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X-ray diffraction (XRD) tomography continuous to be a promising technology for maintaining a high detection probability with low false alarm rates while adding new threat classes. By collecting the diffracted (coherently scattered) X-rays, one realizes several key advantages over transmission-based systems:
• Measure several additional features by which to identify material composition
• Tomographic 3D spatial imaging with only a single view
• Automatic, PC based, explosives detection algorithms for replacing human operators
A key requirement for XRD technology is excellent energy resolution (ER) of the detectors used in the scanner. Existing spectroscopic detectors offer low enough ER (below 6keV) but, unfortunately, operate at rather low count rate (typically under 1kcps/mm2). As a result, commercial XRD scanners, such as the XRD3500, require long scan times. As a result, it is difficult to effectively use these scanners in airports.
A breakthrough in XRD technology was achieved through the use of coded apertures, which increase the signal amplitude by 2-3 orders of magnitude compared to traditional heavy collimated systems. While the brighter resulting XRD signal requires complex signal processing to eliminate increased scatter, it has been shown to produce a much faster XRD scanner response (seconds instead of minutes). A practical implementation of this new approach requires high count rate (between 1kcps/mm2 and 1Mcps/mm2) while maintaining very low ER (below 6keV) and sub-mm spatial resolution required for angular detection precision.
In the last 5 years, Redlen Technologies has developed high-flux CZT detection technology for medical Computed Tomography (CT) that is currently being deployed by major medical OEMs into clinical applications. The technology is based on a 22x34 [748 pixels] 2-D array with a pixel pitch of 330um. The associated high-speed photon counting ASIC that allows for event detection operates up to 250Mcps/mm2. Recently we have found a way to reconfigure that detector technology platform into an XRD platform.
In this paper we will present experimental results of our 2-D 22x32 CZT pixel array that is currently available for deployment into XRD scanner platforms. The CZT sensors used in this platform are 2mm thick with a 330 um pixel pitch and operate without polarization up to 250Mcps/mm2. In the CT mode, the detectors operate in the 16-190 keV range with energy resolution of 6.9 keV and standard deviation of 0.7 keV across 748 pixels. In the XRD mode, the detectors operate in the 12-150 keV range with energy resolution of mean value of 5.6keV and standard deviation of 0.6keV across 748 pixels. We believe these performance levels are more than sufficient to enable operating XRD scanner at the optimum performance levels.
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