Edwin A. Stephan

Study of a high-resolution, 3D positioning cadmium zinc telluride detector for PET

This paper investigates the performance of 1 mm resolution cadmium zinc telluride (CZT) detectors for positron emission tomography (PET) capable of positioning the 3D coordinates of individual 511 keV photon interactions. The detectors comprise 40 mm × 40 mm × 5 mm monolithic CZT crystals that employ a novel cross-strip readout with interspersed steering electrodes to obtain high spatial and energy resolution. The study found a single anode FWHM energy resolution of 3.06 ± 0.39% at 511 keV throughout most of the detector volume. Improved resolution is expected with properly shielded front-end electronics. Measurements made using a collimated beam established the efficacy of the steering electrodes in facilitating enhanced charge collection across anodes, as well as a spatial resolution of 0.44 ± 0.07 mm in the direction orthogonal to the electrode planes. Finally, measurements based on coincidence electronic collimation yielded a point spread function with 0.78 ± 0.10 mm FWHM, demonstrating 1 mm spatial resolution capability transverse to the anodes-as expected from the 1 mm anode pitch. These findings indicate that the CZT-based detector concept has excellent performance and shows great promise for a high-resolution PET system.

Charge collection studies of a high resolution CZT-based detector for PET

Our team is developing the CZT Small Animal PET System, a 1-mm resolution imager for small animal studies. The techniques are also applicable to breast imaging with PET, and Compton imaging for gamma-ray surveillance and threat assessment. CZT detectors are used to achieve 2 – 3 % FWHM energy resolution at 511 keV, which strongly suppresses the effects of Compton scattering on image quality and quantification. The detectors are read out with a novel crossed-strip electrode technique that localizes interaction sites in two dimensions. In the third dimension, charge drift time and the ratio of cathode to anode signals are used for localization. With these capabilities and our electrode design, photon interaction sites are resolved into voxels measuring 1 mm × 1 mm transverse to the incoming direction of an annihilation photon and 5 mm parallel to it, which should enable 1 mm tomographic reconstructed spatial resolution in three dimensions. A steering electrode between the anodes improves charge collection and energy resolution. The detectors’ size is 39 mm × 39 mm × 5 mm and they are read out with RENA-3 ASICs. To study detector and ASIC performance, we developed the Evaluation System that contains four detectors and six RENA-3s. We describe the CZT Small Animal PET System, the Evaluation System, and the detector design. Then we report measurements of the detector’s spatial and energy resolution, the effect of the steering electrode on charge collection, and plans for the future.

High-altitude balloon flight of CdZnTe detectors for high-energy x-ray astronomy: II

Cadmium Zinc Telluride (CZT) is a room temperature semiconductor detector well suited for high energy x-ray astronomy. We have developed a CZT detector with 500 micron crossed strip readout and an advanced electrode design that greatly improves energy resolution. We conducted two balloon flights from Fort Sumner, NM, to study the cross strip detector and a standard planar detector both sensitive in the energy range of 20-350 keV. The flights utilized a total of seven shielding schemes: 3 passive, 2 active and 2 hybrid passive-active. In the active shielding modes, the anti- coincidence shield pulse heights were telemetered for each CZT event, allowing us to study the effect of the shields energy threshold on the spectral shape and magnitude of the background. We are also developing an energy-dependent background rejection technique based on the charge collection properties of the CZT detector. This technique employs the depth of interaction, as inferred by the ratio of cathode to anode pulse height, to reject events inconsistent with incident source x-rays. The long duration of the May flight enabled us to study activation effects. We present result of the effectiveness of each of the shielding schemes on both detectors, the rejection power of depth of interaction technique on the crossed strip detector, inferred aperture background flux and the level of activation after 22 hours as float.

Performance of a prototype CdZnTe detector module for hard x-ray astrophysics

Our collaboration is characterizing a prototype detector module designed for high energy X-ray astrophysics research covering the 20 - 250 keV energy range. The module consists of a three dimensional position sensitive CdZnTe detector, 25 mm X 25 mm X 2 mm, with 1 mm pitch crossed strip electrodes, an interleaved steering electrode, and an Application Specific Integrated Circuit (ASIC) for individual electrode readout. The newly developed readout system is compact, lightweight, has low power consumption and will lead to reduced system electronic noise. The detector is surrounded by a plastic anti-coincidence system for charged particles, and passive shielding that has been optimized based on results from two previous balloon flights. The first balloon flight test of the new detector module is scheduled for Fall 2000. In addition to our continuing balloon studies, we are investigating proton radiation damage effects and present preliminary results. After proton irradiation, the energy resolution is not significantly degraded, calibration photopeaks are down shifted by less than 10% in energy, and the depth of interaction dependence is nearly eliminated.

Study of a high resolution, 3-D positioning cross-strip Cadmium Zinc Telluride detector for PET

As a part of our teams’ efforts in developing ultra-high resolution PET systems, this paper investigates the performance of 1 mm resolution Cadmium Zinc Telluride (CZT) detectors capable of positioning the 3-D coordinates of individual 511 keV photon interactions. The detectors are 40 mm × 40 mm × 5 mm monolithic CZT crystals that employ a novel cross-strip readout with interspersed steering electrodes to obtain high spatial and energy resolution. The study found the best-case single anode FWHM energy resolution of 2.7±0.2% at 511 keV, and neighborsummed FWHM energy resolution of approximately 4.64±0.35% at 511 keV. Improved resolution is expected with properly shielded front-end electronics. Measurements made using a collimated beam established the efficacy of the steering electrodes in facilitating full charge collection across anodes, as well as a spatial resolution of 1 mm in the direction perpendicular to the electrode planes. Finally, measurements based on coincidence electronic collimation demonstrated a spatial resolution of 1 mm transverse to the anodes -as expected from the 1 mm anode pitch. These findings indicate that the CZT-based detector concept has excellent performance and shows great promise for a high resolution PET system.

Position-sensitive CZT detector module

Coded mask imagers for future high energy x-ray astronomy missions will require detector planes with areas of hundreds to thousands of cm2 and position resolutions < 1 mm. Such detectors will enable coded mask imagers to discover and study thousands of high energy x-ray sources. The UCSD/WU/UCR/NOVA collaboration has been developing CZT detector systems with crossed-strip readout to meet these requirements. We report progress on a compact detector module with 41 cm2 area and 0.5 mm spatial resolution. The design includes the bias network and ASIC readout electronics, and allows modules to be combined in large area arrays with very high live-area factors. Results from laboratory and balloon flight tests are presented.

Mapping CZT charge transport parameters with collimated X-Ray and gamma-ray beams

It is desirable to characterize the charge production and transport properties of CZT wafers prior to fabrication into detectors. This allows rejection of undesirable material early in the production cycle as well as acceptance of less-than-perfect material that may be adequate for particular applications. Significant production cost reduction and increased yield will result. We report on the development of techniques to achieve these aims with equipment that may be used in small research groups and companies. Collimators made of high-Z material produce 30-micron to 3-mm pencil beams of X-rays and gamma-rays from radioactive sources and X-ray generators. These are scanned precisely across wafers prepared with simple planar contacts that are read out by a single electrode, the anode in our case. With irradiation on the cathode side, the spatial variation of the signal is mostly due to variations in electron production and transport. Therefore, the position-dependence and bias dependence of the signal’s spectral properties and event rate may be analyzed to determine and map the charge production and electron trapping lifetime, and the 3-D dependence of these parameters. Grain boundaries and other regions of degraded charge production an transport are readily identified. The technique is described in detail, and examples of scanning results are presented.