Improvements of a-Si:H/CsI(Ti) X-Ray Detectors for Radiation Monitoring
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IMPROVEMENTS OF a-Si:H/CsI(Ti) X-RAY DETECTORS FOR RADIATION MONITORING C.MANFREDOTTI, F.FIZZOTTI, M.BOERO, F.CANNISTRACI, E.VITrONE*, A.TORTI, M.TURNATURI'** *Experimental Physics Dept., University of Torino, Via Giuria 1,Torino, Ind Consorzio INFM, Italy "**IRCI,Borgaro Torinese (TO), Italy. ABSTRACT Photosensitive a-Si:H p-i-n diodes, working in photovoltaic mode, have been coupled to CsI(T1) scintillators for dosimetry applications to X-ray monitoring in the energy range from 50 keV up to 15 MeV. A "mesa" approach for p-i-n diodes has been adopted both in order to better define the geometry and to obtain very low dark current. In order to optimize the geometry, a computer program has been created which simulates light generation in the scintillator, the collection by the detector and the photovoltaic current obtained as a function of exposure rate. Measurements have been carried out in the X-rays energy ranges 50-240 keV, at 6 MeV and at 15 MeV. Detectors are linear in response and shows a good sensitivity, with the capability of measuring dose rates as low as 60 mR/h. The agreement between experimental data and simulation outputs can be considered good. INTRODUCTION Hydrogenated amorphous silicon has been extensively investigated as a radiation detector material for high energy and heavy ion physics experiments and for medicine and also industrial X-ray applications [1]. The attractiveness of a-Si:H for these applications essentially stems from the possibility to exploit the already existing photovoltaic and electronic technologies to create highly radiation-resistant, large area and low cost sensor arrays. Photosensitive a-Si:H p-i-n diodes in conjunction with fast scintillators have been already investigated by several groups[2,3,4] for X-ray
and y-ray detection, spectroscopy and imaging. This coupling permits a drastic reduction of the film thickness, then overcoming the problems relating to the peeling off of the films and to the necessity of high growth rate deposition techniques in conjunction with a "detector grade" material quality. Moreover, the (scintillator) / (a-Si:H p-i-n) coupling allows the sensor to work in photovoltaic mode, with the obvious advantages of minimum space requirements, absence of high voltage bias and simplicity of experimental arrangement. These features, together with a very low dark current, allow the realization of position sensitive detectors which can be used as beam profilers, exposure monitors and survey meters. Recently [5], we reported on the results obtained by using an amorphous silicon detector for Xray monitoring and dosimetry, concluding that this approach may reach a good energy sensitivity in the energy region up to I MeV In this work we present the improvements on the performance of a (scintillator/photodiode) package obtained by etching the top doped layer of the a-Si:H film around the electrodes in order to reduce the surface dark current and to increase the signal/noise ratio of the photodiode, by operating in photovoltaic mode. In this way we have matched t
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