Vortextm – a New High Performance Silicon Drift Detector for Xrd and Xrf Applications

A new class of silicon drift detectors (SDD), called “VortexTM”, with a large active area (~ 0.5 cm), high-energy resolution (<150 eV FWHM) and high-count rate capability (>1 Mcps) has been developed for X-ray diffraction (XRD) and X-ray fluorescence (XRF) applications. The VortexTM design allows for a relatively large active area while still maintaining a very low anode capacitance (~ 60 fF). This very small detector capacitance results in a reduction of the seriesnoise component and hence a reduction of the overall inherent electronic noise. The VortexTM detector utilizes novel patent pending structures that have produced very low dark current (both bulk silicon dark current and surface dark current), high electric field, uniform charge collection, low noise and high-sensitivity to low energy X-rays. An energy resolution of 143 eV FWHM was measured at 5.9 keV, 6 μs peaking time; < 250 eV FWHM was achieved at 250 ns with commensurate output count rates of greater than 400 Kcps. The details of the detector performance as a function of amplifier peaking time and input count rates, and as compared to a comparable Si(Li) detector, are discussed. INTRODUCTION The development of charge coupled devices (CCD's) for light-signal imaging, utilizing extremely low capacitance of the detector and readout circuitry, opened up a new chapter in possible nuclear detector designs. This also started a vigorous effort to develop silicon drift detectors for high-energy physics applications [1, 2]. Interest in the development of new structures for X-ray spectroscopy followed [3-6, 12-16]. The beauty of the drift detector design in this regard, is that, unlike traditional planar detectors, the SDD allows for a relatively large active area while still maintaining a very low capacitance (~60 fF) to achieve low noise. In order to take advantage of the low capacitance of the drift detector, the detector must be matched to a low capacitance input transistor. State-of-the-art low noise FETs (field effect transistors) for spectroscopy generally have a capacitance much larger than is optimal for use with the drift detector. In addition, standard techniques for coupling the FET to the drift detector anode typically add stray capacitance. Therefore, one approach has been to design low noise FETs, which can be integrated, or processed, directly on the detector substrate, thereby reducing the overall system capacitance [7-10]. However, it is difficult to achieve high a transconductance internal FET and anecdotal reports from several users of these detectors suggest that the preamplifier involving use of the internal FET may cause severe instability with count rate. Thus we have developed new patent pending techniques for the VortexTM technology using highly optimized coupling of an external FET. Copyright©JCPDS International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 332