Morag Smith

Design and characterization of cylindrical CdZnTe detectors with coplanar grids

This paper describes the development of cylindrical coplanar grid CdZnTe detectors for gamma ray spectroscopy. Cylindrical detector offer a number of advantages over established designs. For example, grid structures for cylindrical detectors are simpler than those for rectangular designs. The goal of our work is to design a cylindrical coplanar grid detector with excellent resolution at low- and high-energy. Information on detector design and manufacturing is presented. Six detectors are characterized. The pulse height resolution of the best detector is 13.5 keV full width at half maximum at 662 keV and 5.5 keV FWHM at 122 keV.

Further studies on the modified two-terminal geometry for CdZnTe detectors

This paper will report on additional studies undertaken on a novel electrode geometry, referred to as CAPtureTM technology, wherein the cathode is extended up the sides of a planar CdZnTe detector. The initial presentation of this development focused on a single geometry, and a simple mechanism was proposed to explain the improved performance. In this presentation, we will describe the results of further electric field and charge transport modeling as applied to this detector configuration, and also compare the calculated performance with experimental results.

Charge collection in multielectrode CdZnTe detectors

Preliminary results of experiments to investigate charge collection in CdZnTe detectors are presented. The experiments support the development of semiconductor- modeling tool for device engineering that will be used to design large volume CdZnTe detectors for gamma ray spectroscopy. Improved diagnostic methods are described, including an automated alpha particle scanner for charge pulse mapping. Semiconductor modeling techniques are presented along with methods to visualize charge transport. Experimental results are compared to a physical model that has been used routinely in research on room temperature devices for gamma ray detection.

The impact of combining nuclear material categories on uncertainty

Abstract There is nearly always some mismatch between the physical properties of items containing nuclear material and standards used to calibrate the assay method. Physical properties include the density and heterogeneity of the items nonnuclear material and the type of interfering species, such as hydrogen in the case of neutron counting. Some assay techniques are less sensitive to variation in physical properties than others and can be used to generate working standards. Provided that a reference assay method 1 (often calorimetry) is available that is well characterized (having negligible or known dependence on varying physical properties), we can assess the total measurement error of another method 2. In this paper we consider the impact of the number of measurement categories on the measurement error standard deviation of method 2. We assume that working standards (traceable to primary reference standards) are provided by the method 1 assay of actual facility items. Given the same number of working standards, we evaluate the tradeoff between using more working standards for each of a fewer number of categories versus using fewer standards in each of more categories. This leads to a method to determine a good allocation of working standards.

Calorimeter prediction based on multiple exponentials

Calorimetry allows very precise measurements of nuclear material to be carried out, but it also requires relatively long measurement times to do so. The ability to accurately predict the equilibrium response of a calorimeter would significantly reduce the amount of time required for calorimetric assays. An algorithm has been developed that is effective at predicting the equilibrium response. This multi-exponential prediction algorithm is based on an iterative technique using commercial fitting routines that fit a constant plus a variable number of exponential terms to calorimeter data. Details of the implementation and the results of trials on a large number of calorimeter data sets will be presented.

Performance of multielement CdZnTe detectors

Single-element CdZnTe detectors are limited in size, and therefore efficiency, by the poor hole transport, even with a coplanar grid. We are investigating the possibility of a 27-element array using 15 mm X 15 mm X 15 mm elements for gamma-ray energies to 10 MeV for NASA planetary missions. We present experimental results for combinations of various size coplanar grid detectors using NIM electronics and energies to 6.1 MeV. Summation of the signals after linear gating and requiring coincidence produces only a small increase in the energy resolution. Our results indicate that good efficiency and spectrum not complicated by a large Compton continuum can be achieved by simply summing the spectra from 15 X 15 X 15 mm3 detectors for gamma-ray energies below about 2 MeV. Above 2 MeV, 2-fold coincidence might be required, depending on the spectrum, to suppress the Compton continuum and escape peaks. We use a Monte Carlo calculation to predict the performance of the 27-elements array for a lunar highlands spectrum. Such ambient-temperature, high-efficiency, good- resolution arrays will facilitate new NASA mission to determine elemental composition of planetary bodies and terrestrial applications requiring high-efficiency, good- resolution portable instruments.

Development of a Liquid Scintillator Neutron Multiplicity Counter (LSMC)

A new neutron multiplicity counter is being developed that utilizes the fast response of liquid scintillator detectors. The ability to detect fast (vs. moderated) fission neutrons makes possible a coincidence gate on the order of tens of nanoseconds (vs. tens of microseconds). A neutron counter with such a narrow gate will be much less sensitive to accidental coincidences making it possible to measure items with a high single neutron background to greater accuracy in less time. This includes impure Pu items with high (alpha,n) rates as well as items of low mass HEU where a strong active interrogation source is needed. Liquid scintillator detectors also allow for energy discrimination between interrogation source neutrons and fission neutrons, allowing for even greater assay sensitivity. Designing and building a liquid scintillator multiplicity counter (LSMC) requires a symbiotic effort of simulation and experiment to optimize performance and mitigate hardware costs in the final product. We present preliminary Monte Carlo studies using the GEANT toolkit along with analysis of experimental data used to benchmark and tune the simulation.