Abstract
Interactions between ultrashort laser pulses with intensities larger than 1013 W/cm2 and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 1015 W/cm2 at pulse durations of ~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.
Highlights
The use of ultrashort laser pulses in air for material processing has many advantages such as the lateral and vertical precision of the surface contours down to the nanometer range and the high reproducibility of the laser-generated structures [1]
Interactions between ultrashort laser pulses with intensities larger than 1013 W/cm2 and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators
The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates
Summary
The use of ultrashort laser pulses in air for material processing has many advantages such as the lateral and vertical precision of the surface contours down to the nanometer range and the high reproducibility of the laser-generated structures [1]. Machining with high-intensity laser pulses can be accompanied by the generation of a near-surface electron plasma due to absorption and ionization of the material, a subsequent plasma heating by the laser pulse, and an interaction of “hot” plasma electrons with the processed material, leading to continuous and characteristic X-ray emissions. The authors registered X-ray dose rates requiring radiation protection measures The amount of this X-ray radiation is determined by the laser parameters (pulse duration, peak intensity, pulse energy, wavelength, and polarization), the workpiece (atomic number and surface preparation), and the laser process management (scanning or stationary regime, laser turning, etc.). The use of laser peak intensities above 1013 W/cm in combination with laser pulse repetition rates in the few 100 kHz range can already lead to X-ray dose rates clearly exceeding the permitted limits for members of the public. Under typical ultrashort pulse laser machining conditions in the intensity range of 1013 to 1015 W/cm, collision-less resonance absorption appears to be the driving X-ray generation mechanism [17]
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