FOCUSING CRYSTAL VON HAMOS SPECTROMETERS FOR XRF APPLICATIONS

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1 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol FOCUSING CRYSTAL VON HAMOS SPECTROMETERS FOR XRF APPLICATIONS D. Clark Turner 1, Larry V. Knight 2, A. Reyes-Mena 1, Paul W. Moody 1, Hans K. Pew 1, James D. Phillips 1, Alexander P. Shevelko 3, Sergei Voronov 1, and Oleg F. Yakushev 3 1 MOXTEK, Inc., 452 West 126 North, Orem, Utah 8457, USA 2 Department of Physics and Astronomy, Brigham Young University, Provo, Utah 8462, USA 3 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow , Russia ABSTRACT In order to increase the energy resolution in XRF devices, it is necessary to use crystal-based spectrometers. We suggest a von Hamos scheme[1] of the spectrometer for this purpose. This type of spectrometer has the following definite advantages over other focusing schemes: high efficiency in a wide spectral range (or energy range); high spectral resolution (in our case E/ûE = 5-); simplicity: wavelength (or energy) scan is realized by a simple linear translation of a detector; the possibility to use mosaic crystals to increase efficiency without sacrificing spectral resolution; and compact size. Two von Hamos spectrometers with mica and graphite crystals (2mm radius of curvature) were designed and built. Absolute efficiency of the spectrometers was measured at a wavelength of about 2 ' using an Fe 55 source. The graphite crystal showed very high reflectivity (3-7 times higher than the mica crystal). An x-ray tube with Ag anode in a transmittance mode was used as an excitation source. A MOXTEK energy-dispersive PIN diode detector was used to reduce the contribution of scattered light from the sample. The spectrometers were used for XRF analysis of sulfur in oil. Experimental results, including detection limits, will be presented for these two crystals. INTRODUCTION In the von Hamos [1] spectrometer scheme a crystal is bent into a cylindrical surface and the source and detector plane lie on the cylinder axis. In this case monochromatic radiation from the source, after diffraction from the crystal arc, is focused to a point lying on the spectrograph axis. The wavelength of radiation diffracted at a given angle is given by Bragg s law: n2dsin (1) where d is the lattice spacing of the crystal and n is an integer. By positioning a detector at position X o (=2Rcot, where R is the radius of curvature of the crystal) in Figure 1, radiation of wavelength can be detected. The relation between exposure I [phot/cm 2 ] on the image plane and the brightness of the monochromatic source L [phot/cm 2 ster] is equal to IB#L (2)

2 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol Figure 1. Ray path in a von Hamos spectrometer in direction of dispersion. The parameter B[ster] defines the spectrograph efficiency[2]: B!##sin 2 (3) where! is the integral reflectivity, is angular aperture of the spectrometer, and is the Bragg angle. It is easy to show that the ratio of efficiencies of the von Hamos spectrograph and a flat crystal spectrograph working at the same geometry is equal to R#/a (4) where R is the radius of curvature of the crystal and a is the source size. This ratio can be thought of as the extra solid angle collected with the curved crystal versus a flat crystal. For small source sizes (a < µm) and typical spectrograph parameters ( 1 rad, R 1 mm) factors of have been observed [2]. Normally, mosaic crystals (see Table 1) have a wide rocking curve width that leads to a low spectral resolving power in crytal spectrometers //tan// (5) where / is the rocking curve FWHM. Nevertheless, it is possible to compensate the mosaic spread of crystals when the source and detector are symmetrically disposed with respect to the

3 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol diffracting planes as described below. Table 1. Parameters of some commonly used crystals. Crystal plane 2d, '!, 1-5 rad /, min // = tan // Ref. Mica ~2-4 ~ -2 [3,4] Quartz ,-1, [4] Graphite [3,4] LiF [3] In the von Hamos geometry, for any orientation / on the crystal surface for which the Bragg condition is valid, reflected rays intersect the spectrometer axis at distance x which is very close to x. If we assume that cot << /, then it can be shown [5] that ûxx x x2r(/) 2 cot. (6) Values of ûx at different angles when / =.5( (graphite crystal) and R = 2mm are presented in Table 2. In the third column the corresponding spectral resolving power // is presented. For a comparison, in the fourth column spectral resolving power, defined by mosaic spread of the graphite crystal, is shown. It is seen that the mosaic spread of the crystal is compensated and the spectral resolution is defined by the source size and not by the mosaic spread (assuming Eq. 6). Table 2. Spectral resolution of graphite crystal., deg ûx, µm // mosaic spread // = tan// The von Hamos spectrometer offers several features that make it a good choice for an analytical instrument: The size can be relatively small, especially compared with existing WDX spectrometers. The high resolving power offers excellent analytical sensitivity. Mosaic crystals with mosaic focusing provide high reflectivity and improved sensitivity. Focusing geometry provides for a wide spectral range. The addition of a moveable detector allows for multiple element determinations. Addition of an energy-dispersive detector allows rejection of multiple order reflections at a given angle. Given these features, an excellent use of the spectrometer would be to determine the levels of sulfur in oil and other petroleum products. Excellent energy resolution is required to separate lines such as the S K. from the Pb M lines (ûe approximately 3 ev), yet there is a limited set of elements that is of analytical interest.

4 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol EXPERIMENTAL Two von Hamos spectrometers with mica (St. Petersburg Mica Factory, St. Petersburg, Russia) and graphite (Optigraph, Moscow, Russia) were designed and built [5]. Preliminary calibration of the crystals was made using an Fe 55 isotope source (=2.13') and a MOXTEK PIN diode detector (Model PF-7, MOXTEK, Orem, Utah). The measured integrated reflectivities [5] are shown in Table 3. The data are in good agreement with reference 2. Note that the integrated refectivity of the graphite crystal is about 3-7 times higher than that of the mica crystal. Table 3. Integrated reflectivity! of crystals measured using an Fe 55 isotope source (=2.13'). Mica (2µm thickness), third-order reflection! = 4.6 x 1-2 mrad (effective 2d = 19.88/3 = 6.61') Graphite (µm thickness), first-order reflection 2d = 6.78' Graphite (2µm thickness), first-order reflection 2d = 6.78'! = 125 x 1-2 mrad! = 278 x 1-2 mrad X-Ray Tube C A D B Cylindrically bent crystal - Mica or graphite R λ 2 λ 3 Sample λ 1 Detector X Figure 2. Spectrometer geometry. Points A and B are on the same cross section of the cylinder. Points A, C and D are on the same line parallel to the cylinder axis.

5 Copyright(c)JCPDS-International Copyright(c)JCPDS-InternationalCentre Centrefor fordiffraction DiffractionData Data21,Advances 21,AdvancesininX-ray X-rayAnalysis,Vol.44 Analysis,Vol.44 Another von Hamos spectrometer was set up using the geometry outlined in Figure 2. The x-ray tube has a transmission target end window with silver anode [Model BS-1, Svetlana-Röntgen, St. Petersburg]. Operating conditions were 25 KV and.3 ma emission current. The detector is a MOXTEK PIN diode with 8µm DuraBeryllium window, moveable within a dectector mounting sleeve. The spectrometer is detailed in Figures 3 to 5. Figure 3. Prototype von Hamos spectrometer. Figure 4. Prototype spectrometer. View showing sample cup for containing the sulfur in oil sample. Sulfur in oil standards were obtained from SPEX [ppm, catalog #DSS8-2Y, and Base Oil 2, SPEX CertiPrep, Metuchen, NJ]. Dilutions of the standard into the base oil were done for 1 and ppm concentrations. After evacuating the spectrometer cavity, the optimum detector position was set while irradiating a pure sulfur Figure 5. Spectrometer crystal cavity. Tubulation at bottom left allows helium purging or vacuum pumping. powder sample. Energy-dispersive spectra of the oil samples were then obtained during 2-minute collection times. Data were collected using both the mica and graphite crystals. RESULTS Some representative spectra are shown in Figures 6 and 7. With the mica crystal it is possible to observe fluorescence of the crystal elements, namely potassium, iron, and copper. But with an energy-dispersive detector these elements do not interfere with the sulfur analysis

6 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol Base Oil Blank - Mica Crystal ppm Sulfur - Mica Crystal 3 25 K 3 25 K 2 15 Fe Cu 2 15 S Fe Cu 5 S Channel # Channel # Figure 6. Spectra collected using mica crystal. Left figure is a base oil blank. Right figure is a ppm sulfur in oil standard. The spectra collected with the graphite crystal are interesting as they demonstrate the improved reflectivity of the crystal. Scattered primary tube radiation, including bremsstrahlung radiation, is reflected from the crystal at the expected Bragg reflection wavelengths. Again, the energydispersive detector allows resolution of these higher order reflections. Base Oil Blank - Graphite Crystal ppm Sulfur - Graphite Crystal 8 Lambda/2 Lambda/ Sulfur 6 4 Lambda/4 2 Lambda Lambda/ X-ray Energy (kev) X-ray Energy (kev) Figure 7. Spectra collected using graphite crystal. Left figure is a base oil blank. Right figure is a ppm sulfur in oil standard. Note the scattered tube radiation at integral divisions of lambda. The intensity of the scattered peaks suggests the shape of the Bremsstrahlung output from the x-ray tube. Minimum detection limits for 1 minute collection times were estimated from the equation MDL3#Conc# Background Peak (7) and the results are reported in Table 4 below.

7 Copyright(c)JCPDS-International Centre for Diffraction Data 21,Advances in X-ray Analysis,Vol Table 4. Estimated detection limits for von Hamos multidispersive spectrometer. Standard Conc. Background Peak MDL Crystal Mica ppm ppm Graphite ppm ppm CONCLUSIONS The von Hamos multidispersive spectrometer with 7.5 watt tube, graphite crystal, and Si PIN diode detector has demonstrated 2 ppm sensitivity for sulfur in oil. With reduced background and scatter the spectrometer should be capable of better than 4 ppb detection limits for sulfur. In addition to sulfur, a moveable detector will allow analysis for lead, vanadium, iron and other elements. Future enhancements will include the use of miniature x-ray tubes to reduce the size of the spectrometer and the use of CCD detectors for simultaneous collection of energy-dispersive spectra and spatial resolution information. REFERENCES 1. Hamos, L.v.; Naturwiss 2, 1932, Shevelko, A.P.; Proceedings SPIE, 346, 1998, Gilfrich, J.V.; Brown, D.B.; Burkhalter, P.G. Applied Spectroscopy, 29, 1975, Alexandropoulos, N.G.; Cohen, G.G. Applied Spectroscopy, 28, 1975, Shevelko, A.P.; Antonov, A.A.; Grigorieva, I.G.; Kasyanov, Y.S.; Knight,L.V.; Reyes-Mena, A.; Turner, D.C.; Wang, Q.; Yakushev, O.F. Proceedings SPIE, 4144, 2, in press.