High energy resolution fluorescence spectrum of dispersive X-ray sources based on microcalorimeters: Perspectives for scanning electron microscopy and space exploration applications

X-ray fluorescence analysis has played a significant role in material elemental identification and charge state measurements during exploration of Mars, the Moon, and asteroids. It also has important applications in advanced light sources and scanning electron microscopy. Depending on the practical scientific application, various excitation sources can be used, including X-ray sources, electron excitation sources, proton excitation sources, and α particle excitation sources. The interaction mechanisms between the various excitation sources and materials are significantly different. The interactions of photons and electrons with atoms, for instance, exhibit distinct differences with regard to selection rules and energy-momentum conservation restrictions, etc. This results in varying processes and probabilities of the excitations for electrons located in different atomic shells when interacting with photons and electrons. In contrast, electrons, unlike photons, do not vanish after interacting with the outer-shell electrons and do not require momentum from the atomic nucleus. Additionally, electrons tend to excite outershell electrons more readily when they interact with atoms. Owing to the disparities in the electron shells and spin states excited by electrons and photons, differences may arise in the transition paths, as well as in the intensity of these transitions, reflecting the differing probabilities of each type of excitation event. Such differences among the excited electrons could lead to variations in the intensities of the characteristic spectral lines emitted by the same element during de-excitation. These variations in transition probabilities and their resultant spectral lines, in terms of both relative intensities and likelihood of occurrence, are critical for interpreting spectral data.

A challenge arises when comparing the spectral lines of the β-line and γ-line series, which exhibit small energy separations (typically less than 100 eV, and in some cases, as minimal as 10 eV) for practical scientific applications. Such narrow energy gaps make it difficult to measure materials and identify elements using conventional detection devices such as silicon drift detectors (SDD). The energy resolution of an SDD is approximately 120 eV, which is insufficient for discriminating the narrowly separated individual spectral lines. A detector using a superconducting transition-edge sensor (TES) can perfectly satisfy these requirements. By utilizing cryogenic X-ray spectrometers equipped with TES detectors, we can more effectively discern and accurately measure spectral features, and relevant measurements can then be performed .In this study, the TES detectors demonstrated superior energy resolution and detection efficiency for X-rays within the energy range of 1.5 keV to 17.0 keV. Therefore, certain spectral lines within this range were selected for analysis. The K characteristic lines of copper (Cu) lie between 8 keV and 9 keV, whereas the L line series of tungsten (W) also falls within the sensitive range of our spectrometer. Therefore, both copper and tungsten are ideal candidates for our comparative study of the spectral differences observed between electron and X-ray excitation.

High resolution detector give more details

To obtain the intrinsic spectra of copper and tungsten using the electron source, it is essential to appropriately adjust the experimental conditions. For copper, with its K-edge at 8.980 keV, we selected voltage settings of 11 kV, 12 kV, 13 kV, and 14 kV to ensure that the beam’s energy surpassed this threshold, facilitating the excitation of the characteristic lines of copper. Similarly, because the L1 , L2 , and L3 edges of tungsten are 12.102 keV, 11.540 keV, and 10.200 keV, the voltage settings of the tungsten target were 15 kV, 16 kV, 18 kV, and 20 kV, respectively. Regarding the use of the X-ray source, for the molybdenum target with the K-edge of molybdenum at 50.000 keV, the most intense Kα1 line energy produced was 17.479 keV, which provided sufficient energy to excite the inner-shell electrons in the studied samples.

The normalization approach applied in Fig. 1 for copper leverages the Kα peaks, comprising Kα1 , and Kα2 . These peaks correlate with the energy-level transitions of KL3 and KL2 in copper, respectively. From our analysis, it is notable that across varying voltages within the intrinsic and XRF energy spectra, the intensities of the left Kβ lines exhibited no significant variations under either the electron or photon excitation sources.

Similarly, Fig. 2 compares the intrinsic spectrum of tungsten elicited by the electron source and the XRF energy spectrum of tungsten obtained using the photon source. A comparative analysis revealed that within the energy range of the characteristic lines of the L-edge of tungsten, the Lα lines of tungsten showed no noticeable variations under different excitation sources. However, distinct differences were observed within the Lβ line series.

Conclusion and outlook

In conclusion, our study provides insight into the spectral responses of copper and tungsten when subjected to various excitation sources. For copper, the experimental data revealed that the intensities of the Kα and Kβ lines remained constant across the electron and photon excitation methods. This observation aligns with theoretical insights into the structure of the K shell, suggesting that the lack of subshell divisions results in uniform transitions that are less sensitive to the type of excitation source used. In contrast, the spectral analysis of tungsten demonstrated distinct behaviors, especially for Lβ lines under varying excitation sources. Although the Lα lines were consistent, the Lβ lines displayed notable variations, which can be attributed to the complex subshell configuration and its influence on the excitation and de-excitation processes.

Looking ahead, the findings of our current study can serve as a springboard for advancing our understanding of atomic excitation dynamics and spectroscopic analysis. With the nuances of spectral line behaviors now in sharper focus, our upcoming research initiatives will aim to refine the methods we use to analyze and interpret data from an expanded spectrum of materials. An electron gun spanning a 1 keV to 30 keV energy range has been procured for the laboratory, and the development of a matching high-vacuum chamber is already in progress. These tools will allow us to extend our investigations to metals beyond copper and tungsten and delve into the intricacies of the Lβ line series across different elements. A TES detector equipped with a sufficiently high-energy resolution and wide-range capability promises a more meticulous deconstruction of individual spectral lines. This development is intended to reveal finer details of material structures, offering a richer dataset for energy-dispersive spectroscopy applications, particularly within the realm of electron microscopy.

Furthermore, experiments are planned using Am-241 as an α particle source to juxtapose and contrast the energy spectra evoked by electron versus α particle interactions. These research endeavors will ultimately establish a comprehensive database. This reservoir of knowledge is envisioned to function in sync with sophisticated space science instruments such as APXS, enhancing both terrestrial and extraterrestrial material analyses. Through these ambitious projects, we aim not only to bolster the foundation of spectroscopy, but also to contribute a significant corpus of data and analytical expertize to the expansive fields of material science, chemistry, and astrophysics.

The complete study is accessible Via DOI: 10.1007/s41365-024-01622-y.