3.1. Characterization of the Laser-induced X-ray source
In this study, we used an Al target with low inner metal ionization condition to demonstrate multilayer materials can be analyzed even with a compact laser source. So it is necessary to investigating a new optimization condition suitable for bremsstrahlung emission and electrons instead of the conventional optimization condition focused on the characteristic emission. To characterize and optimize the laser-induced plasma-based sources (characteristic emission, bremsstrahlung emission, and electron source), the incident angle of the laser beam was varied from 0° to 40° in 5° increments. We used a focused laser source with a power of 1.5 W, beam diameter of 22 µm, and peak power of 1.97 × 1016 W/cm2. The peak power of the laser satisfied the inner metal ionization conditions of the aluminum metal and exceeded the Al Kα flux saturation condition. [23, 26]
Figure 2(a) shows the laser-induced X-ray fluorescence spectrum with the aluminum tape target measured by changing the incident angle. The spectrum exhibits the average value of data which was measured 10 times with an accumulation time of 5 seconds each. The Al Kα line was observed at 1.48 keV, and bremsstrahlung emission was observed for energies of ≥ 4 keV. The pile-up peak was shown in 2.96 keV, which did not affect to results of the fluorescence measurement. The maximum Al Kα peak occurred at the incident angle of 30°, whereas the minimum was at 40°. The Al Kα flux at an angle of incidence of 30° was 2.54 × 107 photon/s∙srad. The incident angle dependence of Al Kα (1.48 keV) and the bremsstrahlung emission (> 4 keV) is shown in Fig. 2(b) and 2(c). It showed the maximum value at an incident angle of 25–30°, and it decreased at the out of the optimized range. This result shows a similar trend to that of previous studies about incident angle dependence of characteristic emission. [23, 24] The incident angle of the focused laser beam affects the plasma scale length in the resonance absorption mechanism, and the Al Kα flux changes by the decrease of incident angle. [19, 30] On the other hand, the bremsstrahlung emission increased with decreasing incident angle and was saturated at ≤ 20°. When the incident angle was changed from 30° to 20°, the bremsstrahlung emission flux improved by 45%.
The bremsstrahlung emission intensity was affected by the bulk electron temperature as follows [22];
where Ibrs is the Bremsstrahlung spectrum, Te is the bulk electron temperature (keV), λ is the wavelength (Å), and A is a constant. The bulk electron temperature does not significantly affect the Kα emission conversion efficiency but can affect the shape of the Kα peak line. It has been reported that the bulk electron temperature difference shifts the Kα spectral line, as the spectral line shifts, there is also a slight change in the Kα intensity. [28, 29] The characteristic emission is occurred by the collision of accelerated electrons to the inner metal shell, while the bremsstrahlung emission is caused by the deceleration of the electrons generated in laser plasma. Because the generation mechanism of these two sources is different, we explored and established new optimization conditions focused on the optimization of the bremsstrahlung emission. X-ray fluorescence measurements were performed by applying the results from the incident-angle dependence analysis.
3.2. X-ray fluorescence measurement with the laser-induced X-ray source
We measured the X-ray fluorescence of the alloy samples by changing the incident angle to identify whether the incident angle optimization condition was well matched to the laser-induced X-ray fluorescence. We also investigated the composition analysis potential of laser-induced X-ray sources by quantitative measurements of alloy samples. The X-ray fluorescence of stainless steel (SUS 303) was measured by laser-induced X-ray sources, with respect to the incident angle changed from 0° to 30° by 10°. The stainless steel (SUS 303) used in the experiment was composed of 72.1% of Fe, 17.1% of Cr, 8.07% of Ni, and 0.580% of Si. Figure 3(a) shows the X-ray fluorescence spectrum of the stainless steel by changing the incident angle, the black line is an incident angle of 10°, and the red line is an incident angle of 30°. The X-ray fluorescence signal of Fe Kα and Cr Kα had higher values at an incident angle of 10° compared to incident angle of 30°. And the X-ray fluorescence signals of the main components of stainless steel were confirmed: the signals for Si Kα, Cr Kα, Cr Kβ, Fe Kα, Fe Kβ, and Ni Kα were observed at energies of 1.73, 5.41, 5.94, 6.40, 7.05, and 7.47 keV, respectively.
Figure 3(b) shows the comparison of the X-ray fluorescence signals of main metal components according to the incident angle. The Fe Kα and Cr Kα fluorescence signals increased by 14% and 48% at an incident angle of 10° compared to an incident angle of 30°, respectively. The X-ray fluorescence signal’s incident angle tendency followed the incident angle dependence of the bremsstrahlung emission of previous experiments. We identified that the optimization condition also applied to the X-ray fluorescence measurement, and it will adopt to improve the X-ray fluorescence signal.
We quantitatively measured three types of stainless steels (SUS 303, SUS 316, and SUS 430) to investigate the possibility of composition analysis of multi-element materials by laser-induced X-ray source. Unlike the conventional X-ray tube source, in this study, we used the bremsstrahlung X-ray source, which is a broadband source that follows a Maxwellian distribution. So, rather than using the conventional quantitative measurement technique focused on the monochromatic X-ray tube source, we performed the quantitative measurement by using calibration with standard methods, using standard samples of accurately known elemental compositions. Based on the X-ray fluorescence signal intensity and composition of alloy sample (SUS 303), the experimental composition of SUS 316 and SUS 430 was measured and calculated. Figure 4 shows the spectrum data obtained by X-ray fluorescence analysis of the 3 types of stainless steel under the same conditions. The fluorescence signals for Fe Kα, Cr Kα, and Ni Kα in the spectrum were at the energy of 6.40, 5.41, and 7.47 keV, respectively. The Fe and Cr contents of the SUS 316 obtained by experimental measurement were 69.9% and 15.8%, respectively, with relative error values compared to the actual composition were 0.590% and 1.84%, respectively. In the case of SUS 430, the Fe and Cr contents by experimental measurement were 87.1% and 14.5% with relative error values compared to the actual composition were 5.32% and 10.2%, respectively. Additional data analysis processes such as the identification of peaks caused by superimposed signals, self-attenuation, and matrix effects can improve the accuracy. It was possible to clearly distinguish the Kα and Kβ lines which are fingerprint signals of metal components. Even with the laser-induced X-ray by compact laser, it was possible to analyze the approximate elemental composition of multi-element materials, which can use to analysis of artwork and cultural heritage.
3.3. X-ray fluorescence measurement with the laser-induced X-ray and electrons.
In the previous chapters, the laser-induced X-ray source was optimized by changing the incident angle and investigated whether the optimization condition matched to actual X-ray fluorescence measurement. And, we identified a clear fingerprint of the alloy sample’s Kα and Kβ lines, the approximate composition of the alloy samples was analyzed. Finally, the X-ray fluorescence of copper-covered stainless steel was measured by laser-induced X-ray and electrons, and analyzed by changing the incident angle. In addition, as a real-setting application of surface and interior analysis on alloy samples, we analyzed three types of Korean 10 Won coins.
Figure 5(a) shows the incident angle dependence of the X-ray fluorescence photon number of copper-covered stainless steel. The black and blue points indicate the Cu Kα and Fe Kα fluorescence photon numbers measured by X-rays and electrons, respectively. The red and green points indicate the Cu Kα and Fe Kα fluorescence photon numbers, respectively, measured by the X-ray source. The Cu Kα fluorescence photon number has maximum values at an incident angle of 10°, regardless of the presence of the electron-removing magnet pair. The Cu Kα fluorescence photon number generated by the X-rays and electrons increased by about 80% when the incident angle changed from 30° to 10°. After removing the electrons using the magnet pair, the Cu Kα fluorescence signals generated by only the X-rays increased by about 50% when the incident angle changed from 30° to 10°. Because of the penetration depth limit of electrons, the Fe Kα fluorescence signal did not significantly differ with electron removal. The Fe Kα fluorescence signal measured only by the bremsstrahlung emission slightly increased at 10° compared to 30°. Figure 5(b) shows the incident angle dependence of the X-ray fluorescence photon number of copper-covered stainless steel measured by electrons. Using the difference in X-ray fluorescence signals before and after the magnet installation, the X-ray fluorescence signals generated only by the electrons were investigated. The black and red points indicate the Cu Kα and Fe Kα fluorescence photon numbers measured by electrons, respectively. The Cu Kα fluorescence signal increased more than twice at an incident angle of 10° compared to 30°. Due to the penetration depth limit of electrons, only the Cu Kα signal on the material surface was detected, and the Fe Kα signal of the material interior was not detected. In this study, we analyzed the surface of the copper-covered stainless steel using the laser plasma produced electrons and optimized the measurement by adjusting the incident angle.
Figure 6(a) shows the laser-induced X-ray fluorescence spectrum of the 3 types of Korean 10 Won coins, which have different metal compositions depending on the minting year: the first coin was minted in 1968 (brass: 88% Cu, 12% Zn; diameter: 22.8 mm; thickness: 1.38 mm), the second in 1999 (brass: 65% Cu, 35% Zn; diameter: 22.8 mm; thickness: 1.43 mm), and the third in 2018 (Cu-plated Al: plating ratio of 11:78:11 of Cu : Al : Cu; diameter: 18.0 mm; thickness: 1.20 mm). The clear fingerprint signals of Cu and Zn’s Kα lines were shown at an energy of 8.04 keV and 8.63 keV, respectively. Using the X-ray fluorescence signal intensities, it was possible to calculate the composition of two different Korean 10 Won coins. The Cu and Zn contents of the mint 1968 coin were 87.5% and 12.4%, respectively, with relative errors compared to the actual composition were 0.550% and 3.83%, respectively. In the case of mint 1999 coin, the composition of Cu and Zn were 67.2% and 32.7%, respectively, and with relative errors compared to the actual composition were 3.52% and 6.57%, respectively. As the energy distribution of the Zn Kα peak slightly overlaps with the Cu K𝛽 peak, the Zn composition data has relatively low accuracy. By separate signal analysis processing such as peak identification due to superimposed signals, the measurement accuracy can be improved. [18] The coins with the same appearance but different composition ratio, for example the Korean 10 Won coins minted in 1970, can be sufficiently distinguished with this quantitative measurement method. Figure 6 (b) and (c) shows the X-ray fluorescence spectrum of the mint 2018 Korean 10 Won (Cu-plated Al) coin by X-ray(b) and electrons(c). The arbitrary units in Fig. 6 were all the same scale. The Cu Kα and Cu Kβ signals were identified by surface measurements, but the Al Kα signal could not be identified; because the Al-Kα energy was too low to penetrate the copper plate. The plating ratio of the mint 2018 coin was 11:78:11, with the approximate copper plating thickness on one side being 132 𝜇m. The transmittance of the Al Kα signal (1.486 keV) for the 132 𝜇m thickness Cu plate was zero. Except when light metal is plated with heavy metal, the internal X-ray fluorescence signal passes through the surface and can be measured.
The experimental results showed that it is possible to analyze the both surface and interior of multi-layer materials even with the compact laser source, and it was possible to distinguish the components of alloy samples with different compositions from each other by quantitatively measurement. The laser-induced X-ray source did not require separate poly capillary optics compared to conventional micro X-ray tube source, and it is not necessary to earn prior background data for surface and interior analysis of multi-layer materials. In addition, we confirmed that the surface and interior analysis of multi-layer material was possible even with a relatively low peak intensity laser source compared to the laser-induced PIXE. The measurement accuracy can be improved by additional factors, such as the identification of peaks caused by superimposed signals, self-attenuation, and matrix effects. In addition, by using the detector gating method, the signal-to-noise ratio of the X-ray fluorescence peak can be improved by measuring at a high intensity in picoseconds.