IEEE zyxwvutsrqpo TRANSACTIONS ON NUCLEAR SCIENCE, VOL. zyxwvut 40, NO. 4, AUGUST 1993 983 Characterization of the response of a double side p-strip silicon detector to X-rays in the diagnostic energy range A. Bandettini", W. Bencivellib, E. Bertolucci", U. Bottigli", M. Conti", A. Del Guerrad, M.E. Fantacci', P. Randaccio', V. ROSSO", P. RUSSO", A. Stefanini" "Dipartimento di Fisica dell'Universith and Sezione I.N.F.N., Pisa, Italy bIstituto di Patologia Speciale Medica e Metodologia Clinica dell'Universit& and Sezione I.N.F.N., Pisa, Italy 'Dipartimento di Scienze Fisiche dell'universitb and Sezione I.N.F.N., Napoli, Italy dDipartimento di Fisica dell'Universit& and Sezione I.N.F.N., Ferrara, Italy eIstituto di Fisica dell'Universit& and Sezione I.N.F.N., Cagliari, Italy Abstract-We are investigating the use of a double side p-strip silicon crystal for X-ray detection. The detector is 300 pm thick and the read-out pitch is 100 pm for both sides. It operates in capacitive charge division mode by means of floating strips between read-out strips. The detector has been irradiated by 241Am and '"Cd sources. Different zones within the 100 pm read-out pitch have been individually exposed. Thus, the following charac- teristics have been studied as a function of the im- pact point of the photon: (a) the charge collection mechanism; (b) the relative detection efficiency; (c) the energy resolution; (d) the spatial resolu- tion. The absolute efficiency of the detector has also been measured at three energy values. 1. INTRODUCTION We are investigating the possibility of using a silicon crystal for X-ray detection in the diagnostic energy range, both by Montecarlo techniques [l] and by direct experi- mental measurements [2]. The final aim of the project is a detector for digital radiography. The detector under in- vestigation is a double side microstrip silicon crystal, orig- inally developed for high energy physics experiments, with a thickness of 300 pm, and a surface of 1.4 x 1.4 cm2. The electrodes, 12 pm wide, are deposited in arrays with 25 pm spacing on the junction (J) side and with 50 pm spacing on the ohmic (0) side [3]. The read-out pitch is 100 pni for both sides. Previous studies have shown that such a detect,or re- sponds to X-rays radiation, and images have been taken to establish its imaging capability zyxwvutsrq [a]. The purpose of this work is to study the charge collection mechanism of the detector and to characterize its properties by measuring the energy a.nd spatial resolutions. 11. EXPERIMENTAL APPARATUS Figure 1 shows the detector. A small surface of the de- tector was irradiated with 60 keV photons from a "lAm source. After amplification the signal of each channel is sent both zyxwvu t30 an ADC and to a discriminator for the sub- sequent trigger formation. Two logical OR's from all the equipped strips of each side are defined and either can be selected to generate a trigger for the acquisition. The out- put of the ADC is registered for all channels. The detector operates in capacity charge division mode, and the loca- tion of each interaction is reconstructed off line by means of the center of mass of the charge collected by two adjacent strips. This set-up has been used to study the detector characteristics as a function of the photon impact point. To measure the detector efficiency, the OR-ed, discrim- inated signals from the involved microstrips have been di- rectly sent to a scaler. This measurement has been per- formed at three energy values, namely 24 and 88 keV, using a lo9Cd source, in addition to 60 keV of the photons from the 241Am source. 111. RESULTS A. Charge Collection Figure 2 shows the detector response to a uniform ex- posure. After verifying that all the cells have an uniform behaviour, all data have been reduced to one cell, 100 pm wide. On the triggering side (figures 2a and 2d), we see two peaks corresponding to the read-out strips, the trig- gering threshold having been set to a fixed level on each single channel rather than on the total charge. On the non- triggering (i.e. non-discriminated) side (figures 2b and 2c) , we see not only the read-out strips but also, in the expected 0018-9499193$03.00 0 1993 EEE