XRF

The New HORIBA XGT-9000:
An Overview

TechnicalNote
XGT01

Sofia Gaiaschi, Jocelyne Marciano
HORIBA FRANCE SAS, Palaiseau, France.
Abstract: In this technical note we present the features of the XGT-9000, the new X-ray microscope from HORIBA. We
show its versatility through some examples of the application domains which could benefit from a micro X-ray fluorescenceanalysis.
Keywords: µ-XRF; X-ray mappings, X-ray fluorescence
The XGT-9000 is the new X-ray analytical microscope fromHORIBA. The versatility of this instrument makes it a perfecttool for industrial applications, to detect failure problems orforeign particles, as well as R&D. The application domains ofEnergy Dispersive X-ray Fluorescence (EDXRF) range fromcosmetics to pharmacy, archaeology, agro-aood, metallurgy,
electronics etc… Moreover, this technique can provide both
qualitative and quantitative bulk composition of solid andliquid samples. In addition, when equipped with a XY table,
large elemental mappings can be provided.

The principle of X-ray fluorescence is schematicallyrepresented in Figure 2. When materials are exposed toX-rays (primary X-rays in Figure 2) the ejection of one ormore electrons from the inner shell of the atom may occur.
The removal of an electron makes the electronic structure
unstable and in order to re-establish the neutrality, electrons
in the outer shells «fall» into the lower ones to fill the holeleft behind. In falling, energy is released in the form of acharacteristic X-ray photon, the energy of which is equal tothe energy difference of the two orbitals involved (Kα, Kβ, Lα,etc, in Figure 2). Thus, the material emits radiation with anenergy characteristic of the atoms present.

Figure 1. The new XGT-9000 microscope

Figure 2. Schematic representation of the X-ray fluorescencephenomenon.

Figure 1 shows the standard instrument, which allows amaximum mapping size of 10x10 cm. A large chamber model(the XGT-9000SL) is also available, providing a maximum
mapping size of 35x35 cm.

In the XGT-9000, the characteristic X-rays are detected thanks
to an energy dispersive silicon drift detector that combines highcount rate with good resolution. Furthermore, no liquid nitrogenis needed as the Peltier effect is used to cool it down.

A typical EDXRF spectrum is illustrated in Figure 3. The
X-ray tube can work up to 50 kV and 1 mA. In theseconditions, high energy characteristic X-Rays (e.g. Ba Kα at
32.194 keV) can be generated, giving the possibility to deal
with some interferences like Ti Kα (4.512 keV) and Ba Lα(4.466 keV). For instance, the high energy Ba Kα is used forthe quantification of Ba. Then, the software, using a fixedpeak ratio approach, subtracts the Ba contribution from theinterfering peak at ~4.5 keV, leaving just the Ti Kα for thecorrect Ti quantification.

Figure 4. Examples of calibration curves for (a) Mg and (b) Si in aluminiummatrix.

Figure 3: X-ray spectrum of barium titanate. In the inset we present thespectrum between 31 keV and 34 keV. In green we present the main Ti
emission lines and in red the Ba ones. Without the high energy Kα lines, itwould be impossible to deconvolute the Ba and Ti interference in the low
energy region.

The primary X-rays are focused on the sample thanks tomono or polycapillary X-Rays Guide tubes from 10 µm up to
1.2 mm. No charging effect is induced by X-Rays irradiation,
allowing to analyse the samples with little or no preparation.
The system is also equipped with several energy filters, toenhance the excitation of X-rays in specific areas of thespectrum, and therefore, to identify elements at detection
limits.
At a given energy, the characteristic X-Ray intensity depends
on the concentration of the emitting element. However, due tointerelement effects, the quantification is indirect, needing touse standard samples or the fundamental parameter method(FPM). When using standard samples, for each element tobe quantified it is required to build specific calibration curves(regression between the concentration and the measured
X-ray intensity, as shown in Figure 4). This method has theadvantage of giving a better accuracy, however the numberof calibrated elements and the calibration range depend on
the available reference materials. On the other hand, theFPM is less precise, but its specificity offers the possibility tomodel the sample as a bulk and to calculate its composition
without the need of any reference sample. Indeed, once theXRF spectrum is acquired, thanks to a suitable simulation
software, equations are set up, containing expressions forprimary and characteristic x-ray excitations for every element.

These complex equations also include physical andhardware fundamental parameters (e.g. x-ray absorption,incident beam energy and intensity, etc). The elemental
concentration is then adjusted and compared with the actual
x-ray intensities of the sample spectrum. Using an iterative
process, the concentration of the sample can be calculated.
The advantage of the micro-XRF is the XY table, which
provides the possibility to acquire a pixel-by-pixel spectrum
from which X-ray elemental images can be reconstructed, as
shown in Figure 5.
(Courtesy of CERATIZIT)

Figure 5. XRF image of circular saw blade (a) Zn raw mapping, (b) W raw
mapping. Using the PS mode allows to remove the W Lα and the Zn Kαinterference, obtaining the deconvoluted mapping (c) for Zn and (d) for W.

The software is equipped with the peak separation (PS)function, which allows the removal of interferences. Figures
5a and 5b show the raw images of Zn and W, for an SEM
preparation of a circular saw blade. The blade is made mainlyof steel, where WC tips are soldered using a 4900 soldering
wire (Ag 49%, Cu 16%, Zn 23%, Mn 7.5% and Ni 4.5%).
In the figures we see that the Zn contribution is not only
detected inside the soldering, but also inside the WC; andconversly, we see a W contribution inside the soldering aswell as in the WC. This is due to the overlapping of the W Lα(8.39 keV) and the Zn Kα (8.63 keV), as shown in Figure 6.
Thanks to the PS function, pixel-by-pixel, the deconvolutionof the general spectrum is performed, and the interference of
these two elements are correctly removed, giving access tothe final images presented in Figures 5c and 5d.

Another feature of the XGT-9000 software is focused on
the possibility to locate and count single particles. Themappings (X-Rays, transmitted or combined, as shown inFigure 8) can be processed through this function for particle
detection. Then, their average size can be measured, and asize distribution (number of particle, with its relative diameter)
can be obtained. Moreover, once all the particle positions
are registered, they can be used to perform point analysis oneach one of them.

Figure 6. Zoom in on the XRF spectrum of the circular saw blade
presented in Figure 5. Thanks to the PS function, the deconvolution of theW Lα and the Zn Kα is possible, providing the correct elemental mapping.

The instrument can be run in full or partial vacuum. The
advantage of the full vacuum is the possibility to detect lightelements, down to Na. However, when working in partial
vacuum it is possible to analyse liquids, biological samples
or hydrated ones without any damage, as they are kept atatmospheric pressure. For example, insects can be easily
characterised (Figure 9).

Furthermore, the addition of a scintillation detector
placed under the sample can provide an image of X-rays
transmitted. Figure 7 shows the optical image (Figure 7a),the Cu Kα (Figure 7b) and the X-ray transmission (Figure7c) of an electronic component. The latter gives access tocomplementary information compared to those provided by
the characteristic X-ray ones, showing the internal structureof the sample and therefore enabling the detection of innerdetails or defects.

Figure 9. X-Rays mappings of a Mosquito (P, K, Ca, Fe, Cu and Zn) andcombined mappings (P, K, Ca and Fe)

Finally, the micro-XRF becomes a suitable non-destructive
technique for the measurement of coating thicknesses.
The relatively large penetration depth of X-rays (typicallyfrom several μm to mm levels, depending on the analyzedmatrix) allows multiple layers to be simultaneously analyzed.
Therefore, with some a priori information about the elemental
composition of each layer and their order, it is possible tomodel samples as a stack of layers. Once the XRF spectrum
is acquired, the iterative process of the FPM allows to adjust
and compare the simulated spectrum with the experimental
one, easily providing both thickness and composition of eachlayer in the stack. However, the accuracy of this method issomewhat limited. For this reason, an alternative solution is touse calibration curves. As it was previously explained for thequantification, if certified thickness standards are provided,
they can be used to build thickness calibration curves.
Figure 8: TiAlV particles – a) Transmitted image; b) Ti X-ray image, c) V
X-ray image d) Combined image (transmitted+Ti+V) and e) Number ofparticles and relative size

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Figure 7. a) optical image, b) Cu Kα and c) transmission image of anelectronic component.