Detection of Charged Particles in Thick Hydrogenated Amorphous Silicon Layers
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DETECTION OF CHARGED PARTICLES IN THICK HYDROGENATED AMORPHOUS SILICON LAYERS I. FUJIEDA*, G. CHO*, S.N. KAPLAN*, V. PEREZ-MENDEZ*, S. QURESHI*, W. WARD*, AND R.A. STREET** *Lawrence Berkeley Laboratory, Berkeley, CA 94720 **Xerox Palo Alto Research Center, Palo Alto, CA 94304. ABSTRACT We show our results in detecting particles of various linear energy transfer, including minimum ionizing electrons from a Sr-90 source with 5-12 micron thick n-i-p and p-i-n diodes. We measured W (average energy to produce one electron-hole pair) using 17keV filtered Xray pulses with a result W=6.0+0.2eV. This is consistent with the expected value for a semiconductor with band gap of 1.7-1.9eV. With heavily ionizing particles such as 6 MeV alphas and 1-2 MeV protons, there was some loss of signal due to recombination in the particle track. The minimum ionizing electrons showed no sign of recombination. Applications to pixel and strip detectors for physics experiments and medical imaging will be discussed. INTRODUCTION Various devices have been used for radiation detection in physics experiments and medical imaging. Gas filled chambers have the longest history and semiconductor detectors are relatively new. Scintillation detectors are intermediate. Each of these devices satisfies different requirements specific to each application. For physics experiments, there is a need for large area, position-sensitive detectors which are also radiation-resistant. Its performance should not be affected by unusual environment such as strong magnetic fields. When these conditions are not met by a single type of radiation detector, we turn to new materials or a combination of existing devices. It is our belief that amorphous silicon (aSi) offers an interesting alternative that satisfies these requirements. The same argument applies to medical imaging although some conditions are less stringent. To summarize advantages of a-Si:H are: (1) large area position-sensitive detectors can be fabricated easily; (2) the material is radiation-resistant and induced radiation damage can be annealed; and (3) relatively low cost. The challenging questions are whether the material can be made thick enough to achieve sufficient signal to noise (S/N) ratio, and to have adequate transport of the charge carriers created by the radiation. We have obtained relatively thick a-Si:H diodes grown on glass substrates by Plasma Enhanced Chemical Vapor Depositioi (PECVD) from Xerox (5-12/pm p-i-n, n-i-p) and from Glasstech Solar (G.S.I.) (12, 27 pim p-i-n diodes). The dangling bond density of the thick i layer of these diodes is k~-- as low as possible with state-of-the-art technology (< 1 x 1015 /cm 3 ). These diodes are reverse-biased, and connected to standard electronics for solid state radiation detectors, i.e. charge-sensitive preamplifier followed by a shaping amplifier. We have exposed these diodes to several kinds of radiation, such as photons, Xrays, and charged particles. The discussion on experiments and results follows. I-V, NOISE CHARACTERISTIC Fig. 1 shows a typic
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