The monolithic X-ray polycapillary lens and its application in microbeam X-ray fluorescence† Xie Jindong,* He Yejun, Ding Xunliang, Pan Qiuli and Yan Yiming Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, China Received 2nd September 1998, Accepted 20th November 1998 A monolithic polycapillary X-ray lens is a single piece glass optic made in a furnace. It is composed of hundreds of thousands of individual capillaries. The X-ray lens can be used to collect divergent X-rays emitted from an X-ray source in a large solid angle and to transmit them with high eYciency by multiple total reflections in the individual capillaries, thus forming an intense focused beam. Recently, new monolithic focusing lenses have been manufactured, tested and used for microbeam X-ray fluorescence (MXRF) analysis in the authors’ laboratory. For the Mo Ka line, the measured focal spot diameter was 30 mm.A gain factor of more than 2000 compared with when an aperture was used instead of the lens was obtained.The application of the lens in MXRF reduced the detection limits for transition elements down to the sub-pg level when a 1 W X-ray source was used. In the last 20 years, microbeam XRF (MXRF) spectrometry thousands of individual capillaries in a single bundle.10 It can be made by single or multiple composition processing of has become an increasingly popular and promising analytical method. This is partly due to the increasing importance of the drawing a bundle of glass capillaries in a drawing tower.In a single processing step a bundle of thousands of monocapillaries detailed and comprehensive analysis of micromaterials and of extremely small samples of bulk materials.1,2 On the other is simply drawn through an oven to form a monolithic lens. Such lenses are designed for application at low X-ray energies, hand, the use of high flux synchrotron sources and the application of capillary X-ray focusing optics oVer very high where large diameters of the channels and a relatively small number of capillaries per lens are needed.For hard X-ray spatial resolution with powerful X-ray microbeams for such analyses. lenses, a much larger number of capillaries (hundreds of thousands) and smaller diameters of the channels (a few There are two types of capillary optics being used today: monocapillaries3–5 and polycapillary focusing systems6,7 microns) are required, so in a first step one has to make the polycapillary bundles by drawing in a drawing tower, followed (Kumakhov lenses).Both are based on multiple total reflection of X-rays on the inner surface of capillaries. The polycapillary by drawing a bundle of polycapillaries to form a hard X-ray lens. X-ray lens is composed of a large number of capillaries and can collect divergent X-rays emitted from an X-ray source in The monolithic X-ray lens is compact, flexible and can easily be installed in available devices.The important characteristic a large solid angle and transmit them in capillaries with high eYciency, forming a very small focal spot (focusing lens, for of a monolithic lens in comparion with an assembled lens is that the cross-section of each channel changes along its axis, use in MXRF) or a quasi-parallel X-ray beam (for X-ray diVraction studies). so that all channels of the optic are independently oriented to the focal point of the lens. In principle, the focal spot size is Among the diVerent types of X-ray lenses, the monolithic X-ray lens has unique advantages in application.The mono- determined only by the divergence of the X-ray beam as it emerges from each individual channel. In practice, of course, lithic X-ray lens is a single piece glass optic made of capillaries in a furnace. Such lenses are very compact, have high trans- it is very diYcult to realize this situation at the current level of technology. mission eYciency and are convenient for utilization. The manufacture and evaluation of monolithic lenses and their The design of an X-ray lens is based on modelling of the multiple total external reflection of X-rays on the inner surface application in MXRF in the X-Ray Optics Laboratory at Beijing Normal University was started 4 years ago, and its of the X-ray capillary channels with simple roughness correction.A polycapillary lens uses curved channels (radius of development is still in progress.In this paper a brief review of the manufacure, measurement curvature R) to cut oV X-rays of high energies which have grazing angles larger than the critical angle of total reflection of basic characteristics and application of the monolithic lens is presented. The possible combination of a polycapillary lens for a given material. A simple geometric calculation yields the following inequality for succesful transmission: with a monocapillary is also briefly discussed.R>2d/hc2 (1) The manufacture and design of the monolithic X-ray where d is the diameter of the channel, R the radius of lens curvature and hc the critical angle of total reflection, which is The development of the X-ray lenses in our laboratory involved linearly proportional to the wavelength of X-ray. X-rays will three steps. The first generation of X-ray lens is the optics not be transmitted when the geometry of the channel does not assembled from monocapillaries,8 the second generation is the satisfy inequality (1).For the monolithic X-ray lens, the optics assembled from polycapillaries9 and the third generation transmission eYciency depends on the photon energy and is the monolithic lens, which is made up of hundreds to geometric parameters of the lens such as the sizes of the channels (diameters and lengths) and the shape and size of the lens. In designing the monolithic lens, first one should deter- †Presented at the Fifteenth International Congress on X-ray Optics and Microanalysis (ICXOM), Antwerp, Belgium, August 24–27, 1998.mine the type of lens and the desired energy range of X-rays J. Anal. At. Spectrom., 1999, 14, 391–394 391according to the requirements of the application. Then one determines the geometric parameters of the lens by a simulation calculation optimizing the performance of the lens. A computer code for designing X-ray monolithic lenses based on simulations of X-ray trajectories by Fresnel reflection and a refraction equation with wall roughness correction and diVerential geometry was developed.11 Characterization and measurement of monolithic lens A monolithic lens can be characterized by its physical parameters, namely the transmission eYciency, gain in power density, focal spot size and equivalent distance.10 The physical properties of the lens are to some extent also dependent on the characteristics of the X-ray source used in the experiments.Here, an REIS-I (Svetlane-Rentgen, St.Petersburg, Russia) X-ray source was employed for the evaluation lenses. The power of the X-ray generator was 5 W and an Mo anode X-ray tube with an 80 mm spot size of X-rays was used. A detailed description of the measurement system was publised previously.8 The whole experimental apparatus was placed along the optical track on an optical table. Several multi- Fig. 2 (a) Knife edge scan at the focal spot of lens F4 using the Mo dimensional holders were placed on the optical track for Ka line.(b) DiVerentiated knife edge scan at the focal spot of lens mounting and aligning the X-ray source, X-ray lens, knife F4 using the Mo Ka line. edge, aperture, X-ray detector, etc. A schematic diagram of the measuring set-up is shown in Fig. 1. Determination of the focal distance and of the focal spot size The lens must be aligned at the beginning of the measurement. The lens alignment can be achieved by changing the distance between the lens and X-ray source and the source position in a transverse direction relative to the axis of lens to maximize the X-ray intensity through the lens.At the maximum throughput, the distance between the source and the input of the lens is defined as the input focal distance. The distance between the output surface of the lens and the focal plane is defined as the output focal distance. The focal spot size can be obtained by measuring the integrated curves of counts of Fig. 3 FWHM of the output beam as a function of the distance from X-rays with a 0.5 mm thick Mo knife edge scanning at diVerent the exit surface of lens F4. distances from the lens output surface [see Fig. 2(a)]. These curves are then diVerentiated to give the space distribution of Table 1 Geometrical parameters and measured results for monolithic lens F4 the power density of the X-ray beam at that particular distance from ouptput surface of the lens. The focal spot size is defined Lens length 52 mm as the minimum full width at half maximum (FWHM) of the Entrance diameter 6.5 mm space distribution of the measured power density.The result Exit diameter 5.0 mm of such a procedure for lens F4 is shown in Fig. 2 and 3. The Input focal distance 45 mm main geometric parameters and physical characteristics of lens Output focal distance 15.65 mm Focal spot size 30 mm F4 are given in Table 1. Transmission eYciency at 17.4 keV 0.8% Gain of power density 2317 Transmission eYciency Equivalent distance 2.34 mm The transmission eYciency of a lens is defined as the ratio of the intensity of X-rays emerging from the lens on the output measuring the direct beam flux (without the lens) at a given side to the intensity of the X-rays incident upon the lens distance and making a correction for solid angle and distance.entrance. The former term can be measured directly at the The transmission eYciency of a lens is a function of the exit of the aligned lens.The latter term can be determined by X-ray energy, since the value of the critical angle of total reflection is dependent upon the X-ray energy. The transmission eYciency of lens F4 was measured to be 0.8% for the Mo Ka line. There is still the possibility of increasing the transmission eYciency by improving the technology and optimization of the lens parameters. It is should be pointed out that misalignment of the X-ray source–lens system in the direction perpendicular to the lens axis can significantly influence the transmission eYciency, as shown in Fig. 4.The experimental data can be fitted by a Fig. 1 Schematic diagram of the measuring set-up. (1) X-ray source; Gaussian curve. The FWHM of this curve depends strongly (2) lens; (3) aperture or knife edge; (4) HpGe detector; (5) five axis holder; (6) three axis holder; (7) optical track. on the critical angle of total reflection of X-rays and the spot 392 J. Anal. At.Spectrom., 1999, 14, 391–394Fig. 5 Spectrum of sputting alloy on Mylar film. Fig. 4 Transmission of the Mo Ka line as a function of source ground, and therefore the signal-to-background ratio can be misalignment for lens F4. higher and the minimum detection limit (MDL) of MXRF can be improved; and (c) compared with monocapillaries, the size of the X-ray source. The transmission curve implies that working space on both the source side and the sample side the X-ray source has to be positioned exactly at an appropriate can be increased, because the monolithic lens usually has an position for maximizing the transmission eYciency, because input focal distance of tens of millimetres and an output focal only the X-rays which can reach all channels under a grazing distance of more than 10 mm.angle less than the critical angle of total reflection will As an example, experimentally determined MDL values contribute substantially to the transmission eYciency.obtained in an MXRF analysis set-up which was described previously8 is discussed. A standard sample was prepared by Gain and equivalent distance of the lens sputtering a steel alloy on to a thin Mylar film in the Synchrotron Laboratory of the Institute of High Energy The gain in power density obtained by the lens is defined as Physics of the Chinese Academy of Sciences. A spectrum taken the ratio of the X-ray intensities at the focal spot of the lens from this alloy sample is shown in Fig. 5. Based on this with and without the lens. It is determined by directly measurspectrum and the nominal contents of the elements in the ing the focused beam intensity and the X-ray intensity with alloy, the MDL values were obtained using the commonly an aperture at the position of the focal spot without the lens. used equation For an isotropic X-ray source the gain (K) can be calculated by means of the following equation: MDL=3.29CNb1/2/Na (4) where C is the mass of each element in the sample, Nb the K(E)=AL f1B2 Sin Sspot g(E) (2) background counts and Na the net counts of peak area.The results are presented in Table 2. where g is the transmission eYciency of the lens for X-rays of All previously measured MDL values which were obtained energy E, L the distance from the X-ray source to the focal during the last 4 years are listed in Table 3 in order to illustrate spot of lens, Sin the entrance area of the lens, Sspot the area of the advances made in MXRF sensitivity using monolithic the focal spot and f1 the input focal distance.focusing X-ray lenses. It can be seen that the MDL for Fe is The gain of the lens directly reflects the amplification of the about 0.5 pg. There is no doubt that the MDL can be improved X-ray beam power density by the lens. Another useful magni- signficantly if a much more intense X-ray source is used. tude is the equivalent distance (Leq). The equivalent distance can be calculated with the equation Table 2 MDLs obtained by MXRF analysis using the lens F4.Conditions: Mo anode, 27 keV, 36 mA, 1000 s Leq=L/K1/2 (3) where Leq is the distance between the X-ray source and the Content Net peak Background MDL/ Element C/pg (counts) (counts) pg point where the power density of X-rays emitted directly by the X-ray source is equal to that in the focal spot of lens when Cr 15.7 4459 1986 0.526 the lens is used. An equivalent distance of 2–3 mm was Fe 52.9 14650 1756 0.498 achieved for lens F4 and the X-ray source REIS-I.This means Ni 12.8 2677 3080 0.873 that one can perform experiments at a point 2–3 mm from the Ga 179.9 16753 2386 1.896 X-ray source when the focusing lens is applied. As 169.1 7113 2471 3.888 Se 302.1 10543 1699 3.886 Application of monolithic focusing lens in MXRF Table 3 Improvement of MDL (pg) for elements obtained by MXRF MXRF analysis is used in many fields of pure and applied using a monolithic focusing X-ray lens science and industry,12–16 such as physics, chemistry, biology, materials science, earth science, life science, enviromental Cr Fe Ni Spot size and sample used Ref.science, medical science, microelectronics industry, metallurgy and even archaeology and forensic medicine for analysis and 84.2 56.6 235 157 mm, mix 05a 17 15.2 7.72 38.5 50 mm ( lens+aperture), 18 mapping of element contents. In recent years, monocapillaries mix 05a (straight, tapered and ellipsoidal ) have been applied in 0.865 0.722 50 mm, sput 02b 19 MXRF.4,5 The application of the monolithic X-ray focusing 0.526 0.498 0.873 30 mm, sput 01b This work lens has several advantages: (a) the lens can produce a very aSample prepared by pipettting a mixed standard solution on to a intense X-ray microbeam (tens of microns) from a relatively thin Mylar film.bSample was prepared by sputtering a steel alloy on weak X-ray source; (b) the lens has a definite bandwith of to a thin Mylar film.transmission, so it can cut oV the high energy photon back- J. Anal. At. Spectrom., 1999, 14, 391–394 3933 D. J. Thiel, D. H. Bilderback, A. Lewis and E. A. Stern, Nucl. Instrum. Methods, Sect. A, 1992, 317, 547. 4 A. Attaelmanan, S. Larsson, A. Rindby, P. Voglis and A. Kuczumow, Rev. Sci. Instrum., 1994, 65, 7. 5 K. Janssens, L. Vincze, B. Vekemans, F. Adams, M. Haller and A. Kno�chel, J. Anal. At. Spectrom., 1998, 13, 339. 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