An Array of CdTe Detectors for Imaging Applications
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AN ARRAY OF CdTe DETECTORS FOR IMAGING APPLICATIONS Y. EISEN AND E. POLAK Soreq Nuclear Research Center, Yavne, Israel 70600 ABSTRACT The performance characteristics of a linear array composed of several hundred miniature spectroscopy grade CdTe detectors are described. The linear array is utilized in a X-ray security imaging system. It operates either in an energy dispersive mode to highlight certain atomic elements in the luggage or in a regular mode for obtaining only the transmission image of the luggage. The array functions in the photon counting mode and tolerates count rates as high as 1o6 counts/sec. The array has high quantum efficiency and good spatial resolution which result in excellent image quality. The dynamic range, the electronic noise, the energy resolution, the homogeneity of response among detectors and the image quality are presented. INTRODUCTION Significant improvements over the last decade in the performance characteristics of CdTe solid state detectors has enabled us to construct a multielement linear array of CdTe detectors for use in a luggage inspection system at airports.
Most imaging systems developed in the past for industrial or medical applications utilize arrays composed of scintillators such as NaI(Tl), CsI(Tl), CdWO 4 or BGO coupled to either photomultipliers or photodiodes. Most of these systems operate in a current mode, where no spectroscopic information is available. However they are able to supply good quality image and function even at very high X or gamma ray fluences. CdTe detectors have several characteristics which make them useful for industrial imaging applications. 1.
High atomic number (48,52)
and high density (6.03
g/cm3 ) which
imply high quantum efficiency for detectors of thickness less than 2 mm and for photon energies lower than 150 keV. 2. Area of detectors can be made small, which enables to obtain good spatial resolution. 3. CdTe detectors have relatively good energy resolution Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
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which enables their use in energy dispersive imaging systems. 4. Have a wide bandgap (1.47 eV) which enables their operation at room temperature with relatively low noise. Current state-of-the-art applications utilizing CdTe detectors are described in ref.[i]. A construction of an array composed of 90 CdTe 3 has been detectors, each having the dimensions of 2 x 2 x 2 mm recently described in ref.[2]. The characteristics needed for an array used for imaging purposes are the following: 2. 1. High intrinsic efficiency for relatively small thicknesses. Large dynamic range. 3. Area of each element in the array should be and defined by the imaging application. sufficiently small 4. Stability at high count rates and negligible polarization ("after 5. Small cross talk between glow") for short sampling times. neighbouring detectors in the array. The linear array composed of several hundred of CdTe detectors and its performance characteristics are described in the present study. THE ARRAY OF CdTe DETECTORS The de
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