A HgI 2 Microstrip Detector Based on Resistive Charge Division Readout
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A HgI 2 MICROSTRIP DETECTOR BASED ON RESISTIVE CHARGE DIVISION READOUT D. GRASSI*, F. MUROLO*, E. PERILLO*, G. SPADACCINI*, M. VIGILANTE*, M. AMANN**, J.M. KOEBEL**, P. SIFFERT** AND W. DUSI*** * Dipartimento di Scienze Fisiche-Universiti "Federico II" and Istituto Nazionale di Fisica Nucleare-Sezione di Napoli, Mostra d'Oltremare pad 20, 80125 Napoli, Italy ** CRN/PHASE, 23, rue de Loess, Strasbourg, France *** CNR/TESRE, via dei Castagnoli, 1, Bologna, Italy ABSTRACT A HgI2 linear array detector with a surface resistive layer between the readout strips has been constructed with the aim of achieving the maximum possible active detection area and a spatial resolution of - 50 pm, by using an analog readout of the strip outputs and a centroid finding algorithm. This device has been developed as a high efficiency position-sensitive detector for use in a Bragg X-ray spectrometer, designed for detecting X-ray photons with energies up to - 30 keV. Calculations performed by taking into account the various sources of electronic noise have allowed the optimization of the interstrip distance and resistance. Fabrication techniques and preliminary results on energy and spatial resolution are reported and discussed. INTRODUCTION Position sensitive detectors (PSD) for X- and y-ray detection made of Mercuric Iodide have been proposed in recent years. Some technological problems related to their use under vacuum limited their applications and these have been solved only recently. These devices show very high detection efficiencies, even at thicknesses of 500 Am, owing to the high density and average atomic number of the bulk material. Microstrip linear array detectors have been investigated in various laboratories [1-5]. Generally, a digital readout of the strip outputs has been used. In this way the active region of detection is essentially limited to that below the strips, since the interstrip events are lost, or, in the best cases, assigned to one of the adjacent strips. In a recent paper [5] it was shown that, even with interstrip distances as small as 50 jam, the charge collection from these regions is largely inefficient when irradiating a 500 jam thick detector with an 241Am source. A shielding technique has been suggested by Iwanczyk era!. [2,3] to completely eliminate these undesired events. On the other hand, Patt et al.. [4] suggested a method to assign a position to this kind of event by taking the ratio of the simultaneous pulses present at two adjacent strips, provided that the sum of the pulses equals the full energy of the incoming y-ray. This technique, requiring an analog readout, led to an enhanced overall efficiency and to a spatial resolution of 350 jm for yray energies of 60 keV, better than that generally obtained by means of the digital readout. For some applications, e.g. an X-ray Bragg crystal spectrometer, a spatial resolution of = 50 jim is required. The energy resolution may be not critical; for the above application, for example, it should just allow separation of the X-rays diffracted from the differ
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