F. Patrick Doty

Gamma- and X-ray detectors manufactured from Cd1−xZnx Te grown by a high pressure bridgman method

Results of recent efforts in the growth of Cd1−xZnxTe crystals by a high pressure Bridgman (HPB) method and their use in gamma- and X-ray detector applications are presented. Evidence for crystals of relatively low defect content include etch pit densities of ≤ 104cm−2, double crystal rocking curve linewidths of 10–15″ and sharp, bright emission lines with excitonic features in low temperature photoluminescence measurements. Resistivities in excess of 1011 ohm cm are achieved without impurity doping. The resulting low leakage currents lead to good energy resolution, <6% at 59.5 keV for example. The dependence of leakage current on temperature from 233 K to 373 K implies a Fermi level at mid-gap for x=0.2. The results of flash X-ray experiments indicate that the high current sensitivity, low leakage current and good temporal response of Cd1−xZnxTe detectors make them attractive candidates for applications involving short pulses at high dose rates.

Connecting structure with function in metal–organic frameworks to design novel photo- and radioluminescent materials

The exemplary structural versatility and permanent porosity of Metal–Organic Frameworks (MOFs) and their consequent potential for breakthroughs in diverse applications have caused these hybrid materials to become the focus of vigorous investigation. These properties also hold significance for applications beyond those traditionally envisioned for microporous materials, such as radiation detection and other luminescence-based sensing applications. In this contribution we demonstrate that luminescence induced by ionizing radiation (also known as scintillation) is common in appropriately designed MOFs and describe how this property can be harnessed to generate novel materials useful for detecting radiation. Through a diverse selection of MOFs, we explore the structural properties of MOFs that give rise to scintillation and photoluminescence in these materials. These results enable us to define a new structure-based hierarchical system for understanding luminescent properties in MOFs. Finally, we describe some performance metrics for MOF-based scintillation counters, such as luminosity and resistance to radiation damage, and discuss how these materials relate to the current state of the art in scintillation counters.

Results for aliovalent doping of CeBr3 with Ca2

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr3. The structural, electronic, and optical properties of CeBr3 crystals were studied using the density functional theory within the generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr3, including the dielectric function, were calculated.Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown, i.e., 1.9% of the CeBr3 molecules were replaced by CaBr2 molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr2 added to 222.14 g of CeBr3. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the...

Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property nonuniformity that causes low manufacturing yield. Although tremendous efforts have been made in the past to reduce Te inclusions/precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbations and the associated property nonuniformities. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here, we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals.

Recent developments in CdZnTe gamma-ray detector technology

A discussion of recent results in the preparation of Cd1-xZnxTe crystals by a high pressure Bridgman (HPB) method and use of the crystals for gamma- and x-ray detectors is presented. Resistivities in excess of 1011 ohm-cm are achieved in Cd1-xZnxTe without impurity doping. The consequential low detector leakage currents lead to excellent energy resolution, 8.4% (FWHM) at 30 keV, for example. Useful energy spectroscopy can be performed at temperatures up to 100 degree(s)C. The dependence of leakage current on temperature from -40 degree(s)C to 100 degree(s)C implies a Fermi level at mid-gap for x equals 0.2. HPB grown Cd1-xZnxTe crystals exhibit relatively low defect content, as evidenced by etch-pit-densities <EQ 104 cm-2, double-crystal-rocking-curve linewidths of 10 - 15 arc-seconds and sharp, bright emission lines, with excitonic features, in low temperature photoluminescence measurements. Results of flash x ray experiments indicate high current sensitivity, low leakage current, and good temporal response. Preliminary results of Cd1-xZnxTe imaging detector array studies are discussed.

A refined parameterization of the analytical Cd–Zn–Te bond-order potential

This paper reports an updated parameterization for a CdTe bond order potential. The original potential is a rigorously parameterized analytical bond order potential for ternary the Cd–Zn–Te systems. This potential effectively captures property trends of multiple Cd, Zn, Te, CdZn, CdTe, ZnTe, and Cd1-xZnxTe phases including clusters, lattices, defects, and surfaces. It also enables crystalline growth simulations of stoichiometric compounds/alloys from non-stoichiometric vapors. However, the potential over predicts the zinc-blende CdTe lattice constant compared to experimental data. Here, we report a refined analytical Cd–Zn–Te bond order potential parameterization that predicts a better CdTe lattice constant. Characteristics of the second potential are given based on comparisons with both literature potentials and the quantum mechanical calculations.

Effect of Humidity on Scintillation Performance in Na and Tl Activated CsI Crystals

Time dependent photoluminescence and radioluminescence for sodium (Na) and thallium (Tl) activated cesium iodide (CsI) single crystals exposed to 50% and 75% relative humidity (RH) has been investigated. These results indicate that Tl activated crystals are more robust than the Na activated crystals against humidity induced scintillation degradation. The development of “etching pits” and “inactive” domains are the characteristics of deteriorated Na activated CsI crystals. These “inactive” domains, bearing a resemblance to a polycrystalline appearance beneath the crystal surface, can be readily detected by a 250 nm light emitting diode. These features are commonly observed at the corners and deep scratched areas where moisture condensation is more likely to occur. Mechanisms contributing to the scintillation degradation in Na activated CsI crystals were investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS depth profiles indicate that Na has been preferentially diffused out of CsI crystal, leaving the Na concentration in these “inactive” domains below its scintillation threshold.

Atomistic models for scintillator discovery

A2BLnX6 elpasolites (A, B: alkali; Ln: lanthanide; X: halogen), LaBr3 lanthanum bromide, and AX alkali halides are three classes of the ionic compound crystals being explored for γ-ray detection applications. Elpasolites are attractive because they can be optimized from combinations of four different elements. One design goal is to create cubic crystals that have isotropic optical properties and can be grown into large crystals at lower costs. Unfortunately, many elpasolites do not have cubic crystals and the experimental trial-and-error approach to find the cubic elpasolites has been prolonged and inefficient. LaBr3 is attractive due to its established good scintillation properties. The problem is that this brittle material is not only prone to fracture during services, but also difficult to grow into large crystals resulting in high production cost. Unfortunately, it is not always clear how to strengthen LaBr3 due to the lack of understanding of its fracture mechanisms. The problem with alkali halides is that their properties decay rapidly over time especially under harsh environment. Here we describe our recent progress on the development of atomistic models that may begin to enable the prediction of crystal structures and the study of fracture mechanisms of multi-element compounds.

Use of metal organic fluors for spectral discrimination of neutrons and gammas.

A new method for spectral shape discrimination (SSD) of fast neutrons and gamma rays has been investigated. Gammas interfere with neutron detection, making efficient discrimination necessary for practical applications. Pulse shape discrimination (PSD) in liquid organic scintillators is currently the most effective means of gamma rejection. The hazardous liquids, restrictions on volume, and the need for fast timing are drawbacks to traditional PSD scintillators. In this project we investigated harvesting excited triplet states to increase scintillation yield and provide distinct spectral signatures for gammas and neutrons. Our novel approach relies on metal-organic phosphors to convert a portion of the energy normally lost to the scintillation process into useful luminescence with sub-microsecond lifetimes. The approach enables independent control over delayed luminescence wavelength, intensity, and timing for the first time. We demonstrated that organic scintillators, including plastics, nanoporous framework materials, and oil-based liquids can be engineered for both PSD and SSD.

Order and charge collection correlations in organic materials for neutron detection

Organic materials, and in particular, poly(p-phenylene vinylene)s, are being investigated for solid state neutron detection. Semiconducting organics can offer direct detection because of high resistivity, high dielectric strength, natural gamma discrimination due to low Z, and room temperature operation. However, the effective charge collection is dependant on several material processing variables, including solvent choice and concentration, substrate, deposition method and conditions, post-deposition processing, and other factors, all of which can influence the local and bulk order of the material. We have investigated the effects of processing variables on the material order through infrared dichroism. The charge collection of the device was measured with visible laser excitation, and related to the order.