Carolyn E. Seifert

A Prototype Three-Dimensional Position Sensitive CdZnTe Detector Array

A new CdZnTe gamma-ray spectrometer system that employs two layers of modular detector arrays Is being developed under the collaboration between the University of Michigan and the Pacific Northwest National Labaratory (PNNL). Each layer can accommodate up to three by three 3-dimensional position sensitive CdZnTe gamma-ray spectrometers. This array system is based on the newly developed VAS_UM/TAT4 ASIC readout electronics. Each of the nine detector modules consists of a pixellated CdZnTe detector and a VAS_UM/TAT4 ASIC front-end board. Each 1.5times1.5times1.0 cm3 CdZnTe detector employs an array of 11 by 11 pixellated anodes and a planar cathode. The energy depositions and 3-dimensional positions of individual interactions of each incident gamma ray can be obtained from pulse amplitude, location of each pixel anode and the drift time of electrons. Ten detectors were tested individually and half of them achieved resolution of <1.0% FWHM at 662 keV for single-pixel events (~30% of all 662 keV full energy deposition events). Two of them were tested in a simple array to verify that the upgrade to an array system does not sacrifice the performance of individual detectors. Experimental results of individual detectors and a two-detector array system are presented, and possible causes for several worse performing detectors are discussed.

Real-Time Compton Imaging for the GammaTracker Handheld CdZnTe Detector

We are currently developing a handheld radioisotope identifier containing 18 position-sensitive CdZnTe crystals. In addition to isotope identification, the device performs basic Compton imaging to determine the location of suspected sources. This paper presents two computationally efficient algorithms for this purpose. The first algorithm traces individual Compton cones onto the unit sphere, whereas the second algorithm computes the intersection of two Compton cones and the unit sphere. Simulations demonstrate that the algorithms are suitable for determining the directionality, even with features such as uncertainty calculations omitted. The one-cone algorithm works more efficiently at high count rates, but the two-cone algorithm generates fewer image artifacts.

Isotope identification in the GammaTracker handheld radioisotope identifier

GammaTracker is a portable handheld radioisotope identifier using position sensitive CdZnTe crystals. The device uses a peak-based method for isotope identification implemented on an embedded computing platform within the device. This paper presents the run-time optimized algorithms used in this peak-based method of analysis. Performance of the algorithms is presented using measured data from gamma-ray sources.

Electronics system for the GammaTracker handheld CdZnTe detector

We are currently developing a handheld radioisotope identifier containing eighteen position-sensitive CdZnTe crystals. In addition to isotope identification, the device performs basic Compton imaging to determine the location of suspected sources. This paper gives an overview of the electronics system we have designed for this instrument. We use specialized application-specific integrated circuits to preprocess the outputs of each CdZnTe crystal. A low-power microprocessor running Windows CE drives the user interface and implements the isotope identification and directionality computations. Finally, we use a field-programmable gate array to perform the computationally intensive imaging tasks in real time.

Using Atmospheric Dispersion Theory to Inform the Design of a Short-lived Radioactive Particle Release Experiment.

AbstractAtmospheric dispersion theory can be used to predict ground deposition of particulates downwind of a radionuclide release. This paper uses standard formulations found in Gaussian plume models to inform the design of an experimental release of short-lived radioactive particles into the atmosphere. Specifically, a source depletion algorithm is used to determine the optimum particle size and release height that maximizes the near-field deposition while minimizing both the required source activity and the fraction of activity lost to long-distance transport. The purpose of the release is to provide a realistic deposition pattern that might be observed downwind of a small-scale vent from an underground nuclear explosion. The deposition field will be used, in part, to study several techniques of gamma radiation survey and spectrometry that could be used by an On-Site Inspection team investigating such an event.

PRex: An Experiment to Investigate Detection of Near-field Particulate Deposition from a Simulated Underground Nuclear Weapons Test Vent.

AbstractA radioactive particulate release experiment to produce a near-field ground deposition representative of small-scale venting from an underground nuclear test was conducted to gather data in support of treaty capability development activities. For this experiment, a CO2‐driven “air cannon” was used to inject 140La, a radioisotope of lanthanum with 1.7‐d half-life and strong gamma-ray emissions, into the lowest levels of the atmosphere at ambient temperatures. Witness plates and air samplers were laid out in an irregular grid covering the area where the plume was anticipated to deposit based on climatological wind records. This experiment was performed at the Nevada National Security Site, where existing infrastructure, radiological procedures, and support personnel facilitated planning and execution of the work. A vehicle-mounted NaI(Tl) spectrometer and a polyvinyl toluene-based backpack instrument were used to survey the deposited plume. Hand-held instruments, including NaI(Tl) and lanthanum bromide scintillators and high purity germanium spectrometers, were used to take in situ measurements. Additionally, three soil sampling techniques were investigated and compared. The relative sensitivity and utility of sampling and survey methods are discussed in the context of on-site inspection.

DC Ionization Conductivity of Amorphous Semiconductors for Radiation Detection Applications

DC ionization conductivity measurements were used to characterize the electrical response of amorphous semiconductors to ionizing radiation. Two different glass systems were examined: a chalcopyrite glass ( CdGe_xAs₂; for <i>x</i> = 0.45-1.0) with a tetrahedrally coordinated structure and a chalcogenide glass ( As₄₀Se_(60-x)Te_x; where <i>x</i> = 0-12 ), with a layered or three dimensionally networked structure, depending on Te content. Changes in DC ionization current were measured as a function of the type of radiation (alpha or gamma ), dose rate, applied field, specimen thickness and temperature. The greatest DC ionization response was measured with CdGe_0.85As₂ at -40degC from an alpha source (which is the first reported result for radiation response from an amorphous chalcopyrite semiconductor). Avalanche gain was observed in As₄₀Se₆₀ with exposure to alpha radiation at fields ges 7times10³ V/cm. These results demonstrate the potential of these materials for radiation detection applications.

Prospects for Thermal Neutron Detection and Imaging with the GammaTracker Handheld Radioisotope Identifier

We present measured data on the neutron detection response of the GammaTracker handheld radioisotope identifier. Two neutron detection modes are discussed: measuring absorption gamma rays from the cadmium present in the Cd-Zn- Te spectrometers, and measuring the absorption gamma rays from moderator material present in the environment. In both cases, the capture gamma rays can be imaged to help locate a shielded neutron source. In this work, we discuss the total neutron detection efficiency of the GammaTracker instrument, and we present measured images of shielded neutron sources. Prospects for gamma-ray rejection are discussed.

Three-Dimensional Position Sensitive CdZnTe Detector Array for PNNL

A new CdZnTe gamma-ray spectrometer system that employs two layers of modular detector arrays is being developed under the collaboration between the University of Michigan and PNNL. Each layer can accommodate up to three by three 3-dimensional position sensitive CdZnTe gamma-ray spectrometers. This array system is based on the newly developed VAS_UM/TAT4 ASIC readout electronics. Each of the nine detector modules consists of a pixellated CdZnTe detector and a VAS_UM/TAT4 ASIC front-end board. Each 1.5times1.5times1.0 cm3 CdZnTe detector employs an array of 11 by 11 pixellated anodes and a planar cathode. The energy depositions and 3-dimensional positions of individual interactions of each incident gamma ray can be obtained from pulse amplitude, location of each pixel anode and the drift time of electrons. This is the first system that multiple 3D CdZnTe detector modules can be tiled together to achieve detection volume greater than 40 cm3. This configuration allows the expansion of detector area to achieve higher detection efficiency while maintaining the excellent energy resolution. Experimental results of individual detectors and the array system are presented and discussed.

Estimating radiological background using imaging spectroscopy

Optical imaging spectroscopy is investigated as a method to estimate radiological background by spectral identification of soils, sediments, rocks, minerals and building materials derived from natural materials and assigning tabulated radiological emission values to these materials. Radiological airborne surveys are undertaken by local, state and federal agencies to identify the presence of radiological materials out of regulatory compliance. Detection performance in such surveys is determined by (among other factors) the uncertainty in the radiation background; increased knowledge of the expected radiation background will improve the ability to detect low-activity radiological materials. Radiological background due to naturally occurring radiological materials (NORM) can be estimated by reference to previous survey results, use of global 40K, 238U, and 232Th (KUT) values, reference to existing USGS radiation background maps, or by a moving average of the data as it is acquired. Each of these methods has its drawbacks: previous survey results may not include recent changes, the global average provides only a zero-order estimate, the USGS background radiation map resolutions are coarse and are accurate only to 1 km - 25 km sampling intervals depending on locale, and a moving average may essentially low pass filter the data to obscure small changes in radiation counts. Imaging spectroscopy from airborne or spaceborne platforms can offer higher resolution identification of materials and background, as well as provide imaging context information. AVIRlS hyperspectral image data is analyzed using commercial exploitation software to determine the usefulness of imaging spectroscopy to identify qualitative radiological background emissions when compared to airborne radiological survey data.