A method for constructing an x-ray telescope with exceedingly hgh spatial resolution is to use a pair of coaxial, Fresnel zone plates aligned with an imaging x-ray detector. This combination allows the high sensitivity imaging of x-ray and gamma-ray sources ranging in energy from 1 keV to several hundred keV over a field of view of several degrees with spatial resolution of a fraction of an arc minute. We have implemented a version of such a telescope using several relatively new technologies. These include specialized techniques for constructing Fresnel zone plates from thin sheets of tungsten, a 64-element, avalanche photodiode (APD) array coupled to a matching, segmented, CsI(T1) scintillator, a new ASIC which provides 16-channels of low noise amplification, and image processing software that provides the user not only with localized intensity information, but also with localized spectral information.
The Avalanche Photodiode (APD) is a unique device that combines the advantages of solid state photodetectors with those of high gain devices such as photomultiplier tubes (PMTs). APDs have internal gain that provides a high signal-to-noise ratio. APDs have high quantum efficiency, are fast, compact, and rugged. These properties make them suitable detectors for important applications such as LADAR, detection and identification toxic chemicals and bio-warfare agents, LIDAR fluorescence detection, stand-off laser induced breakdown spectroscopy (LIBS), and nuclear detectors and imagers.
Recently there have been significant technical breakthroughs in fabricating very large APDs, APD arrays, and position sensitive APD arrays (PSAPD). Signal gain of over 10,000 has been achieved, single element APDs have been fabricated with active area greater than 40 cm2, monolithic pixelated arrays with up to 28 x 28 elements have been fabricated, and position sensitive APDs have been developed and tested. Additionally, significant progress has been made in improving the fabrication process to provide better uniformity and high yield, permitting cost effective manufacturing of APDs for reduced cost.
Development of monolithic arrays of multiplexed, high-gain avalanche photodiodes suitable for use in a spectroscopic radiation-imaging device is underway at RMD. To dramatically reduce the electronics required to support a large array of discrete pixels, we have utilized a unique property of avalanche photodiodes and the method in which they are produced to develop a relatively simple readout scheme using row-column addressing. By adding a step to the avalanche photodiode creation, it is possible to place two, separate diode contacts onto the back of each photodiode in the array. These isolation diodes allow the readout of an entire row or column of photodiodes through a common readout line. A data-decoding matrix uniquely determines the position in the array while simultaneously supporting the goal of reducing the number of signal readout lines and support electronics. This approach reduces the number of pre-amplifiers, pulse-shaping circuits, and sample-and-hold stages from n2 to 2n (n pixels on a side) per array. Recent research has been carried out with 14 × 14 pixel, planar-processed avalanche photodiode array having pixels 2.00 mm on a side with 2.25-mm pitch. These arrays will be paneled to form the photodetector of a radiation imager of approximately 100-cm2 photosensitive area. To reduce the contribution of noise from each pixel to the common readout lines, research is being carried out to develop a discriminator with an adjustable threshold for each avalanche photodiode readout connection. Initial performance results from multiplexed arrays, a discussion of the active discrimination contacts, and the current status of the imager research project are given.
Radiation Monitoring Devices has carried out research to develop a monolithic array of multiplexed, high-gain avalanche photodiodes suitable for use in a spectroscopic radiation-imaging device. To dramatically reduce the electronics required to support a large array we have utilized a unique property of avalanche photodiodes, and the method in which they are produced, to develop a relatively simple readout scheme using row-column addressing. Results indicate that there is good charge sharing between diode contacts on the back of a photodiode, feedback and crosstalk are minimal between avalanche photodiodes connected to a common data line using the diode-contact approach, and that reducing the noise contribution of each separate avalanche photodiode to the common data lien remains a critical issue to be examined further.
Monitoring nuclear materials that is dangerously radioactive, remotely located, or difficult to access is a challenging task. The necessary research required to develop a system capable of remotely monitoring radioactive materials has been undertaken at Radiation Monitoring Devices, Inc. We report on a system utilizing a spectroscopic gamma-ray imager for real-time observation of sensitive nuclear materials over the Internet or dedicated networks. Research at RMD has produced a spectroscopic gamma-ray imager centered on a position-sensitive photomultiplier tube coupled to scintillation crystal and a coded aperture. A gamma-ray intensity pattern from the detector is stored and processed by a portable computer workstation and then mathematically corrected to yield the original radiation-source image. The pseudo-color, radiation-source image is overlaid on a co-registered video picture of the same area captured by a high-resolution charge-coupled device. The combined image is displayed as an accurate map of gamma-ray sources in the physical environment. Recent developments involve instrument control and data transmission through computer networks. Alarm triggers based on changes in the video image, the radiation image, the energy spectrum are under development. Work to remotely control alarm sensitivity and type, as well as the image update frequency, has also been examined.
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