Research Papers

A New Improved Silicon Drift Detector (SDD) for Microanalysis and X-Ray Mapping Applications

Shaul Barkan*, Valeri D. Saveliev*, Jan S. Iwanczyk**, Liangyuan Feng*, Carolyn R. Tull**, Bradley E. Patt**, Dale E. Newbury*** and John A. Small*** * SII NanoTechnology USA Inc, 19355 Business Center Drive, Suite 8, Northridge, CA 91324 ** Photon Imaging, Inc. *** National Institute of Standards and Technology A new class of silicon drift detectors (SMCD) has been developed for microanalysis and X-ray mapping applications [1,2]. The SMCD has a large active area (~0.5 cm2), high energy resolution, and high count rate capability. The detector utilizes novel structures that have produced very low dark current, high electric field, uniform charge collection, low noise and high sensitivity to low energy X-rays [3,4]. A custom designed spectrometer package has been built with a long probe to fit a JEOL 840 scanning electron microscope (SEM). In bench-top measurements with an 55Fe radioisotope source, an energy resolution of 129 eV FWHM was repeatedly measured with the 0.5 cm2 SMCD (at 5.9 keV, 6 – 12 µs peaking time, cooled to -70 oC). The spectrometer package, shown in Figure 1, contains a 0.5 cm2 SMCD detector, cooled by using a small Peltier element. The spectrometer model shown in Figure 1 was designed specifically to be used with a JEOL 840 scanning electron microscope (SEM) at the National Institute of Standards and Technology (NIST). The probe diameter is ¾” and its length is 12”. An atmospheric thin window (ATW) from Moxtek (Orem, UT) is placed at the window edge and allows good transmission of very low energy X-rays while the detector is kept in a high vacuum condition. An electron trap is placed on the window to deflect electrons scattered from the sample. The detector spectral response was evaluated using an 55Fe radioisotope source, as well as by fluorescing materials with an X-ray source. Figure 2 shows an 55Fe spectrum showing an energy resolution of 129 eV FWHM at 5.9 keV (collected at 12 µs peaking time, -70 oC). To evaluate the high count rate X-ray performance, which is very important for fast X-ray mapping, a Cu sample was fluoresced using a Rh anode X-ray tube. Figure 3 shows the Cu spectra collected at: (a) 3000 cps input, with 206 eV FWHM at 8.04 keV (0.5 µs peaking time), and (b) at > 1 Mcps with an energy resolution of 214 eV FWHM and output count rate exceeding 350 kcps. The photopeak position and energy resolution are virtually independent of count rate. The detector is scheduled to be assembled on a JEOL 840 SEM at NIST at the end of February 2004. We will measure the X-ray energy resolution and count rate performance under microanalysis conditions. In addition, we will evaluate the spectral response to very low energy X-rays, including C and B. The SEM results will be presented. References: [1] S. Barkan, et al., “Vortex® – A new high performance silicon drift detector for XRD and XRF Applications”, Advances in X-Ray Analysis, Vol. 46 (2003) 332-337. [2] L. Feng, et al., “A New High Performance Silicon Drift Detector for XRD and XRF Applications”, Hard X-ray and Gamma-ray Detector Physics V, Proceedings of SPIE, International Society of Optical Engineering, Vol. 5198, (2004) 103-110. [3] J. S. Iwanczyk, et al, “Large Area Silicon Drift Detectors for X-Rays- New Results”, IEEE Trans. Nucl. Sci. vol. 46 (1999) 284-288. [4] U.S Patent #6,455,858 B1 “Semiconductor Radiation Detector”, 2002. Acknowledgements: This work was supported in part by NIST. Download this article in Adobe PDF format.
Figure 1. Photograph of the first generation prototype, spectrometer designed for the JEOL 840 SEM. Probe length is 12”.
Figure 2. Spectral response of SMCD to 55Fe (12 µs peaking time)
Figure 3 (a). Spectrum of Cu sample at low count rate (206 eV FWHM at 8 keV, 0.5 µs peaking time and 6% dead time)
Figure 3 (b). Cu spectrum at high count rate (>1 Mcps input, 350 kcps output, 214 eV FWHM at 8 keV, 0.5 µs peaking time, 68% dead time).

VORTEX® – A New High Performance Silicon Drift Detector for XRD and XRF Applications

J. Liangyuan Feng, Shaul Barkan and Carolyn R. Tull SII NanoTechnology USA Inc., 19355 Business Center Drive, Suite 8, Northridge, CA 91324 Jan S. Iwanczyk and Bradley E. Patt Photon Imaging, Inc., 19355 Business Center Drive, Suite 8, Northridge, CA 91324

ABSTRACT

Vortex®, a high performance silicon drift detector, has been developed and extensively tested for potential X-ray diffraction (XRD) and X-ray fluorescence (XRF) applications. As a type of silicon drift detector, it utilizes our patented structure design [1] and achieves very low capacitance and very low leakage current with a relatively large active area (~50 mm2). The detector operates at near room temperature with thermoelectric cooling and is thus very compact in size. These features make it ideal for many XRD and XRF applications. Results will be presented to demonstrate its superior performance over conventional cryogenic Si(Li) detectors, especially with respect to energy resolution and throughput at short amplifier peaking times. Keywords: Vortex®, Silicon Drift Detector, Silicon Drift Detector, FET, Peltier Cooler, X-ray Diffraction, X-ray Fluorescence, Multi-channel Analyzer, Single-channel Analyzer, Digital Pulse Processor, Sealed Proportional Counter, Graphite Monochromator, SMCD, SDD, XRD, XRF, Si(Li), SCA, MCA, DPP, SPC, FWHM, ICR, OCR Download this article in Adobe PDF format.

High Efficiency Silicon X-Ray Detectors

C. R. Tull, Member IEEE, J. S. Iwanczyk, Senior Member IEEE , B. E. Patt, Senior Member IEEE, S. Barkan, and L. Feng

ABSTRACT

Thick silicon drift detectors (SMCD) for high efficiency X-ray detection have been designed, fabricated and tested. These thick detectors (up to 1.5 mm thick) extend the practical X-ray detection range from the current level of ~20 keV, up to ~40 keV, while still maintaining the low noise and high count rate performance of the thinner (~0.3 mm) SMCD technology. The increase in X-ray detection efficiency at higher energies will have a significant impact on practical uses of these detectors in a wide variety of X-ray fluorescence (XRF) applications. In addition to increasing the detection efficiency for X-rays, the thick silicon detectors will offer improved efficiency for high energy electrons, alphas and other light particles in nuclear physics and astrophysics applications. Very high resistivity float zone material was used for the substrates to minimize the operating voltages required. Multiguard ring structures were designed to prevent the premature breakdown of the devices at the voltages required to fully deplete the thick detectors. We have measured 172 eV and 158 eV FWHM energy resolution at 5.9 keV (at 4 µs and 12 µs peaking time, respectively, -55 oC) on 1 mm thick prototype detectors. Spectral performance, energy resolution, efficiency and count rate performance are presented. Index Terms: X-ray detector, silicon, drift detector, synchrotron. Download this article in Adobe PDF format.

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