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New two-dimensional (2D) position sensitive solid state pixel detectors with dedicated multi-channel mixed-signal front-end readout integrated circuits (ICs) have been developed for x-ray and gamma ray imaging. These pixel detectors were designed to be versatile and accommodate many types of position-sensitive solid-state sensors. These 2D pixel detectors are designed to address the needs of a wide range of photon counting applications with high energy resolution imaging, high spatial resolution and for fast photon counting with simultaneous multiple energy binning capability. The first family of these pixel detectors is intended for spectroscopy applications with sensors such as Si, Ge, GaAs, HgI2, PbI2, Se, CdTe and CdZnTe in a variety of configurations to detect and image x-rays and gamma-rays of energies up to 1.3 MeV. The second family is developed for fast photon counting with simultaneous energy binning. They can be used with similar solid state sensors. The third family is for high spatial resolution imaging with resolution down to 50 times 50 micron.

New, high spatial resolution CdZnTe (CZT) and silicon (Si) pixel detectors are developed for high angular and high energy resolution X-ray astronomy. These detectors are proposed for use in wide field-of-view, imaging All-sky X-ray, and Gamma-ray Astronomy Monitor (AXGAM) in a possible future space mission. The high stopping power of CZT detectors combined with low-noise front-end readout electronics ASIC (application specific integrated circuit) makes possible an order of magnitude improvement in spatial (angular) and energy resolution in X-ray detection. The AXGAM instrument will be built in the form of a fine coded mask placed over two-dimensional, high spatial resolution and low energy threshold silicon and CZT pixel detector arrays. The preliminary test results obtained from our first silicon and CZT pixel detectors are presented. These detectors may also be used with or without modification for medical and industrial imaging.

Hybrid pixel detector arrays that convert X-rays directly into charge signals are under development at NOVA for application to digital mammography. This technology also has wide application possibilities in other fields of radiology and in industrial imaging for applications in nondestructive evaluation and inspection. These detectors have potentially superior properties compared to either emulsion based film, which has nonlinear response to X-rays, or phosphor-based detectors in which there is an intermediate step of X-ray to light photon conversion. Potential advantages of direct conversion detectors are high quantum efficiencies (QE) of 98% or higher (for 0.3 mm thick CdZnTe detector with 20 keV X-rays), improved contrast, high sensitivity and low intrinsic noise. These factors are expected to contribute to high detective quantum efficiency (DQE). The prototype hybrid pixel detector developed has 50/spl times/50 microns pixel size, and is designed to have linear response to X-rays, and can support a dynamic range of 14 bits. Modulation Transfer Function (MTF) is measured on a l-mm silicon detector system where 10% or better modulations are obtained at 10 lp/mm spatial frequency. Preliminary DQE measurements of the same system yields a value of 55% at zero spatial frequency. Here, the authors report data of their first full size prototype readout ASIC chips hybridized with both silicon and CdZnTe detector arrays and present initial MTF and DQE measurement results as well as some test images.

A wide field-of-view, arcsecond imaging, high energy resolution X-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to detect and find counterparts at other wavelengths for /spl gamma/-ray bursts (GRBs). Detection of GRBs requires wide field-of-view (/spl pi/ to 2/spl pi/ field-of-view) and high sensitivity. This will be achieved by using high quantum efficiency CdZnTe pixel detectors with a low energy threshold (few keV) to observe the larger flux levels at lower energies, and large effective area (625 to 1000 cm) per coded aperture imaging module. Counterpart searches can only be done with ultra-high angular resolution detectors (10 to 30" FWHM) which gives 1 to 5" position determination especially for strong GRBs. A few arcsecond size error box is expected to contain at most one object observed at another wavelength. This will be achieved by using ultra-high spatial resolution pixel detectors (50/spl times/50 to 100/spl times/100 /spl mu/m) and a similar resolution coded aperture to achieve the required angular resolution. The low energy range may provide important new information on GRBs and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. The major scientific goals of the proposed mission in addition to continuously monitoring GRBs, is to observe AGN, transient phenomena, isolated and binary pulsars, and solar flares. A space-deployed AXGAM detector is expected to observe a few hundred GRBs per year.

A high sensitivity electronically collimated gamma camera is under development at NOVA. The proposed detector has electronic collimation and will measure the direction of the incident /spl gamma/-ray directly, hence eliminating the need for a physical collimator. This is accomplished by using the Compton double scatter technique. A feasibility study was successfully completed and prototype fabrication started. The new detector is expected to have 3 to 10 times improvement over the present gamma cameras. The high sensitivity is expected to allow fast imaging on time scales of 1 to 2 min and may open the way for dynamic metabolism visualization and Multi Gated Acquisition (MUGA) studies. The prototype is designed to have a high spatial resolution which is 2 to 3 times better compared to present gamma cameras. It is expected to have improved energy resolution by a factor of 2 to 4. Good energy resolution will enable discrimination against and scattered /spl gamma/-ray background with higher effectiveness than presently available. This will improve the signal-to-noise ratio and improve image quality. The detector can be designed as a single gamma camera or formed into a cylindrical detector and used as a tomographic SPECT imager. A modified smaller version may be directly applied to scintimammography.