Lanthanum and cerium bromides and chlorides form isomorphous alloy systems with the UCl<sub>3</sub>
type structure. These scintillating alloys exhibit high luminosity and proportional response, making
them the first scintillators comparable to room temperature semiconductors for gamma spectroscopy;
Ce(III) activated lanthanum bromide has recently enabled scintillating gamma ray spectrometers
with < 3% FWHM energy resolutions at 662 keV. However brittle fracture of these materials
impedes development of large volume crystals. Low fracture stress and perfect cleavage along
prismatic planes cause material cracking during and after crystal growth. These and other properties
pose challenges for material production and post processing; therefore, understanding mechanical
behavior is key to fabricating large single crystals, and engineering of robust detectors and systems.
Recent progress on basic structure and properties of the lanthanide halides is reported here,
including thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and
fracture toughness. Observations including reversible hydrate formation under atmospheric pressure,
loss of stoichiometry at high temperature, anisotropic thermal expansion, reactivity towards common
crucible materials, and crack initiation and propagation under applied loads are reported. The
fundamental physical and chemical properties of this system introduce challenges for material
processing, scale-up, and detector fabrication.
Analysis of the symmetry and crystal structure of this system suggests 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. This proposed failure mechanism and
its implications for scale up to large diameter crystal growth are also discussed.
IN the past few years, there has been an increasing need for new methods to detect and identify suspect materials in packages and containers used for transport and shipment of goods. Nuclear techniques are playing an important role in the design of such instruments and in this paper we discuss several new applications. While the methods for detecting nuclear radiation have become quite well established over the past few decades, it is interesting to see how some of the properties of these classical sensor are being improved and modified with new technology. In this short review, we review several novel applications for nuclear sensors and measurement techniques that are used in new security instrumentation designs.
We have fabricated several monolithic x-ray detector arrays from Cd<SUB>0.9</SUB>Zn<SUB>0.1</SUB>Te material. The principle advantages of this material are its relatively high stopping power and room temperature operation. Recently fabricated linear arrays exhibit improved spatial and energy resolution as compared to our previous versions. Each array is 25.3 mm long with 32 independent detector elements, yielding a center-to-center spacing of 0.8 mm. Gaps of less than 100 micrometers , but with greater than 2 gigaohms of resistance, separate the pixel contacts. Our results indicate that smaller spacings and gap widths are possible with our current fabrication methods. The arrays have been operated in a pulse counting mode with photon energy discrimination. Results for energy resolution and spatial response are presented. Examples of low dose x ray transmission images obtained with a prototype linear scanner utilizing multiple arrays are shown here. By using the energy discrimination capability of the arrays and electronic readout, we have also been able to generate dual-energy images of various samples.
We have designed and fabricated an x-ray linear scanner based on monolithic CdZnTe arrays. The arrays are 1.0 inch long with 16 detectors each. To increase the x-ray stopping power of the arrays, they are operated with the x-ray photons incident normal to the E-field direction. A photon-counting pulse mode read-out is utilized to produce high dynamic range images at low incident flux rates. Our read-out electronics are composed of independent 16-channel modules including charge pre-amplification, pulse shaping discriminated scaler, and buffered I/O. The linear packing density for the electronics is 16 channels per inch, commensurate with the array pitch. Results from measurements of spatial response generated by scanning a fine beam of gamma rays across the arrays, and x-ray images produced in a linear-scan mode are presented.
Current approaches to digital radiography and tomography are dominated by the use of Scintillator-Photodiode arrays as detectors. To improve the quality of the data for such measurements it is desirable to increase the efficiency of the device both for the absorption of incoming x rays as well as the ratio of current produced per unit dose. In order to be of practical use, such detectors must maintain a high signal to noise performance and level of dark current stability in the presence of large radiation fluxes. In this laboratory, we are exploring the use of monolithic linear arrays that directly convert ionizing radiation into charge without the intervening photo-emission step. We have evaluated detectors made from CdTe as well as CdZnTe intrinsic material with a variety of contact methods. Our studies have shown that the relative efficiency of charge collection of the holes within the pulse shaping time is the most significant parameter governing their use. Data have been collected on this property from several devices. CdZnTe solid state devices produce over ten times the current per absorbed dose than a typical scintillator-photodiode. Recent advances in raw material production and contact technology provide detectors which can maintain their operating characteristics over kilo-rad of dose. Readout methods that use pulse counting mode operation have been evaluated. Results are shown on the sensitivity and spatial resolution of these detectors. Examples of results taken with multi-element, monolithic devices fabricated thus far are demonstrated with some estimates on the possibility for the production of larger arrays.
Conference Committee Involvement (5)
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIII
22 August 2011 | San Diego, California, United States
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XII
2 August 2010 | San Diego, California, United States
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XI
3 August 2009 | San Diego, California, United States
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X
11 August 2008 | San Diego, California, United States
Hard X-Ray and Gamma-Ray Detector Physics IX
27 August 2007 | San Diego, California, United States