Radiation Detection with Evaporated Csl(TI) Coupled to a-Si:H Photodiode Layers
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RADIATION DETECTION WITH EVAPORATED CsI(TI) COUPLED TO a-Si:H PHOTODIODE LAYERS I. FUJIEDA*, G. CHO**, J. DREWERY**, T. GEE**, T. JING** S.N. KAPLAN**, V. PEREZ-MENDEZ** AND D.WILDERMUTH** *Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304 "**Lawrence Berkeley Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720 ABSTPACT CsI(TI) layers 100-1000 gIm thick were evaporated on glass substrates from a crystal CsI(TI). When they were exposed to calibrated X-ray pulses, their scintillation properties were found to be comparable to those of a crystal CsI(TI). Single P3 particles from radioisotopes were successfully detected by these layers coupled to a crystalline Si photodiode. The light spread inside evaporated CsI(TI) was measured by an amorphous Si (a-Si:H) photodiode array coupled to evaporated CsI(TI) layers. Monolithic X-ray detectors were fabricated by evaporating CsI(TI) on a-Si:H photodiodes directly. The signal yield and noise of this prototype were 1.5x10+4 electrons/MeV and 3x10+ 4 electrons FWHM, respectively. Larger signal size and lower noise are expected by optimizing the photodiode design. INTFCDUCTfON Radiation detection with a-Si:H layers has been investigated for applications in high energy physics experiments, medical imaging, material and life science [1,2,3,4]. Thick a-Si:H layers of good quality can be depleted up to 50 gm by a high external bias and detection of minimum ionizing particles has been demonstrated by smallarea prototype detectors. However, it takes a long time to grow thick layers over a large area by the conventional Glow Discharge method and the deposited layers have a large built-in stress. These issues related to film growth must be addressed before the use of thick layers become practical. An alternative approach is to use a scintillator material coupled to a thin a-Si:H layer. Scintillators such as CdWO4, ZnS(Ni) Gd 20 2S:Tb and Gd20 2S:Pr:Ce:F have been investigated for X-ray fluence detection [5,6,7,8,9]. Choice of a scintillator depends on specific requirements for each detector application, such as detection efficiency, count-rate capability and spatial resolution in case of imaging applications. Some properties of scintillators commonly-used in high energy physics experiments and medical imaging can be found in ref [10,11]. For many radiation imaging applications using a scintillator, thickness of a scintillator is determined by the trade-off between the detection efficiency and the spatial resolution. A built-in light guide structure inside a scintillator can ease this compromise by reducing the light spread. Among the many scintillators available today, CsI is particulary interesting because vacuum evaporated Csl(Na) forms columns parallel to the evaporation direction, which restrict the scintillation light spread [12,13]. It is used for image intensifier application due to its good emission spectrum matching to the photocathode sensitivity. It has been proposed to couple evaporated Csl layers to a-Si:H photodiodes for applications that require a high s
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