Fred Duttweiler

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.

First use of a high-sensitivity active pixel sensor array as a detector for electron microscopy

There is an urgent need to replace film and CCD cameras as recording instruments for transmission electron microscopy (TEM). Film is too cumbersome to process and CCD cameras have low resolution, marginal to poor signal-to-noise ratio for single electron detection and high spatial distortion. To find a replacement device, we have tested a high sensitivity active pixel sensor (APS) array currently being developed for nuclear physics. The tests were done at 120 keV in a JEOL 1200 electron microscope. At this energy, each electron produced on average a signal-tonoise ratio about 20/1. The spatial resolution was also excellent with the full width at half maximum (FWHM) about 20 microns. Since it is very radiation tolerant and has almost no spatial distortion, the above tests showed that a high sensitivity CMOS APS array holds great promise as a direct detection device for electron microscopy.

Future directions for camera systems in electron microscopy.

Publisher Summary Charge-coupled device (CCD) invented in 1970, soon became the sensor of choice in many imaging applications, particularly for video cameras and camcorders. This chapter reviews current efforts to scale up lens-coupled CCD camera and make a system capable of exceeding the spatial resolution of film, while maintaining single-electron sensitivity. This lens-coupled CCD system represents the current state-of-the-art in CCD-based systems, and it also demonstrates the great engineering effort required to achieve these key performance benchmarks when the detector is based on a resolution-limiting scintillation screen. The chapter discusses the development of a parallel effort to produce a radiation-tolerant system that can withstand direct electron bombardment. It also describes efforts required to adapt the pixel array detector (PAD) that is commonly used in X-ray diffraction, and discusses the development of a groundbreaking prototype system based on an active pixel sensor (APS). This early implementation of an APS-based direct detection detector (DDD) has already delivered unprecedented performance in many areas exceeding the fundamental capabilities of CCD-based systems.

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.

A new direct detection camera system for electron microscopy

High resolution electron imaging is very important in nanotechnology and biotechnology fields. For example, Cryogenic Electron-Microscopy is a promising method to obtain 3-D structures of large protein complexes and viruses. We report on the design and measurements of a new CMOS direct-detection camera system for electron imaging. The active pixel sensor array that we report on includes 512 by 550 pixels, each 5 by 5 μm in size, with an ~8 μm epitaxial layer to achieve an effective fill factor of 100%. Spatial resolution of 2.3 μm for a single incident e- has been measured. Electron microscope tests have been performed with 200 and 300 keV beams, and the first recorded Electron Microscope image is presented.

The intermediate size direct detection detector for electron microscopy

In a longstanding effort to overcome limits of film and the charge coupled device (CCD) systems in electron microscopy, we have developed a radiation-tolerant system that can withstand direct electron bombardment. A prototype Direct Detection Device (DDD) detector based on an Active Pixel Sensor (APS) has delivered unprecedented performance with an excellent signal-to-noise ratio (approximately 5/1 for a single incident electron in the range of 200-400 keV) and a very high spatial resolution. This intermediate size prototype features a 512×550 pixel format of 5&mgr;m pitch. The detector response to uniform beam illumination and to single electron hits is reported. Radiation tolerance with high-energy electron exposure is also impressive, especially with cooling to -15 °C. Stable performance has been demonstrated, even after a total dose of 3.3×106 electrons/pixel. The characteristics of this new detector have exciting implications for transmission electron microscopy, especially for cryo-EM as applied to biological macromolecules.

High altitude balloon flights of position sensitive CdZnTe detectors for high energy X-ray astronomy

Cadmium Zinc Telluride (CZT) is a semiconductor detector well suited for high energy X-ray astronomy. The High-Energy X-ray Imaging Spectrometer (HEXIS) program is developing this technology for use in a hard X-ray all-sky survey and as a focal plane imager for missions such as FAR_XITE and Constellation X. We have designed a novel electrode geometry that improves interaction localization and depth of interaction determination. The HEXIS program has flown two high altitude balloon payloads from Ft. Summer, NM to investigate background properties and shielding effects on a position sensitive CZT detector in the energy range of 20–350 keV.