A study on the femtosecond Laser-induced X-ray characteristics and X-ray fluorescence by laser-produced plasma sources

Abstract

By the advancement of short-pulse laser sources, generating laser-produced plasma has become easily achievable even at the laboratory scale. The electrons, ion beams, X-rays, and extreme ultraviolet (EUV) radiation produced by laser-produced plasmas have been extensively investigated in many research fields. Among these, the laser-induced X-rays has unique characteristics inherited from the laser pulses used for generation, such as a short pulse width, a high repetition rate, and a point-like source. Due to its advantages of short pulse width and high repetition rate, the laser-induced X-ray was used in the field of material science to observe atomic-level structural changes in materials. Laser-induced X-rays are also utilized in X-ray absorption, diffraction, radiography, microscopy, and phase-contrast imaging. By employing detector gating, the signal-to-noise ratio can be improved compared to conventional continuous X-ray sources, such as X-ray tubes. And this point-like source can be alternative to conventional micro X-ray tube sources for high-resolution imaging applications. In order to use laser-induced X-ray sources for commercial purpose, it is necessary to improve the flux and conversion efficiency of sources. Several research groups are conducting studies on enhance the efficiency by modifying several physical parameters, or changing the shape of targets.

This study aims to generate femtosecond laser-induced X-ray sources with an aluminum tape and investigate their characteristics by optimizing various physical parameters. Additionally, metal-protein compounds and multi-layer materials were measured for X-ray fluorescence using the laser-produced plasma sources, such as the characteristic emission, bremsstrahlung emission and electrons. The experiment was performed with a double-amplified CPA femtosecond Ti: Sapphire laser with a pulse width of 36 fs, a repetition rate of 1 kHz, a power of 10 W, and a wavelength of 800 nm. This short pulse was focused by an off-axis parabolic mirror with a diameter of 22 μm and a peak power of 1016 W/cm2. The focused beam was irradiated onto a rotating Al tape target in a vacuum chamber at a pressure of 10-4 Torr. Intensity of focused beam satisfied the inner metal ionization condition, and the characteristic emission and bremsstrahlung emission were generated from the laser-produced plasma. By adjusting the pulse power, pulse width, focusing position, and incident angle of the laser source, the optimal conditions for X-ray emission were determined, and the differences in optimized conditions between characteristic emission and bremsstrahlung emission were observed.

In Chapter 3, using the laser-produced plasma sources, the X-ray fluorescence of metal compounds and metal-protein materials was measured with high stability and accuracy for microgram unit samples. In Chapter 4, the surface and interior information of multi-layer materials were individually analyzed using electrons from the laser-produced plasma. X-ray fluorescence of three types of stainless steel, copper-coated stainless steel, and three types of Korean 10 Won coins were quantitatively measured using laser-produced plasma sources, such as characteristic emission, bremsstrahlung emission, and electrons. It demonstrated the information of surface and interior and composition of multi-layer materials can be analyzed even with a compact laser system.