structure, Lutetium oxide (Lu2O3) has been extensively researched for scintillating applications. Present manufacturing methods, such as hot pressing and sintering, do not provide adequate resolution due to light scattering of polycrystalline materials. Vapor deposition has been investigated as an alternative manufacturing method. Lutetium oxide transparent optical coatings by magnetron sputtering offer a means of tailoring the coating for optimum scintillation and resolution. Low crystallization in the sputtering process adversely affects transparency and scintillation, and is critical in growing stress free, thick coatings. The effect of process parameters on the crystallization is being investigated via x-ray diffraction (XRD), scanning electron microscopy (SEM) and emission spectroscopy and will be presented and discussed.

We demonstrate, based on first-principles calculations, that chemical bonding in deltahedral boron hydrides, BnHn2− also known as closo boranes, can be understood within the three-center two-electron (3c2e) bonding model in line with other families of boranes. We show that bonding in the triangular lattice of BnHn2− cages can be described by delocalized resonant 3c2e bonding. We also find that the reason for all the BnHn2− to be dianions can be attributed to the reduction of boron coordination number in the deltahedral cage structure from that of boron sheet with triangular lattice.

Surface passivation of device-grade radiation detector materials was investigated using x-ray photoelectron spectroscopy in combination with transport property measurements before and after various chemical treatments. Specifically Br-MeOH (2% Br), KOH with NH4F/H2O2 and NH4OH solutions were used to etch, reduce and oxidize the surface of Cd(1-x)ZnxTe semiconductor crystals. Scanning electron microscopy was used to evaluate the resultant microscopic surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and core lines were used to evaluate the surface chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the Schottky barrier height for improved radiation detector devices.

A review is presented of recent ceramic scintillator R&D. Attention is focussed on Ce doped gamma-ray scintillators for medical imaging applications. Ceramic scintillators discussed in detail include SrHfO3:Ce and Lu2Hf2O7:Ce. These materials combine a high density and high atomic number with fast emission and a good light yield and may find practical application in medical imaging modalities such as Positron Emission Tomography and Computed Tomography.

CdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ∼10 μm for CdTe crystal and ∼8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm−2 for CdTe and ≤ 3×104 cm−2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ∼1.5×1010 Ω·cm for CdTe and 2-5×1011 Ω·cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe=2×10−3 cm2/V (μτh=8×10−5 cm2/V) and μτ3-6×10−3 cm2/V (μτh=4-6×10−5 cm2/V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using 241Am (60 keV) and 137Cs (662 keV) energy sources and the results are presented in this paper.

Cerium and lanthanum tribromides and trichlorides form isomorphous alloys with the hexagonal UCl3 type structure, and have been shown to exhibit high luminosity and proportional response, making them attractive alternatives for room temperature gamma ray spectroscopy. However the fundamental physical and chemical properties of this system introduce challenges for material processing, scale-up, and detector fabrication. In particular, low fracture stress and perfect cleavage along prismatic planes cause profuse cracking during and after crystal growth, impeding efforts to scale this system for production of low cost, large diameter spectrometers. We have reported progress on basic materials science of the lanthanide halides. Studies to date have included thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and fracture toughness. The observed mechanical properties pose challenging problems for material production and post processing; therefore, understanding mechanical behavior is key to fabricating large single crystals, and engineering of robust detectors and systems. Analysis of the symmetry and crystal structure of this system, including identification of densely-packed and electrically neutral planes with slip and cleavage, and comparison of relative formation and propagation energies for proposed slip systems, suggest possible mechanisms for deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of residual process stress to cleavage planes, initiating fracture. Ongoing work to develop fracture resistant lanthanide halides is presented.

Organic semiconductors are increasingly common in electronics and sensors, and are now under investigation for a novel type of radiation sensors at Sandia National Laboratories. This class of materials can offer wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths, suggesting they may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, primarily in the family of poly(p-phenylene vinylene)s, or PPVs. The high ratio of hydrogen to carbon offers neutron sensitivity, while the low Z material provides some natural gamma discrimination. Compared to existing detectors, these materials could potentially offer large-scale radiation detection at a substantially reduced cost.

While PPVs hold promise for radiation detection, the mechanical and electrical properties must be optimized and the processing effects understood. Polymers can offer significantly simplified processing compared to the more common crystals used in solid state detection, which can be size limited and fragile. However, organic semiconductors are very sensitive to processing conditions, and mobility can be affected by orders of magnitude by processing variables, without altering any chemistry. Additives can also have dramatic effects on both electrical and mechanical properties. We report on nanoparticle additives that cause an increase in photoresponse of approximately three orders of magnitude as compared to a polymer film without additives. We separately show an order of magnitude increase in photoresponse by exposing the polymer/fullerene composite to sub-bandgap light.

Future work will analyze the feasibility of single particle detection and various geometries for optimization. Additional processing variables will also be investigated for further improvement of mobility and reduction of trap density.

Synthetic CdZnTe or “CZT” crystals can be used for room temperature detection of α- and γ-radiation. Structural/morphological heterogeneities within CZT, such as twinning, secondary phases (often referred to as inclusions or precipitates), and polycrystallinity can affect detector performance. As part of a broader study using synchrotron radiation techniques to correlate detector performance to microstructure, x-ray topography (XRT) has been used to characterize CZT crystals. We have found that CZT crystals almost always have a variety of residual surface damage, which interferes with our ability to observe the underlying microstructure −for purposes of crystal quality evaluation. Specific structures are identifiable as resulting from fabrication processes and from handling and shipping of sample crystals. Etching was found to remove this damage; however, our studies have shown that the radiation detector performance of the etched surfaces was inferior to the as-polished surface due to higher surface currents which result in more peak tailing and less energy resolution. We have not fully investigated the effects of the various types of inducible damage on radiation detector performance.

Crystals are the central materials element of most gamma radiation detection systems, yet there remains surprisingly little fundamental understanding about how these crystals grow, how growth conditions affect crystal properties, and, ultimately, how detector performance is affected. Without this understanding, the prospect for significant materials improvement, i.e., growing larger crystals with superior quality and at a lower cost, remains a difficult and expensive exercise involving exhaustive trial-and-error experimentation in the laboratory. Thus, the overall goal of this research is to develop and apply computational modeling to better understand the processes used to grow bulk crystals employed in radiation detectors. Specifically, the work discussed here aims at understanding the growth of cadmium zinc telluride (CZT), a material of long interest to the detector community. We consider the growth of CZT via gradient freeze processes in electrodynamic multizone furnaces and show how crucible mounting and design are predicted to affect conditions for crystal growth.

The influence of γ-irradiation on the dielectric losses (tg δ) and permittivity (ε′) of recycled high density polyethylene (HDPER) thermoplastic dynamic vulcanizates (TDV) has been investigated. The materials and composites that were studied include HDPER:EPDM, HDPER+EPDM+GTR, HDPER+EPDM+GTR/plast; EPDM (ethylene-propylene diamine).

The experimental data obtained shows significant differences between dielectric properties different samples, as well as between the unirradiated and γ-irradiated samples.

Conjugated polymers are largely intact by gamma exposure but can be energized in the presence of high-Z compounds. The resulting alteration of the polymer's high optical density and photoluminescence efficiency can be exploited for the detection of gamma radiation with high sensitivity. Diiodobenzene and conjugated polymers mix on the molecular level in solid thin films. Composite films of various thicknesses were conveniently cast from solution and exposed to gamma radiation. The responses of the films to gamma dosage were observed with dramatic changes in ultraviolet-visible absorption and photoluminescence.

Applications of first principles methods to understand properties of several known and potential scintillators for radiation detection are described. These include results for rare earth and Pb-based phosphates, rare-earth trihalides, ZnO, perovskites and tungstates.

The limitation of energy resolution of scintillation detectors are discussed with a special emphasis on non-proportionality response of scintillators to gamma rays and electrons, which is of crucial importance to an intrinsic energy resolution of the crystals. Examples of the study carried out with different crystals and particularly those of tests of undoped NaI and CsI at liquid nitrogen temperature with the light readout by avalanche photodiodes are presented suggesting strongly that the non-proportionality of the halide crystals are not their intrinsic property. Moreover, the influence of slow components of the light pulses on energy resolution and non-proportionality are discussed.