VORTEX A NEW HIGH PERFORMANCE SILICON DRIFT DETECTOR FOR XRD AND XRF APPLICATIONS

Transcription

1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume VORTEX A NEW HIGH PERFORMANCE SILICON DRIFT DETECTOR FOR XRD AND XRF APPLICATIONS Shaul Barkan, Jan S. Iwanczyk, Bradley E. Patt, Liangyuan Feng, and Carolyn R. Tull Photon Imaging, Inc., Business Center Drive, Suite # 8, Northridge, CA ABSTRACT A new class of silicon drift detectors (SDD), called Vortex, with a large active area (~ 0.5 cm 2 ), 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 Vortex 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 Vortex 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 Vortex technology using highly optimized coupling of an external FET.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume VORTEX DETECTOR The electrical and mechanical packaging for the Vortex detector was designed to accommodate the detector, Peltier cooler platforms, heat exchange assemblies and utilized vertical assembly techniques for interconnecting the detector with the electronics to ensure minimum microphonics and noise. The Vortex detector housing is shown in Figure 1. Figure 1. A Photo of the outer housing of the Vortex detector. The coin in front of the detector window is a quarter. The detector is optimally coupled to a special low noise FET and a custom-designed low noise preamplifier. Tennelec TC 244 and Canberra 2026X amplifiers, a Nucleus PCA multi-channel analyzer, and XIA DXP-X10P were used to characterize the detector response to various radioisotope sources. Spectral Response An 55 Fe spectrum, obtained with the Vortex SDD detector is shown in Figure 2. An energy resolution of 143 ev FWHM was obtained for the 5.9 kev photopeak, at an amplifier peaking time of 6 µs.

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume mm 2 SDD - Fe 55 Spectrum, 143eV 10,000 1,000 Counts Chanel Figure 2. An 55 Fe spectrum obtained with a 0.5 cm 2 Vortex SDD at 6 µs peaking time. Count Rate Performance Energy resolution as a function of amplifier peaking time for both a cryogenically cooled 30 mm 2 Si(Li) detector and the Vortex detector is shown in Figure 3. The rapid degradation in resolution for the Si(Li) detector at shorter peaking times is due to the larger capacitance of the Si(Li) detector compared with the SDD detector. Fe 55 FWHM of 30mm 2 Si(Li) and 50mm 2 SDD FWHM mm2 Si(Li) mm2 SDD Peaking Time [microsec] Figure 3. Comparison of the energy resolution as a function of amplifier peaking time, for a 30 mm 2 Si(Li) and the 0.5 cm 2 SDD Vortex detector, in response to 55 Fe. Figures 4 and 5 show Cu spectra from a Vortex SDD and the Si(Li) detector, respectively collected at 1 µs peaking time. The resolution advantage of the SDD at the short time constant is clearly apparent.

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Figure 4. Cu spectrum from a 50 mm 2 SDD at 1 µs peaking time, with 60% dead time; Cu K α line is 200 ev FWHM (linear scale). Figure 5. Cu spectrum from a 30 mm 2 Si(Li) detector, at 1 µs peaking time, with 50% dead time; Cu K α line is 340 ev FWHM (linear scale). Using an X-ray tube excitation makes it possible to test the performance of this special detection unit (Vortex SDD plus DPP MCA) under very severe count rate conditions. In particular, we were most concerned about the throughput, resolution and peak shift performances. In the test we used a Cu foil sample irradiated with varying X-ray fluxes. Spectra were acquired at different peaking times under various input count rates (ICR) and output count rates (OCR). Table 1 shows the results obtained. Table 1. Cu Kα Resolution, Peak Shift vs. Throughput PeakingTime ICR DT OCR CuKα FWHM CuKα Peak Pos. [µs] [KCPS] [%] [KCPS] [ev] [KeV] Note: The peak position and resolution were determined using Gaussian Peak Fitting method. Background correction was applied for resolution calculations.

6 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume CONCLUSIONS The Vortex SDD has superior performance characteristics at high count rates and short amplifier shaping times compared with traditional Si(Li) and Ge detectors, and can be operated using only thermoelectric cooling, which enables compact and portable instrumentation. Energy resolutions of 143 ev FWHM at 6 µs peaking time and 250 ev FWHM at 250 ns peaking time (at 5.9 kev) makes the instrument ideal over a wide range of count rates for X-ray spectroscopy applications. The Vortex SDD package, as shown in Fig. 1 can be used for both XRD and XRF applications. The Vortex SDD can be mounted into the vacuum chamber of a scanning electron microscope or transmission electron microscope for standard chemical analysis or fast X-ray mapping applications [11]. In the very high count rate (> 1 Mcps) regime, with associated good energy resolution (<300 ev FWHM at Cu K α 8.4 kev), the Vortex SDD is also ideal for X-ray diffraction applications. ACKNOWLEDGEMENTS This work was sponsored in part by the DOE SBIR #DE-FG03-97ER82450, DOE SBIR #DE- FG03-99ER82853 and NIH SBIR #1R44-RR REFERENCES [1] E. Gatti & P. Rehak, Semiconductor Drift Chamber An Application of a Novel Charge Transport Scheme, Nucl. Instr. And Meth. In Phys. Res. 225 (1984) 608. [2] W. Chen, H. Kraner, Z. Li, P. Rehak, E. Gatti, A. Longni, M. Sampietro, P. Holl, J. Kemmer, U. Faschingbauer, B. Schmitt, A. Woner and J.P. Wurm, "Large Area Cylindrical Silicon Drift Detector," IEEE Trans. on Nucl. Sci. V39 (1992) [3] G. Bertuccio, A. Castoldi, A. Longoni, M. Sampietro and C. Gautheir, "New electrode geometry and potential distribution for soft X-ray drift detectors," Nucl. Inst. and Meth. in Phys. Res. A312 (1992) [4] P. Jalas, A. Niemela, W. Chen, P. Rehak, A. Castold, A. Longoni, "New Results with Semiconductor Drift Chambers for X-Ray Spectroscopy," IEEE Trans. on Nucl. Sci., V41 (1994) [5] E. Pinnoti, A. Longoni, M. Gambelli, L. Struder, P. Lechner, C.V. Zanthier, & H. Kraner, Room Temperature High Resolution X-Ray Spectroscopy with Silicon Drift Chambers, IEEE Trans. on Nucl. Sci., V42 (1995) 12. [6] J.S. Iwanczyk, B.E. Patt, G. Vilkelis, L. Rehn, J. Metz, B. Hedman & K. Hodgson, Simulation and Modeling of a New Silicon Drift Chamber X-ray Detector Design for Synchrotron Radiation Applications, Nucl. Instr. & Meth. in Phys. Res. A380 (1996) [7] M. Sampietro, L. Fasoli, P. Rehak and L. Struder, Novel p- JFET embedded in silicon radiation detectors that avoids preamplifier feedback resistor, IEEE Elect. Dev. Lett., 16 (1995) [8] G. Cesura, N. Findeis, D. Hauff, N. Hornel, J. Kemmer, P. Klein, P. Lechner, G. Lutz, R. Richter and H. Seitz, New pixel detector concepts based on junction field effect transistors on high resistivity silicon, Nucl. Instr. Meth. A377 (1996)

7 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume [9] P.F. Manfredi, V. Re and V. Speziali, Monolithic JFET preamplifier with nonresistive charge reset, IEEE Trans. Nucl. Sci. 45 (1998) [10] K. Misiakos and S. Kavadias, A silicon drift detector with a p-type JFET integrated in the n-well anode, Nucl. Instr. Meth. A458 (2001) [11] J.S. Iwanczyk, Bradley E. Patt, Carolyn R. Tull, and Shaul Barkan, High-Throughput, Large Area Silicon X-Ray Detectors For High-Resolution Spectroscopy Applications, Proceedings of the Microscopy and Microanalysis 2001 Conference, Long Beach, CA. Aug , Invited paper [12] J.S. Iwanczyk, B.E. Patt, C.R. Tull, and L.M. MacDonald, Novel X-Ray and Gamma-Ray Drift Detectors Based on Silicon and Compound Semiconductors, Proceedings of SPIE, Vol (1999) [13] J.S. Iwanczyk, B.E. Patt, C.R. Tull, J.D. Segal, C. Kenney, J. Bradley, B. Hedman, and K.O. Hodgson, Large Area Silicon Drift Detectors for X-Rays-New Results IEEE Trans. Nucl. Sci. Vol. 46, No. 3 (1999) [14] J.S. Iwanczyk and B.E. Patt, "New Detectors for XRF Analysis", Advances in X-Ray Analysis, V41 (1998) [15] J.D. Segal, C.H. Aw, J.D. Plummer, C. Kenney, S.I. Parker, G. Vilkelis, J.S. Iwanczyk, and B.E. Patt, "A Vertical High Voltage Termination Structure for High Resistivity Silicon Detectors", IEEE Trans. Nucl. Sci. Vol. 45, No. 3 (1998) [16] J.D. Segal, B.E. Patt, J.S. Iwanczyk, G. Vilkelis, J.D. Plummer, B. Hedman, and K.O. Hodgson, "A New Structure for Controlling Dark Current Due to Surface Generation in Drift Detectors, Nucl. Instr. Meth. A 414 (1998)