Evaluation of Silicon Photodiode as X and Gamma-Ray Detector at Room Temperature
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EVALUATION OF SILICON PHOTODIODE AS X AND GAMMA-RAY DETECTOR AT ROOM TEMPERATURE
V.CIMPOCA, GH.CARAGHEORGHEOPOL, D.LAZAROVICI, C.LAZAROVICI, R.RUSCU Institute for Atomic Physics, Bucharest 76900, P.O.Box MG-6, Romania
ABSTRACT A silicon photodiode 10 sqmm area designed for 0,45 A/W and maximum sensitivity at 900 gm, was tested as X and gamma-ray detector in the 6-122 keV range. The simple structure, good manufacturing and passivating for a NIP photodiode (6 Kn cm,p- type material) gives a yield of 20% from batch with suitable characteristics for low energy X and gamma-ray detection. A cheap nuclear detector for technical measurements results. Evaluation and analysis of parameters in X and gamma detection is made for these silicon structures; they are mounted on TO-8 holders without entrance lenses. The applications proposed namely the composition test of alloys (with known matrix) by X-ray fluorescence analysis and in radiation safety field (absorbed dose survey), ex. in radiological clinics defectoscopic laboratories or in front of video displays. INTRODUCTION The utilization of silicon structures for X and gamma ray detection at room temperature is a goal of nuclear radiation dosimetry and spectrometry, because these detection systems are developed and easy to use. Silicon detector (in spite of not having detection efficiency due to a small thickness), works stable and are easy to use in measurements. The reverse bias is low (for high resistivity silicon) and the associated electronics for applications is now available in a various convenient standard equipment. The criteria for a spectrometric grade detector is the capacity to separate the energetic lines of radionuclide emission, that is to have a good resolution. For dosimetry, the detector must give a signal to noise ratio of minimum 10 (taking into account a high threshold for noise discrimination), as high as possible efficiency and flat response to radiation in the energetic range of interest. Our silicon photodiode was designed for visible light and infrared range. The obtained parameters (especially the reverse dark current), allowed us to select a fraction of the manufactured batch as radiation detector for soft X and gamma rays. The cost of such a detector is slightly higher than that of a photodiode (increased only by some special measurements for selection as nuclear radiation detector). The whole photodiode batch is used: a fraction as detectors for nuclear techniques and the emaining one as infrared sensors. PROCESS OF PHOTODIODE MANUFACTURE The semiconductor material used was P-type silicon with 6 Kx~cm resistivity, maximal dislocation density (EPD) 500/cm 2, lifetime greater than Ims, crystal orientation (111). The photodiode is square and has 10 mm2 active area; his structure is n+pp+ with diffused
Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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junction and contacts. Around the sensitive zone, a guard ring, diffusion made, is provided (as unpolarized p+n junction). A MOS planar process was designed to obtain dark
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