Influence of ultrashort laser-processing parameters and target material on the onset of hazardous X-rays

The identification of the peak intensity threshold for X-ray generation during ultrashort pulse laser material processing is critical for ensuring operational safety and regulatory compliance. Thus far, laser peak intensities below 10 13 W/cm 2 have been regarded as safe, typically resulting in negligible or undetectable X-ray emissions under most experimental conditions. In this study, an industrial-grade ultrashort pulse laser setup delivering 71.9 W maximum average power was employed in materials processing. Both single-pulse irradiations and various laser burst modes were investigated. A laser beam at two different laser wavelengths, 1030 and 515 nm, was investigated. In addition, the pulse duration was varied, ranging between 250 fs and 10 ps. The maximum X-ray dose rates for representative metallic materials, semiconductors and insulators are presented. Notably, as a key finding, the study reveals that X-ray emissions exceeding the German legal limit of 1 µSv/h in 100 mm distance may occur at laser peak intensities below 10 12 W/cm 2 . This is significantly lower than previously assumed for safe laser processes. At the chosen measurement distance of 40 cm, the highest X-ray dose rates (>500 mSv/h) were detected in the 2-pulse burst mode regime during irradiation of tungsten at peak intensities up to 8.2 × 10 14 W/cm 2 .