Response of a-Si:H Detectors to Protons and Alphas
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RESPONSE OF a-Si:H DETECTORS TO PROTONS AND ALPHAS J. DUBEAU*, T. POCHET*, A. KARAR**, L.A. HAMEL*, J.P. MARTIN*, S.C. GUJRATHI* AND A. YELON****.
B. EQUER***,
*Laboratoire de Physique Nucl~aire, UniversitE de Montrial, MontrEal, Canada. **Laboratolre de Physique Corpusculaire, Collage de France, Paris, France. "***Laboratoire de Physique des Interfaces et des Couches Minces, Ecole Polytechnique, Palaiseau, France. ****Dgpartement de Genie Physique, Ecole Polytechnique, MontrEal, Canada. ABSTRACT We measured the response of reverse-biased a-Si:H diodes, to charged particles as a function of in the p-i-n geometry, in the ionization, detector bias (V) and angle of incidence, of 10-300 keV/pm. In the two cases where the ionization range namely at low biases or at low was weak, amount of signal ionizations, the signal was extracted from the noise using a was found to be roughly technique. The response coincidence proportional to 07 , at all values of dE/dx, in the bias range of 7 V to 50 V. The dependence of the charge collection efficiency on the ionization density indicates a strong plasma the detector. This collection efficiency increases effect in steeply when the ionization is decreased but no plateau has yet been reached, indicating that the plasma effect is still important at 10 keV/pm. INTRODUCTION Crystalline Si and Ge have been used as particle detectors since the early 1960's. Many types of devices were developed for various applications and, due to their excellent energy resolution and timing characteristics, they became the basic tools of nuclear spectroscopy. However single crystals are expensive, limited in size and sensitive to radiation damage. When particles of higher energies are studied, larger detectors are needed, and for many applications, large area, position-sensitive detectors are required. Detectors based on amorphous semiconductor technology could provide these features along with improved radiation hardness. But lower mobilities, shorter carrier lifetimes and screening of the applied field by the charge produced along the track affect the charge collection. It is therefore essential to understand the mechanisms of signal formation in order to develop reliable amorphous silicon detectors. Successful detection of protons and alphas with reverse-biased a-Si:H diodes has already been reported for ionizations (conveniently measured by the stopping power dE/dx) in the range of 25-300 keV/pm [1-41. These studies revealed a strong plasma effect which hinders the charge collection. We report here measurements of the response of 3pm thick p-i-n devices to protons and alphas as a function of the detector bias, the angle of incidence and the energy of the particle. The measurement of a-Si:H detector signals in coincidence with a thin transmission crystalline detector enabled the study of the Mat. Res. Soc. Symp. Proc. Vol. 118. '1988 Materials Research Society
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response at low biases or low ionizations where the signal is buried in the noise. DETECTOR FABRICATION The p-i-n detectors used in thi
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