MICROANALYSIS WITH A POLYCAPILLARY IN A VACUUM CHAMBER

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1 THE RIGAKU JOURNAL VOL. 20 / NO. 2 / 2003 MICROANALYSIS WITH A POLYCAPILLARY IN A VACUUM CHAMBER CHRISTINA STRELI a), NATALIA MAROSI, PETER WOBRAUSCHEK AND BARBARA FRANK Atominstitut der Österreichischen Universitäten, Vienna University of Technology, Stadionallee 2, A-1020 Vienna, Austria Microanalysis using a polycapillary as a focusing device was performed under vacuum conditions. The polycapillary was implemented in a vacuum chamber so that also low energy radiation could be used for sample excitation. A Mo tube with a 125 mm window was used, offering the possibility to employ Mo-L radiation (2.3 kev) for the excitation of lower Z elements. A Silicon drift detector (KETEK, Munich) with 5 mm 2 active area and a 7 mm Duraberyllium TM window allows the detection of low energy radiation down to Na. The operation in vacuum also reduces the background originating from scattering in air and thereby helps to improve the detection limits for medium Z elements as well. Results from an area scan around a droplet multi-element sample (distribution of elements on the surface) is presented, and measurements on soda lime glass containing Na and Al, as well as a comparison between air and vacuum conditions are shown. Detection limits of 4600 mg/g for Na in soda lime glass have been obtained. 1. Introduction Single and polycapillaries have been used widely in the recent years to produce intensive microbeams for performing microanalysis (EDXRF) with a spatial resolution of about mm [1 5] but with a few exceptions [6] these experiments have been only carried out in air. The detectable elemental range is thereby limited to elements above S (E Ka 2.3 kev), because the air path between sample and detector about 1 cm absorbs most of the fluorescence radiation with lower energies. The advantages of measuring lower Z elements with the spatial resolution obtainable by using a polycapillary initiated the idea to design a microbeam vacuum chamber. The Atominstitut X-ray group has a long experience in building vacuum chambers, however mainly for TXRF [7 10]. This experience was the backing for the actual construction of a vacuum chamber with all important components such as the polycapillary, sample holder and detector situated inside and fully adjustable with proper instrumentation. The main goal was to reduce absorption of the fluorescence radiation between sample and detector as well as to reduce the scattering in air, whereby the spectral background is minimized and the Ar-K line practically removed. The detector used was suitable for the detection of low energy radiation from Na-Ka upwards. a) Electronic mail: streli@ati.ac.at Polycapillaries offer an additional advantage in the low energy region due to the total reflection at the inner walls of the capillaries, whereby the higher energies are absorbed and only the lower energies reflected (cut-off effect), leading to efficient excitation of light elements. 2. Experimental Setup Figure 1 shows the scheme of the experimental setup and Fig. 2 a picture of the vacuum chamber with all components. A long fine focus diffraction Mo-tube (Philips PW 1043/01) of 3 kw maximum power with a point focal spot of mm was used. The thickness of the Be window is only 125 mm, so that Mo-L radiation (E 2.3 kev) is efficiently transmitted. A polycapillary manufactured by XOS [11] was used (borosilicate glass, single capillaries of 9 mm inner diameter) offering the following characteristics: focal distance from source to the entrance: 53 mm, length: 62 mm, diameter: 8mm, focal distance at exit: 13.5 mm, resulting in a total distance from source to sample of 128 mm. The focal spot size as specified for Mo- Ka radiation (17.48 kev) is 32 mm for a source size of mm. The intensity gain in comparison to a pinhole of 50 mm diameter for a source size of mm and the same energy is 39 [11]. The polycapillary was mounted inside the chamber in a brass tube which was fixed at the entrance wall. The entrance window was made of 8 mm Kapton TM, which allows high transmission also of lower energy radiation. Vol. 20 No

radiation protection between tube and chamber) until the irradiated spot on a ZnS screen was at its maximum intensity.

2 Fig. 1. Schematic setup with polycapillary in the vacuum chamber. Fig. 2. Experimental setup, view from top. The adjustment was performed without any special devices such as gimbals or rotation or translation stages, simply by moving the X-ray tube together with the adapter unit (which also serves as a radiation protection between tube and chamber) until the irradiated spot on a ZnS screen was at its maximum intensity. This spot was also checked using a CCD camera without optics, just by monitoring the X-rays directly on the chip after passing the glass protection and an Al foil which absorbs the light. After optimization the tube was fixed in its position. The detector used is a silicon drift detector (KETEK GmbH, Munich) [12] with 5 mm2 area and a thickness of 300 m m. The entrance window is 8 m m DuraberylliumTM, offering a transmission of 25% for Na-Ka. The detector was cooled electrically by Peltier effect so that no LN2 is required, which made the detector handsome and versatile to use. The amplification is performed by a Canberra 2025 amplifier with the shaping 26 time set to 0.5 m s. An optical microscope is used to control the position at which the sample is hit by the beam. As the sample is situated inside the chamber and the microscope is mounted outside, the view is performed through a glass window in the chamber wall. The working distance of the microscope is about 186 mm and both, direct view or transfer to a computer via a CCD camera is possible (Fig. 2). The sample is mounted on a slide frame, which can be inserted in a holder which in turn is mounted on an x, y, z stage. This allows scanning of the sample surface, but also moving the sample in the X-ray focal spot, which can be controlled by the focused optical image seen by the optical microscope. The scanning as well as all data handling between MCA and stepper motors involved in the scanning are performed under computer control. The setup was designed and tested within the frame of the diploma thesis of N. Marosi [13]. The Rigaku Journal

3. Results and Discussion First of all the size of the focal spot achieved with this particular polycapillary was determined by moving a knife wedge (razor blade) in 5 m m steps through the beam.

Differentiation yields the intensity variation: A Gaussian curve was fitted through the data, leading to a FWHM of 69 m m.

3 3. Results and Discussion First of all the size of the focal spot achieved with this particular polycapillary was determined by moving a knife wedge (razor blade) in 5 m m steps through the beam. Figure 3 shows the resulting intensity as a function of the position of the wedge. Differentiation yields the intensity variation: A Gaussian curve was fitted through the data, leading to a FWHM of 69 m m. This value differs from the value specified by the manufacturer; the reason seems to be the different focal spot sizes of the X-ray tubes. As a test for the capability of the new system the distribution of a multi-element solution dried on a polished TXRF quartz reflector was analyzed. 5 m l of the solution containing 5 m g/g of Cd, Co, Cu, Cr, Mn, Ni, Pb, V, Zn, and Ga were pipetted on a quartz reflector. The dried residue was investigated over an area of mm with a step width of 300 m m horizontally and vertically. The measuring time was 50 s for each scanned point with 30 kv and 20 ma. The distance between sample and detector was about 10 mm for the setting of optimum focusing. The distribution of V, Cr and Mn across the sample can be seen in Fig. 4. It is not homogenous but shows accumulations at specific points forming a ring like structure. However, all elements show their maximum intensity at the same spot. The capability of determining low Z elements with the new system was tested by measuring a soda lime glass (NIST 621 standard reference material). Na and Al are clearly visible; the concentration of Na2O is about 13% (cna 10%) and that of Al2O3 about 3% (cal 2%). The obtained Fig. 3. Vol. 20 No Determination of the focal spot-size. spectrum is shown in Fig. 5. Table 1 lists the detection limits obtained in soda lime under 30 kv and 10 ma. A comparison between operation in air and vacuum was made by measuring a multi-element sample dried on a KaptonTM foil of 8 m m Fig. 4. Distribution of elements of a multi-element solution dried on a TXRF quartz reflector. 27

4 Fig. 5. Spectrum of a soda lime glass (NIST 621) excited with Mo anode X-ray tube, 30 kv, 10 ma, 1000 s counting time. Table 1. Data obtained from NIST 621, standard reference material soda lime. Excitation conditions: 30 kv, 10 ma, measuring time: 1000 s. Index N denotes net counts and index B background counts. Vac: N N Vac: N B LLD (mg/g) Na Al Fig. 6. Multi-element sample on Kapton TM foil, Mo 30 kv, 10 ma, 50 s, (a) under vacuum conditions, (b) in air. 28 The Rigaku Journal

5 Table 2. Net and background counts, N N and N B, for measurement in air and under vacuum conditions as well as the detection limits (LLD) for 1000 s measuring time. Sample: 5 mg/g K and Ca, 100 mg/g Cr. A 5 ml droplet was pipetted on Kapton foil. 30 kv, 10 ma, 50 s. Air: N N Air: N B LLD (mg/g) Vac: N N Vac: N B LLD (mg/g) K Ca Cr thickness. The result is shown in Fig. 6. The intensity of the low energy fluorescence radiation under vacuum conditions is significantly improved. The background changes roughly in proportion to the increasing intensity. Particularly significant is the improvement of the exciting Mo-L radiation from the X-ray tube (as seen by the scatter peak) as well as the fluorescent Mg-K line (1.25 kev), Rb and Sr-L lines (1.83 kev), K and Ca-Ka lines. The Ar-K line is visible only in air. Table 2 shows the differences in net and background counts for K, Ca and Cr. Generally, the net counts are enhanced under vacuum conditions and the peak to background ratio is improved. The dried residue forms roughly a spot of 1 mm diameter; the illuminated area has a diameter of about 70 mm 2. Detection limits obtained under vacuum conditions for K are 8 ng/g. 4. Conclusions The low energy performance of a system operating a polycapillary in a vacuum chamber using a detector suitable for the detection of low energy X-rays was studied. The detection of low energy photons down to Na (1.04 kev) was shown. Detection limits of 4600 mg/g for Na in soda lime glass have been obtained, as well as 8 ng/g for K measured as dried residue on a Kapton by using a Mo-target X-ray tube with a thin window which allowed efficient transmission of Mo-L radiation. References [ 1] Kumakhov: SPIE, 116 (1996), [ 2] Hoffman, Thiel, Bilderback: Nucl. Instr. Meth., A347 (1994), 384. [ 3] B. Vekemans, K. Janssens, G. Vittiglio, F. Adams, L. Andong, Y. Yiming: JCPDS, (1999), [ 4] G. Havrilla: Adv. X-ray Anal., 41 (1999), 234. [ 5] Kumakhov: X-Ray Spectrom., 29 (2000), [6] N. Gao, I. Yu. Ponomarev, Q. F. Xiao, W. M. Gibson, D. A. Carpenter: Enhancement of microbeam x-ray fluorescence (MXRF) analysis using monolithic polycapillary focusing optics., [7] R. Rieder, W. Ladisich, P. Wobrauschek, C. Streli, P. Kregsamer: Nucl. Instr. Meth., A327 (1993), 594. [ 8] C. Streli, P. Wobrauschek, H. Aiginger: Spectrochim. Acta, 48B (1993), 163. [9] C. Streli, P. Wobrauschek, E. Unfried, H. Aiginger: Nucl. Instr. Meth., A334 (1993), 425. [10] C. Streli, P. Wobrauschek, V. Bauer, P. Kregsamer, R. Görgl, P. Pianetta, R. Ryon, S. Pahlke, L. Fabry: Spectrochim Acta, 52B (1997), 861. [11] Specification by XOS. [12] X-ray Optical Systems, Inc., Greenbush, N.Y., USA [13] N. Marosi: Diploma thesis of the Atoministitute, Vienna Univ. of Technology (2002). Vol. 20 No

MICRO XRF OF LIGHT ELEMENTS USING A POLYCAPILLARY LENS AND AN ULTRA THIN WINDOW SILICON DRIFT DETECTOR INSIDE A VACUUM CHAMBER

Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. 229 MICRO XRF OF LIGHT ELEMENTS USING A POLYCAPILLARY LENS AND AN ULTRA THIN WINDOW SILICON DRIFT