Yvan A. Boucher

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.

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.

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.

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.

Thermal neutron source location using a 3-D position-sensitive CdZnTe detector array

A method for the location of a far-field thermal neutron source using an array of eighteen 2 × 2 × 1.5 cm3 position-sensitive CdZnTe detectors has been proposed. The relative intensity of 558 keV 113Cd capture γ rays emitted from the outward-facing surfaces of each detector in the array was estimated via Compton imaging. The detector response to thermal neutrons was simulated using Geant4 and stored in a response matrix. The estimated location was the least squared difference between simulated and measured detector responses. Eight measurements of a 1 Ci 239PuBe neutron source moderated by paraffin were used to demonstrate the performance of this method, which was limited by the chosen size of the system matrix and mismatch between measurement and simulation.

Event classification in 3D position sensitive pixelated CdZnTe detectors

Pixelated CdZnTe detectors with 3D position sensitivity have undergone significant development in the past decade. A CdZnTe detector with a pixelated anode is expected to provide excellent spectroscopy and to form a Compton image of the gamma-ray source distribution. However, there are additional event classification capabilities that are inherent to this device configuration, many of which are presented in this work. The majority of the classification effort has been directed toward gamma-rays. Seperating Compton escape from photoelectric absorption on an event-by-event basis is difficult to achieve but potentially very rewarding if achieved. This work discusses using interaction locations as well as number of interactions to increase the probability of observing the photoelectric effect relative to Compton scatter. Pair production events can be classified by the capture of 511 keV photons in coincidence with another energy deposition elsewhere in the detector. Also, the geometry of the interactions for one event can be used to identify pair production. Correctly identifying the number of interactions that occur for one event is critical for passing valid information to the Compton image reconstruction algorithm. This work illustrates the challenges involved in determining the number of interactions when there is little spatial separation. If a Compton scatter and a photoelectric event occur in the same pixel or in adjacent pixels then the shape of the waveforms induced on the preamplifier must be examined in order to determine the true number of interactions. This work also outlines several additional event identification capabilities. Muon tracks can be identified because they will induce a large charge on many pixels that reveal a trajectory through the detector. The cadmium content makes CdZnTe an excellent thermal neutron absorber. The prompt gamma-ray emission following neutron capture in cadmium-113 results in a peak 558.6 keV. While this device is not an ideal neutron detector, as the only discrimination against gamma-rays is based on the energy deposited, the presence of a peak at 558.6 keV is a clear indicator of a neutron source. Finally, alpha and beta interactions can be eliminated for the purpose of low background experiments by excluding events that occur near the surface of the device. Experimental evidence of each classification capability is presented along with the limitations of the current state of the art detector systems.

Low background measurements using 3-D position-sensitive CdZnTe detectors

There are numerous applications for CdZnTe detectors that require a well characterized background. One such application is the study of neutrino-less double beta decay, which requires a very low background environment. The background has two main contributors, the natural background surrounding the detector system and the internal background due to the natural radioactive decay from components in the system. In order to understand the internal background present in the Polaris 18-detector CdZnTe array system, a lead cave was designed and built at the University of Michigan. The system was well-calibrated and measurements were taken over the course of several months to better characterize the internal background from the components of the Polaris system.

Measurements of gamma rays above 3 MeV using 3D position-sensitive 20×20×15 mm 3 CdZnTe detectors

Measurements of gamma rays above 3 MeV have been made using a system of 20×20×15 mm3 Cadmium Zinc Telluride semiconductor detectors and the GMI VAS_UM2.3TAT4 application specific integrated circuit (ASIC). These detectors are pixellated with 121 anode pixels and use a grid to help steer the charge carriers to the collecting anode. The measurements looked to explore both the spectroscopic and imaging performance of a system for gamma ray sources above 3 MeV. The ASIC has a dynamic range from about 30 keV up to 3 MeV for the cathode signal and each anode pixel signal. Therefore, measurements of sources above 3 MeV add several challenges, including having to differentiate between events that had a signal reaching the maximum for a single channel and those that did not because of multiple interactions with a single crystal or between multiple crystals. Additionally, events with charge sharing or consisting of interactions in more than one detector constituted a large fraction of the events depositing over 3 MeV in the system and led to degradation of the system performance.

Criteria selection for 20×20×15 mm 3 pixelated CdZnTe semiconductor detectors

Large volume, pixelated Cadmium Zinc Telluride detectors have been developed over the past decade. In November of 2009, Redlen Technologies began manufacturing 130 20×20×15 mm3 CdZnTe detectors, each with a common grid between the 121 pixels, for the Polaris Project. Results for the first 65 detectors were studied to determine a set of criteria that could be used to help streamline the detector testing procedure for large volume CdZnTe detectors. By rejecting detectors with >5% uncorrected FWHM, >2 nA/V of total grid-to-anode leakage current, and excessive anode noise detectors with over 1.2% corrected single pixel FWHM at 662 keV were correctly identified 88% of the time. Those with under 1.2% corrected single pixel FWHM at 662 keV were correctly identified 80% of the time.

Criteria selection for 202015 mm3 pixelated CdZnTe semiconductor detectors

Large volume, pixelated Cadmium Zinc Telluride detectors have been developed over the past decade. In November of 2009, Redlen Technologies began manufacturing 130 20×20×15 mm3 CdZnTe detectors, each with a common grid between the 121 pixels, for the Polaris Project. Results for the first 65 detectors were studied to determine a set of criteria that could be used to help streamline the detector testing procedure for large volume CdZnTe detectors. By rejecting detectors with >5% uncorrected FWHM, >2 nA/V of total grid-to-anode leakage current, and excessive anode noise detectors with over 1.2% corrected single pixel FWHM at 662 keV were correctly identified 88% of the time. Those with under 1.2% corrected single pixel FWHM at 662 keV were correctly identified 80% of the time.