Willy Kaye

Optimal Background Attenuation for Fielded Radiation Detection Systems

Radiation detectors are often placed in positions difficult to shield from the effects of terrestrial background. This is particularly true in the case of Radiation Portal Monitor (RPM) systems, as their wide viewing angle and outdoor installations make them susceptible to terrestrial background from the surrounding area. A low background is desired in most cases, especially when the background noise is of comparable strength to the signal of interest. The problem of shielding a generalized RPM from terrestrial background is considered. Various detector and shielding scenarios are modeled with the Monte-Carlo N Particle (MCNP) computer code. Amounts of nominal-density shielding needed to attenuate the terrestrial background to varying degrees are given, along with optimal shielding geometry to be used in areas where natural shielding is limited, and where radiation detection must occur in the presence of natural background. Common shielding solutions such as steel plating are evaluated based on the signal to noise ratio and the benefits are weighed against the incremental cost.

Results From Testing of 145 3D Position-Sensitive, Pixelated CdZnTe Detectors

Testing of 20 × 20 × 15 mm3 3D position sensitive, CdZnTe detectors grown by Redlen Technologies Inc. has shown that 98 out of the 145 detectors analyzed achieved sub-1% FWHM at 662 keV for single-pixel events. Improvements in the detector performance over time reflect the improvements in detector growth and fabrication over the past several years. In addition to the spectroscopic performance of the detectors, the imaging performance of each detector was also quantified. The results show that imaging is weakly correlated to the spectroscopic performance, and more strongly correlated to the accuracy of the depth reconstruction. Other detector issues such as gain deficit and gain variation are also discussed.

Study on effect of charge sharing events in common-grid pixelated CdZnTe detectors

Due to the steering grid that aids the collection of electrons by the anode pixels, the common-grid pixelated CdZnTe (CZT) detectors have demonstrated better performance than simple pixel detectors. For a detailed analysis of charge sharing and weighting potential cross-talk that occurs near and underneath the gap and energy resolution degradation in common-grid CZT detectors, a detailed detector system modeling package has been developed and a related experiment using a sub-pixel interaction position estimation method has been performed. A 20×20×15 mm3 CZT crystal with an 11×11 common-grid pixel anode array with a 1.72 mm pixel pitch was modeled. The VA S UM/TAT4 chip is used for ASIC readout. Through system modeling and measurement, the energy resolution degradation and system properties of common-grid CZT detectors has been studied quantitatively.

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.

Calibration and operation of the polaris 18-detector CdZnTe array

The Polaris system is an array of 18 position-sensitive detectors, using 6 cm3 pixelated CdZnTe crystals from Redlen Technologies and readout electronics from Gamma-Medica Ideas (GMI). Many detectors received from Redlen Technologies are capable of achieving 1% energy resolution or better at 662 keV for all single-pixel events during room temperature operation. Furthermore, 3D position reconstruction capability enables 4p Compton imaging of gamma rays that undergo multiple interactions within a single detector or between multiple detectors in the array. However, there are many factors that can negatively influence the performance of this array. Measurements at elevated ambient temperatures indicate that the energy resolution degrades rapidly as the detector crystals are operated above 30°C. Furthermore, the electron mobility and trapping change with temperature which degrades the accuracy of the position dependent signal reconstruction parameters. The peak hold circuitry in the GMI ASIC provides a time dependent output, resulting in smaller recorded pulse heights for events with longer delay times between charge collection and system readout. This peak hold drop introduces depth dependent non-linearity for single pixel events and degrades the energy resolution of multiple pixel events. This effect is corrected by measuring the test pulse amplitude as a function of readout delay time. The problem of false triggers must be addressed carefully, as Polaris has 2178 anode and 18 cathode electrodes each independently capable of triggering the system. Collimation studies have been used to evaluate the accuracy of the depth reconstruction techniques and to identify the relative position of each detector crystal in the array, which ultimately affects image reconstruction.

Experimental demonstration of coded aperture imaging using thick 3D-position-sensitive CdZnTe detectors

3D-position-sensitive CdZnTe semiconductor detectors have demonstrated 4π Compton imaging capability and excellent energy resolution at room-temperature operation. However, Compton gamma-ray imaging is not feasible at low energies due to the small Compton-scatter cross-section. This work extends the current imaging capabilities to lower energies by utilizing coded aperture masks. Multiple coded aperture masks are applied to a single detector system of four 20mm×20mm×15mm CdZnTe detectors. Near-4n coded aperture imaging has been demonstrated through Monte Carlo simulation. The correct source direction is consistently identified using measured data with one mask above the cathode side and another mask above the non-cathode side of the detector. Challenges related to electric field distortion due to space charge in the detector are discussed. The focus of this research is to image near-4n field of view using coded apertures, ultimately, combining both Compton and coded aperture imaging techniques to expand the range of gamma-ray imaging.

A Simulation Framework for Evaluating Detector Performance in Cargo Screening Applications

Deployed radiation portal monitor systems (RPMs) screen gamma-ray signatures of cargo at international border crossings with the goal of detecting illicit radiological materials. Estimating the detection sensitivity of these systems requires an in-depth understanding, and quantification, of RPM response to both benign and illicit sources. Benign sources of radioactivity include background, alterations of the background due to the presence of vehicles and cargo, as well as sources that frequently cause nuisance alarms. These nuisance sources, for example those consisting of naturally occurring radioactive materials (NORM) and medical isotopes, frequently limit system performance. Advanced detector technology promises to increase the capability of deployed systems to discriminate illicit from nuisance sources. Presented here is a framework developed to assess the performance of these passive detection technologies. Due to the difficulty in obtaining empirical data for emerging technologies, the foundation of this comparison framework lies on a simulated benign source population to create a comprehensive set of data representing cargo vehicles driving through the RPM. Quantification of performance stems from injecting simulated signatures from illicit sources and comparing probabilities of detection via case and population studies.

Study of Long-Term CdZnTe Stability Using the Polaris System

The first Polaris system was built in September 2010 and has gone through significant testing at multiple DOD and DOE facilities as well as at the University of Michigan. It has been biased almost continuously for the 18 months of its current lifespan without any replacement of its detectors. The second Polaris system was built in June 2011 and has also been almost continuously biased during its lifespan. The continuous operation of both devices has provided the best opportunity to study the long-term stability of pixelated CdZnTe detectors. Spectroscopic performance has been monitored throughout each systems lifespan to study their performance over time and the impact on long term operation of these devices. It is shown that the charge transport properties and spectroscopic performance of these CdZnTe detectors are stable over time and devoid of fabrication or assembly issues. It is expected that future detectors will exhibit excellent stability.

4π coded aperture imaging using 3d position-sensitive CdZnTe detectors

3D position sensitive CdZnTe semiconductor detectors have demonstrated 4π Compton imaging ability and excellent energy resolution at room-temperature operation. An array of such detectors would provide unique detection capabilities. However, Compton imaging is challenging at low energies due to the small Compton-scatter cross-section. This work extends the imaging capabilities to lower energies by utilizing multiple coded aperture masks. Each coded aperture mask is applied to one of five sides of a single 2cmx2cmx1.5cm CdZnTe detector via simulation. These simulations show that multiple weak, low energy sources with background can be imaged simultaneously. The ultimate objective is to apply multiple masks to cover 4π space, using an 18 detector array from the Polaris project.

Analysis of system-dependent factors affecting pixelated CdZnTe detector performance through simulation

Charge sharing and weighting potential cross-talk between pixels are important factors that degrade the energy resolution in multiple-pixel events. However, simulation studies indicate that these factors cannot fully explain the measured energy resolution degradation for multiple-pixel events nor the Compton image distortion observed in some detector systems. This work presents simulation-based analysis of additional factors that degrade the performance of pixelated CdZnTe detectors coupled to the Gamma Medica Ideas (GMI) VAS UM/TAT4 ASIC and the H3Dv2 ASIC from Brookhaven National Laboratory (BNL). The peak-hold circuitry in the GMI ASIC cannot properly hold the signal amplitude while the BNL ASIC does not have this problem. The impact of this peak-hold drop on the energy resolution is studied for multiple-pixel events and a calibration technique to overcome this effect is validated. Many CdZnTe detectors have non-uniform performance on a pixel-by-pixel basis. The impact of single-pixel energy resolution non-uniformity is studied for multiple-pixel events. The GMI ASIC uses a simple threshold time pick-off with the common start of the system triggered by the cathode, while the BNL ASIC uses the peaking time with the common start triggered by the anode. The impacts of these schemes are isolated and analyzed through simulation. Overall, the simulation package considers gamma-ray interactions within the CdZnTe crystal, charge induction, electronic noise, pulse shaping, peak-hold drop and ASIC triggering procedures. The simulation package considers different timing pick-off techniques for both the GMI VAS UM/TAT4 ASIC and the H3Dv2 ASIC.