Spatial resolution of confocal XRF technique using capillary optics
© Dehlinger et al.; licensee Springer. 2013
Received: 15 November 2012
Accepted: 6 May 2013
Published: 7 June 2013
XRF (X-ray fluorescence) is a powerful technique for elemental analysis with a high sensitivity. The resolution is presently limited by the size of the primary excitation X-ray beam. A test-bed for confocal-type XRF has been developed to estimate the ultimate lateral resolution which could be reached in chemical mapping using this technique. A polycapillary lens is used to tightly focus the primary X-ray beam of a low power rhodium X-ray source, while the fluorescence signal is collected by a SDD detector through a cylindrical monocapillary. This system was used to characterize the geometry of the fluorescent zone. Capillary radii ranging from 50 μm down to 5 μm were used to investigate the fluorescence signal maximum level This study allows to estimate the ultimate resolution which could be reached in-lab or on a synchrotron beamline. A new tool combining local XRF and scanning probe microscopy is finally proposed.
KeywordsX-ray fluorescence Polycapillary Monocapillary
X-ray fluorescence (XRF) is a highly sensitive, non-destructive technique that is able to detect element traces for material elemental analysis. It is now widely used in various fields of science such as material processing, cultural patrimony, archaeology, medical and biology, environment, etc. Two approaches are possible to increase the XRF lateral resolution for chemical mapping. First, the primary probe diameter can be decreased as the detector aperture is increased to keep a significant signal-to-noise ratio. This is the general tendency both for in-lab classical XRF and in synchrotron environment where 30-nm resolution can be offered on few beamlines (see example in). The second solution consists in keeping the primary beam diameter constant and decreasing the detector input aperture. In this latter case, it must be approached as much as possible towards the surface to keep a significant XRF signal detection. However, the detector steric hindrance impedes approaching at sub-millimetre distance from the surface without primary beam shadowing. A solution is to use a sharp monocapillary to collect the XRF signal near the surface. The XRF signal is proportional to the primary source brightness and thus, in both modes, the higher is the brightness, the higher the signal-to-noise ratio can be expected.
Thanks to the development of new focusing optics like polycapillary lens[7, 8], micro-XRF analysis became possible using laboratory and even portable X-ray sources. In this case, the lateral resolution of the technique is essentially provided by the primary beam geometry and still leads to numerous works in a huge variety of domains[1, 10]. Later, equipping the detector with a second polycapillary lens, a new concept based on a confocal configuration was proposed. Indeed, the detected signal comes from the intersect between the volume excited nearby the source lens focal plane and the analyzed volume in the vicinity of the detector lens focal plane[11–15]. The spatial resolution of the confocal micro-XRF technique is thus enhanced compared to the classical configuration.
However, it is possible to further enhance the spatial resolution of the technique, further shrinking the detector acceptance, and approaching virtually towards the surface using a thin cylindrical capillary. In this work, we have built a test-bed for feasibility demonstration using single cylindrical glass capillaries of 50- down to 5-μm radius equipping an EDX detector. XRF escaping from a Co sample irradiated by a focused micro-X-ray source was measured by these means. From the detected flux values, extrapolation gave low flux values that should be realistically measurable with the same detector equipped with a 0.5-μm radius cylindrical capillary.
Results and discussion
Consequently, point A’ in Figure 4 is positioned at a distance rA’ = 1.7 rspot from the beam centre. To compare the expected and measured values of Φa, we have thus replaced 2 rspot in Equation 1 by distance A’B = 1.7 rspot + rspot. With these considerations, Φa values of 258, 208, 178 and 168 μm are expected for a capillary radius of 50, 25, 10 and 5 μm, respectively. These values are in good agreement with the experimental values of Φa = 240, 205, 172 and 168 μm.
Is it possible to increase this signal by decreasing WD?
If WD is further decreased, the solid angle θ3 under which the capillary nozzle is seen from the point source is higher than θc (Figure 7d). The collected signal is no more limited by the capillary acceptance: the capillary gain as well as the collected signal remain constant. Because the WDc value depends on the capillary radius and the smallest value of WDc is 1 mm for the capillaries tested in this work, this optimum value was chosen and taken constant in all these experiments.
Because the fluorescent emitting source in the experiments is not punctual, we have started simulations to estimate the flux collected with a 0.5-μm radius capillary positioned at a WD of 1 mm. These simulations are based on a finite element method calculation from fundamental parameter equations and will be presented elsewhere. Figure 5 shows the dependence of the collected signal with the capillary radius in the range of 0.5 to 50 μm. The calculated values are in good agreement with the experimental ones. The estimated flux with a 0.5-radius capillary is 0.07 photons/s. This value is obtained at 1 mm WD. However, the maximum signal should be reached at 100 μm WDc value. For this WDc value, about 0.7 counts/s flux can be expected. Note that increasing the acquisition time should lead to significant signal level enhancement with our EDX-SDD device. These results show that it is possible to collect the fluorescence signal using a thinner capillary without any loss on the signal level if it is close enough to the surface. Of course, using a brighter primary source such as a rotating anode or a liquid-metal jet anode electron-impact X-ray source, a significantly higher signal (up to 100 times) can be expected Moreover, replacing the cylindrical capillary at the entry of the detector by an elliptical one would lead to an extra gain of 20[21, 22]. Thus sub-micro-resolution XRF would be possible with an in-lab excitation source. Of course, working with a synchrotron source would lead to higher signal magnitude which could allow to further shrink the capillary radius, and a sub-100-nm lateral resolution could probably be reached. The short capillary-sample working distance suggests that the cylindrical capillary could act as a scanning probe microscope tip to acquire simultaneously sample topography and chemical mapping by XRF analysis, as already demonstrated for simultaneous SNOM-XAS XEOL apparatus. Moreover, within this perspective, the spatial resolution of the detection would not be limited by the critical angle θc because the extremity of the glass tube would be approached in mechanical near-field interaction with the sample.
In this work, we have developed a test-bed consisting in a low power Rh-source focused with a polycapillary lens on a cobalt sample and in a cylindrical capillary to collect the fluorescence signal at the vicinity of the surface. Both capillaries are positioned in a confocal-like configuration. The primary beam has been first characterized, and the lateral profile of the X-ray spot was found to be a Gaussian which radius and magnitude depend on the X-ray energy range. The average radius measured at 1/e is 22 μm. Then, a cobalt sample was placed in the focal plane of the lens, and the generated fluorescence was collected through a cylindrical capillary fixed on a SDD EDX dectector. The thin detection capillary was then scanned across the sample fluorescence emitting zone. Significant signal was collected over a total capillary travel in very good agreement with what can be deduced from simple geometrical considerations. The fluorescence signal magnitude increases as rcap1.8 where rcap is the capillary radius. The extrapolated value for a 0.5-μm radius capillary suggests that sub-1-μm resolution XRF should be possible with a laboratory source. Of course, increasing the source brightness, i.e. working with liquid-metal or synchrotron sources could probably lead to reach 100-nm resolution. Operating at short working distances will allow the increase of the signal level detection. Furthermore, it could lead to a new generation of instrument, coupling XRF and scanning probe microscopy, allowing to simultaneously combine chemical analysis of a sample and topography.
This work was supported by the CEC EUREKA-EUROSTAR program (‘LUMIX’ project E4383) and by the French program CNano-PACA (‘nano-XRF’ project).
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