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Silicon Avalanche Detectors for X-Ray Analysis
Abstract
The note demonstrates the potential capabilities for X-ray analysis by avalanche detectors with relatively thick drift regions. The experiments showed that Si AD can be used in X-ray analysis of the K lines of all elements from the first half of the Periodic Table and of the L and M lines of almost all elements. The standard planar technology used for detector manufacturing can reduce their price.
The performance of n- and p-type silicon detectors manufactured by a recently developed planar technology for the detection of β-particles, conversion electrons and γ-rays is reported. Two structures are tested: an ion implanted detector with p+-n-n+ structure and a diffusion detector with n+-p-p+ structure.The experiments show that planar detectors can be used for high energy electron counting, for γ-ray detection and for identification of low energy β-sources.The phenomenon of total absorption of high energy conversion electrons in thin silicon detectors is observed and explained. It is demonstrated that the peaks of total absorption can be used for the estimation of the thickness of the depleted region of the detector.
The application of reach-through avalanche photodiodes (R'APD) as a photodetector for scintillator tiles has been investigated. The light collected by WLS fibers (0.84mm and 1mm diameter) embedded in the scintillator has been transmited to the 0.5mm2 active surface of APD by clear optical fibers and optical connectors. A low noise charge sensitive preamplifier (approximately 400 electrons equivalent noise charge) has been used to gain the photodiode signal. Various configurations of tile-fibre systems, suitable for CMS and LHCb experiments at LHC have been studied using cosmic muons and muon beam at SPS at CERN. In order to optimize the performance of APD, measurments in the temperature range from -10C to +25C have been done. The MIP detection efficiency and electron/MIP separation have been estimated in order to determine applicability of the readout for LHCb preshower. Comment: 20 pages,13 figures
The relative efficiency of a Si(Li) detector has been experimentally determined over the energy range from 1.5 to 5 keV. The method used is described in detail. It is based on a comparison between experimentally measured Kα characteristic X-ray intensities emerging from “thick” standards, irradiated with an 55Fe annular source, and theoretical intensities calculated for corresponding standards, excitation source and geometry used. The theoretical model is based on a fundamental parameters method and on a Monte Carlo simulation of geometry of the experimental setup used. Data obtained in this way are finally fitted with the Maquardt linear fitting procedure to find the optimal thicknesses for the beryllium window, the evaporated gold layer and the silicon dead layer. Obtained data were used to calculate the relative detector efficiency for photon energy between 1 and 5 keV. The determined efficiency uncertainties are 3–5% for energies below the Si absorption edge (1.735 keV) and only 1–3% for higher energies.
The working principle of Si radiation detectors is described, choosing a p+nn+ structure as a typical device. The physical laws for interaction of corpuscular and gamma radiation with matter are discussed and the conditions for energy spectroscopy derived. For position determination of radiation resistive charge division, integrated diode arrays, charge drift and charge transfer in CCDs are considered. The need for nuclear radiation detectors in research and technology is demonstrated.
Three types of silicon avalanche detectors for low energy gamma- and X-ray counting were manufactured using planar and planar-epitaxial technology. The detectors were tested at room temperature with 57Co and 241Am. Besides the characteristic lines of the sources additional peaks were observed due to X-rays of elements included in the matrices and holders of the sources.For 6.4 keV the energy resolution is approximately 10%.
A planar process is described, which enables the fabrication of low noise high quality Si radiation detectors, by using ion implantation for doping and oxide passivation for reduction of the leakage current. Several technological problems are discussed associated with detector fabrication. Finally, the technical possibilities and economical limits are considered.
By applying the well known techniques of the planar process: oxide passivation, photo engraving and ion implantation, Si pn-junction detectors were fabricated with leakage currents of less than 1 nA cm−2/100 ωm at room temperature. Best values for the energy resolution were 10.0 keV for the 5.486 MeV alphas of 241Am at 22°C using 5 × 5 mm2 detector chips.
Avalanche diodes which are sensitive enough to detect low energy x-rays and beta particles have been fabricated and tested. They operate at room temperature and their structure is based on large area avalanche photodiodes. X-ray response has been measured over the energy range of 4 keV to 26 keV. At 5.9 keV a FWHM of 10% was obtained. In addition, multi-element sensors have been fabricated. Beta particles were detected from a variety of isotopic sources including C1-36, Sr-90, C-14 and H-3. The tritium beta particles, with 5 keV average energy, were detected with a 20% counting efficiency from a calibrated source. Tritium beta particles from biological samples were also measured. The high sensitivity to low energy x-rays and beta particles makes this detector useful for many industrial, military and commercial applications, and in particular for biomedical research.
An avalanche detector radioisotope X-ray fluorescence analyzer has been developed that has rapidly, accurately, and confidently measured the calcuim and potassium content of U. S. Geological Survey silicate rock standards. The measurements were made at room temperature with percentage errors of ±5 percent. The interference due to X-rays from adjacent elements was minimal. The precision of measurement with the solid-state instrument was such that at least 90 to 95 percent of the readings were within 1¿ of the mean value.