Pulse Processing for Planar Cadmium Telluride Detectors
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PULSE PROCESSING FOR PLANAR CADMIUM TELLURIDE DETECTORS M.Richter, M.Hage-AlI, Z.T.Kuznicki, P.Siffert Centre de Recherches Nuc6aires (IN2P3), LaboratoirePHASE (UPR du CNRS no 292), B.P.20, F 67037 Strasbourg Cedex, France Planar CdTe detectors with different performances for gamma radiation detection have been investigated. The detector internal electric field was analyzed by means of time-of-flight experiments. The correlation of the results with the current-voltage characteristics of the samples, their risetime distribution for different gamma irradiation and, eventually, the shape of detected gamma spectra led us to a detector model based on the theory of space charge limited current. Using this model, we try to explain operation and failure of trapping loss correction methods for the different detector groups. 1.Introduction The development of circuits which compensate the trapping loss in planar Cadmium Telluride (CdTe) and Mercuric Iodide (HgI 2) detectors has opened the way to novel applications. It has been possible to enhance energy resolution without losses of detection efficiency [1]. The user can obtain spectra with a higher quality in shorter measurement time. The operation of these pulse processors bases on the convolution of the rise-time and the height of an output pulse of the charge sensitive preamplifier. A linear relationship between relative charge loss and charge collection time has been found with some planar CdTe detector. This fact facilitates the design of suitable spectroscopic amplifiers including a simple analog computer which calculates the correction term k*trise. The output 1s the maximum voltage can be expressed as v. =v (1+kt .s) where v pulse height of the disturbed pulse,' out te value of tIe corrected output pulse height, trise the pulse rise time and k, a gain factor for correction. But, in the mean time we have already found detectors with a nonlinear correction function or devices, for that charge loss correction does not cause any improvement. To characterize the differences between various detector groups we investigated planar CdTe detectors by means of two-parametric spectroscopy and time of flight techniques. Furthermore, we measured their current-voltage characteristics and rise-time distributions and tested eventually their gamma ray detection properties. Based on the data set we found some typical attributes for discrimination between distinct groups of planar CdTe detectors. We applied the model of space-charge limited current to explain some of the observed phenomena. 2. Ballistic loss If we consider factors which Influence the spectral resolution, we can not neglect the so called ballistic deficit in the shaping amplifier. It is defined as the variation of the maximum pulse height at the output of a shaping network in dependence on the rise-time of a unity height input pulses. Most of articles dealing with this subject Investigate the processing of signals coming from cooled, large volume, planar germanium detectors [2J[3]. Since the mobility of electrons and hole
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