Michael C. Browne

CdZnTe gamma ray spectrometer for orbital planetary missions

We present the design and analysis of a new gamma ray spectrometer for planetary science that uses an array of CdZnTe detectors to achieve the detection efficiency needed for orbital measurements. The use of CdZnTe will provide significantly improved pulse height resolution relative to scintillation-based detectors, with commensurate improvement in the accuracy of elemental abundances determined by gamma ray and neutron spectroscopy. The spectrometer can be flown either on the instrument deck of the spacecraft or on a boom. For deck-mounted systems, a BGO anticoincidence shield is included in the design to suppress the response of the CdZnTe detector to gamma rays that originate in the spacecraft. The BGO shield also serves as a backup spectrometer, providing heritage from earlier planetary science missions and reducing the risk associated with the implementation of new technology.

Development of high efficiency, multi-element CdZnTe detectors for portable measurement applications

We describe the development of detector arrays and electronics for large-volume, hand-held CdZnTe detectors with the same counting efficiency as portable NaI(Tl) detectors presently used for nuclear material measurement applications. The pulse-height resolution of the multi-element detectors is at least three times better than NaI(Tl) over a wide energy range (from 100 keV to several MeV), enabling more accurate measurements of gamma-rays emitted by special nuclear material. Arrays of up to eight coplanar grid detectors can be combined to make detectors ranging in size from 4 to 14 cm3. Because the number of spectroscopy channels is small, low-power, hand-held detectors can be manufactured with conventional printed circuit board technology, thus keeping the cost of multi-element detectors to a minimum. The design and performance of an 8-element detector is presented.

Characterization of a large-volume multi-element CdZnTe detector

In this paper, we present results of experiments to characterize a large-volume (4 cm3), multi-element CdZnTe detector for high-efficiency, gamma-ray spectroscopy. The module includes an array of eight 0.5 cm3 coplanar grid detectors manufactured by eV Products. An eight-channel data acquisition system with list mode output is used to record gamma-ray events for each detector in the array. The list mode data are analyzed to determine the efficiency for coincidence events and to demonstrate different modes of operation (e.g., Compton suppression). The total efficiency of the array is found to match Monte Carlo calculations to within a few percent; however, the full-energy (photopeak) efficiency is significantly lower than predicted by Monte Carlo. The observed difference is probably caused by a combination of electrode design and charge transport properties. Approaches to improve full-energy efficiency are proposed.

Development of prototype induced-fission-based Pu accountancy instrument for safeguards applications

Prototype safeguards instrument for nuclear material accountancy (NMA) of uranium/transuranic (U/TRU) products that could be produced in a future advanced PWR fuel processing facility has been developed and characterized. This is a new, hybrid neutron measurement system based on fast neutron energy multiplication (FNEM) and passive neutron albedo reactivity (PNAR) methods. The FNEM method is sensitive to the induced fission rate by fast neutrons, while the PNAR method is sensitive to the induced fission rate by thermal neutrons in the sample to be measured. The induced fission rate is proportional to the total amount of fissile material, especially plutonium (Pu), in the U/TRU product; hence, the Pu amount can be calibrated as a function of the induced fission rate, which can be measured using either the FNEM or PNAR method. In the present study, the prototype system was built using six (3)He tubes, and its performance was evaluated for various detector parameters including high-voltage (HV) plateau, efficiency profiles, dead time, and stability. The systems capability to measure the difference in the average neutron energy for the FNEM signature also was evaluated, using AmLi, PuBe, (252)Cf, as well as four Pu-oxide sources each with a different impurity (Al, F, Mg, and B) and producing (α,n) neutrons with different average energies. Future work will measure the hybrid signature (i.e., FNEM×PNAR) for a Pu source with an external interrogating neutron source after enlarging the cavity size of the prototype system to accommodate a large-size Pu source (~600g Pu).

High-energy characterization of two large-volume multielement CdZnTe detectors

We present results of experiments to characterize two large- volume, multi-element CdZnTe detectors for gamma-ray spectroscopy at high energy. The first detector consisted of four 1.5cm x 1.5cm x 0.75cm coplanar grid detectors. The measurements for the four-element design were performed with various configurations. The second detector consisted of eight 1 cm x 1 cm x 0.5 cm coplanar grid detectors arranged in a 2x2x3 array. The high-energy gamma-ray sources included 60Co(1332), 228Th(2614), 244Cm/13C(6129), and Fe(n,(gamma) )(7645). The front end-electronics consisted of eight spectroscopy-grade preamplifiers/shapes/pulse stretchers, built on circuit boards close to the arrays. For the four-element measurements the shapers/pulse stretchers were replaced with commercial amplifiers. An eight-channel data acquisition system with list mode output was used to record gamma-ray events for each detector element in each array. The list mode data were analyzed to produce coincidence and single spectra and efficiencies for the various sources. The Compton continuum and the escape peaks are suppressed in the coincidence spectra relative to the singles spectra. We compare these spectra and efficiencies at high energy to results at lower energies and to Monte Carlo predictions.