New a-Si:H Photo-Detectors for Long-Term Charge Storage
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NEW a-Si:H PHOTO-DETECTORS FOR LONG-TERM CHARGE STORAGE H. LEE, G. CHO, J. S. DREWERY, W. S. HONG, T. JING, S. N. KAPLAN, A. MIRESHGHI, V. PEREZ-MENDEZ AND D. WILDERMUTH Lawrence Berkeley Laboratory, Berkeley, CA 94720 ABSTRACT Using the high light absorption properties of amorphous silicon, we developed a new device configuration that can detect photons and store the induced charges for relatively long time. This device, coupled to a scintillator such as CsI(T1) in an array form, could be used as a scintillation camera, or for long-term photo-detection such as radionuclide labeled chromatography. The detector has a simple sandwich structure consisting of a scintillator followed by a top metal layer, p-i-n layers of hydrogenated amorphous silicon (a-Si:H), a second metal layer, a thin insulating layer and a bottom metal layer. The electron-hole pairs generated in the i-layer by the interaction with the incident light will be separated by the imposed electric field and be stored in the central metal-insulator interface. Readout will be done by switching the external bias to ground after the storage time, which depends on the needs for the specific application. Prototype devices were fabricated and tested. The performances of the devices were analyzed in connection with the storage time and the background signal produced by the thermally generated charges. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) devices have been described for radiation detection applications. [1] Due to the large area and high radiation resistivity, a-Si:H radiation detectors have some advantages over the crystalline devices. There are many applications of a-Si:H radiation detectors which include detection of visible light, x-rays, 7-rays, charged particles and neutrons.[1-4] For example, for the imaging of y-ray distribution in nuclear medicine we can replace a conventional scintillation camera with an a-Si:H pixel detector combined with a scintillator. CsI(Tl) is known to be an appropriate scintillator for this purpose because of its high light yield and high interaction efficiency for y rays.[4] While the intrinsic spatial resolution of conventional scintillation cameras is more than 3 mm, a-Si:H pixel devices can be made to have much higher spatial resolution if required. Moreover, these devices do not utilize photomultiplier tubes (PMTs) unlike the conventional cameras and accordingly have the advantage of compactness and light weight. With ordinary p-i-n a-Si:H pixel detectors for this purpose we would have to record each incoming y ray by low noise multistage amplifiers at each pixel because of the low counting rate. Storage and integration of the individual counts to give a larger signal can eliminate the need for pixel level amplifiers thus making the readout scheme much simpler. In this paper we describe a device which is appropriate for this purpose. This consists of a scintillator placed on the top transparent conducting layer followed by pi-n a-Si:H layers, a second metal layer, an insulating layer and a bottom metal layer
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