Christopher G. Wahl

Gamma-Ray Point-Source Detection in Unknown Background Using 3D-Position-Sensitive Semiconductor Detectors

Three-dimensional-position-sensitive semiconductor detectors provide both energy and locations of gamma-ray interactions in the detector. From these data, Compton-imaging methods can reconstruct the spatial source distribution around the detector as a function of energy. Here, the generalized likelihood ratio test (GLRT) and the source-intensity test (SIT) are applied to automate detection and identification of point sources of radiation in distributed background by using the combined energy and direction information from these detectors. Throughout, the model makes no assumptions of the background intensity or energy distribution. However, directional information enables detection and identification without prior knowledge of the background spectrum and the detection of isotopes also appearing in the background. Spatially, background is modeled as uniform, but only slight degradation in performance was observed when the background was twice as intense in one hemisphere. Experimental detection and identification performance as a function of false-alarm probability was measured for three weak isotopes using experimental data collected on a 20 mm × 20 mm × 15 mm CdZnTe detector. Both the GLRT and SIT performed better than the commercial software Genie 2000 (Canberra), which uses only spectral information, when the possible source direction was unknown. Prior knowledge of the possible source direction further improves both detection and identification.

Maximum-Likelihood Deconvolution in the Spatial and Spatial-Energy Domain for Events With Any Number of Interactions

In previous works, maximum-likelihood expectation-maximization deconvolution for two-interaction events within a single CdZnTe detector with dimensions of was implemented. This deconvolution method is capable of estimating the source image for each energy range as well as the incident spectrum for each direction around the detector. To improve the detection efficiency and the image resolution, we have built a four-detector array system; each detector has dimensions of . Using this detector-array system, from a Co-60 measurement, 41.5% of recorded events in the energy window from 1100 keV to 1200 keV are two-interaction events. The goal of this work is to increase the efficiency of this deconvolution algorithm by extending the calculation of the system response functions to events with other number of interactions. We first analytically extend the system response function calculation to three-interaction events by deriving the probability density function, considering the measurement noise, and integrating over the digitization and pixelation volume. The system response function is then simplified, modularized, and extrapolated to events with other numbers of interactions from an array system. By including events with any number of interactions in the system model, imaging makes use of all recorded events, and the angular resolution is improved. This deconvolution algorithm is applicable to any gamma-ray detector system that has the capability of recording 3D interaction location and energy deposition for each interaction.

Sensitivity of gamma-ray source detection using 3D-position-sensitive semiconductor detectors

Three-dimensional-position-sensitive semiconductor detectors provide both the energy and the locations of each gamma-ray interaction. From these data, Compton-imaging methods can reconstruct the source distribution around the detector as a function of energy. Known isotopes are detected using the generalized likelihood ratio test (GLRT), and the performance is presented as a function of false-alarm probability for the cases of known and unknown background intensity. In addition, the localization performance is presented using two different algorithms as a function of measurement time for a point source in background using experimental data from a 20 mm × 20 mm × 15 mm CdZnTe detector. The effect of energy resolution on detection performance is also noted. Imaging information is hence used along with spectroscopic information to improve the search for a hidden source.

Maximum likelihood estimation maximization deconvolution in spatial and combined spatial- energy domains for a detector array system

In previous work, our research group has developed gamma-ray imaging methods using two-interaction Compton events recorded from a single 1.5 cm x 1.5 cm x 1 cm 3D position-sensitive CdZnTe detector. The first of these is a maximum likelihood estimation maximization (MLEM) algorithm to estimate the spatial source distribution from measured events. The other, energy-imaging integrated spectral deconvolution, takes place in integrated spatial and energy space using the MLEM algorithm, allowing it to deconvolve the source image at any specific energy, as well as the spectrum for any specific direction. By including Compton-scattering events in the system model, it also estimates the true incident gamma-ray spectrum from each direction free of the Compton continuum. In order to improve efficiency at high energies as well as angular resolution, a new system (Polaris) is under construction using two layers of 3 x 3 modular 3D CdZnTe detectors, each with dimensions of 2 cm x 2 cm x 1.5 cm. Both MLEM methods, which used two-interaction Compton events, have been generalized to the new array system. Moreover, the system response function for both methods have been extended to make use of three-interaction events occurring in the system. The system response function for MLEM in the spatial domain has also been extended to use single-interaction events. Since it is impossible to pre-calculate the system response function due to the large number of possible events, analytic formulas are derived to allow the calculation of the system response function as reconstruction occurs. Finally, representative results are presented.

Model-based reconstruction of spectral and spatial source distribution from objects with known motion

Radiation imaging has many applications ranging from health care to homeland security and defense, and source motion is present in many of these applications. When the motion profile of the source is known or otherwise estimated, one can use motion-compensation techniques to reduce blur in the reconstructed image. In this paper, we present a model-based source-intensity reconstruction in the energy and spatial domains using list-mode data. The model includes separate parameterization for objects moving with known motion that is independent of the stationary backdrop. This approach corrects for object motion without smearing stationary sources in the backdrop space. The goal is to simultaneously obtain an estimate of the incident energy and spatial distribution of the radiation field for the stationary backdrop and for each moving object. Experimental Compton-imaging results using an 18-detector array of 3-D-position-sensitive CdZnTe detectors show that the method can successfully reconstruct the source intensity of moving objects while also revealing stationary sources in the backdrop. Also, by modeling the possibility of partial photon energy deposition in the detector, the incident energy spectrum is reconstructed more accurately.

Point-source detection using energy and imaging information from 3D-position-sensitive semiconductor detectors

Three-dimensional-position-sensitive semiconductor detectors provide both the energy and the locations of each gamma-ray interaction in the detector. From these data, Compton-imaging methods can reconstruct the source distribution around the detector as a function of energy. Automated detection and identification methods have been developed to use this combined energy and imaging information to detect the presence of point sources of radiation in a background environment with an unknown energy spectrum and a uniform spatial distribution, as well as to identify the nuclide, localize its position, and estimate the source strength. Experimental performance was better than the commercial software Genie 2000 (Canberra) that uses only spectral information. Performance for all methods is shown as a function of false-alarm probability using experimental data collected on a 20 mm × 20 mm × 15 mm CdZnTe detector.

Detecting shielded sources using 3-D CdZnTe detectors

Three-dimensional-position-sensitive CdZnTe detectors have excellent energy resolution along with imaging capability. In this work, a 2 cm × 2 cm × 1.5 cm CdZnTe detector was used to detect shielding in front of sources. By using the energy-imaging integrated spectral deconvolution (EIID) algorithm, the shielding has a unique signature in the reconstructed image.

Gamma-ray source location by attenuation measurements

A method has been proposed to locate a point source by measuring the first moment of the count distribution in a 3D position-sensitive radiation detector. Measurements were performed using a Hgl2 crystal. A gamma source at 662 keV could be located to within a quadrant after fewer than 100 events due to the high atomic number of the detector. The performance of the method is studied through simulations and measurements. The method performs well even when few spatial bins are available. The presence of a thick inactive layer around the crystal degrades performance.

Performance of five-or-more-pixel event sequence reconstruction for 3-D semiconductor gamma-ray-imaging spectrometers

The performance of sequence-order reconstruction for five- and six-interaction events is investigated for a single 20 mm × 20 mm × 15 mm CdZnTe (CZT) detector as well as a 3 × 3 × 2 array of these detectors. Simulations are performed for both systems and experimental results are presented for a single 3-D-position-sensitive CZT detector. Two sequence reconstruction methods are evaluated. A deterministic method is based on the probability of each interaction to occur for a given event. A minimum squared difference (MSD) technique compares the scatter angles calculated from geometry and from deposited energies. These two methods are evaluated based on the percentage of reconstructed events that produce a correct Compton cone for image reconstruction. Since the scatteringangle uncertainty decreases in the array system due to the larger average separation between interactions, sequence reconstruction for the MSD method performs better for the array system than the single detector.

A portable 2 × 2 digital 3D CZT imaging spectrometer system

A new portable gamma-ray imaging spectroscopy system using three dimensional position-sensitive CdTeZe has been built at the University of Michigan. This system uses a first generation digital VAD_UM ASIC jointly developed by Integrated Detector Electronics AS (IDEAS) and the University of Michigan and allows four 20 × 20 × 15 mm3 CdZnTe detectors to be simultaneously read out in either a full readout mode (where all channels from the triggered ASIC are read out to a personal computer) or in sparse readout mode (only the triggered channel and its neighbors are read out). The performance of the new system in terms of noise, spectroscopy, and isotope identification will be reported. The portable system is able to use standard AC power and is contained in a pelican case 54 × 30 × 32 cm3.