Silicon Detectors Highly Compensated by Neutron Induced Deep Levels for Low Energy X-Ray Detection *
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SILICON DETECTORS HIGHLY COMPENSATED BY NEUTRON INDUCED DEEP LEVELS FOR LOW ENERGY X-RAY DETECTION* Zheng Li and H. W. Kraner Brookhaven National Laboratory, Upton, New York 11973 ABSTRACT Fast neutron radiation damage in silicon results in defect levels which are predominantly acceptor-like at low fluences and may be used to compensate high resistivity ntype material to create very high effective resisitivity material. Compensated material to the order of Neff below 1011/cm3 enables depletion of diode thicknesses > 1mm at reasonable biases (< 100V), yielding diodes of reasonable area and capacitance< 1 pF which are suitable for low noise applications such as X-ray spectrometry. Although exposure to fluences of this order will greatly increase the generation current and require cooling, most high resolution Xray spectrometry systems are routinely operated at reduced temperature to achieve low noise operation of the front end electronics. Silicon p+ /n- /n+ implanted devices (area :0.25 cm 2 ) made on high resistivity FZ silicon have been irradiated by 1 MeV neutrons to fluences of a few times 1012 n/cm 2 . Thick n- substrates (d=630 14m and 1000 14m) were used to achieve detector capacitances eco/d in the range of 1 pF. After a neutron fluence of 4n=2.9x102 12 n/cm2 , the total depletion of a p+/n-/n+ detector, 1040 14m thick and an area of 0.1 cm , is reached at about V=50 Volts, with a Cd of 1 pF and a neutron induced leakage current of about 300 nA at room temperature. A total depletion of an 680 sm thick detector was reached after the fluence of 2-5x1012 n/cm 2 at a voltage of 20 volts, and the capacitance of a 0.25 cm 2 diode is 4.5 pF The resistivities of the compensated detector substrates are in the range of 100 K Ql-cm, and are not inverted to "p" type. The trapping of collected charge by neutron induced deep levels is modeled and simulated, and is found to be less than a few percent; with no obvious effect on peak shape. Using a resistive feedback preamplifier of modest noise contribution (225 eV), resolution of the Mn Ka X-ray was 255 eV (FWHM) with 3 jtsec shaping time constants . Other effects of uncollected charge will be discussed and comparisons between this type of detectors and Li-drift silicon detectors will be made. INTRODUCTION Semiconductor materials, such as silicon and germanium have been widely used to fabricate X-ray and y-ray radiation detectors. One of the most important material consideration is the detector depletion thickness or the impurity or more appropriately, net or effective impurity concentration Neff of the substrate material. Larger depletion thickness gives larger active region for charge collection and lower capacitance for reduced first stage noise from the preamplifier, a critical issue or low energy X-ray spectroscopy. Traditionally, X-ray and y-ray radiation detectors are made on high purity germanium (HPGe) and silicon compensated by lithium drifting (Si(Li))[l 5 ]. The detector configurations for HPGe and Si(Li) are typically surface barrier devices,giving a thin entr
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