Expected X-ray dose rates resulting from industrial ultrafast laser applications

Abstract

An analytical model is presented, which allows estimating the expected dose rates resulting from X-ray emission from ultrashort-pulse laser-produced plasma under industrial conditions. The model is based on the calculation of the Bremsstrahlung spectrum in the X-ray region between about 5 keV and 50 keV, which is created by the hot electrons in the plasma. The model was calibrated with both spectral and dose rate measurements. The scaling of the hot-electron temperature and the fraction of hot electrons in the plasma served as calibration values. The agreement between experiments and model for the investigated irradiances in range from 10 12 to 10 15 W/cm 2 is excellent. The expected Ḣ (0.07) and Ḣ (10) dose rates at a distance of 20 cm from the process in air were calculated for upcoming lasers with 1 kW of average power. Although the dose rates close to the plasma significantly exceed the allowed dose of 50 mSv per year for an irradiance exceeding about 2·10 15 W/cm 2 , the calculations show that shielding with a 2-mm sheet of iron already at a distance of 20 cm attenuates the radiation to a safe value below 0.4 µSv/h.

Study on X-ray Emission Using Ultrashort Pulsed Lasers in Materials Processing

“…The monitored Bremsstrahlung spectra represent qualitatively the energy distribution of the X-ray photon flux at 100 mm distance and can be approximated by a Maxwell–Boltzmann distribution [ 9 , 12 ] according to where k B represents the Boltzmann constant, T e is the electron temperature, and E is the energy of the emitted X-ray photons.…”

“…However, recent works have shown that the X-ray photon emission increases proportionally with higher average laser power, and actual X-ray emission dose rates can cause serious health risks for the laser operators [ 9 , 10 , 11 , 12 , 13 , 14 ]. In particular, this is the case when high-pulse repetition frequency lasers will be used in materials ablation as the X-ray emission per pulse can accumulate to harmful X-ray photon dose over time.…”

X-ray Dose Rate and Spectral Measurements during Ultrafast Laser Machining Using a Calibrated (High-Sensitivity) Novel X-ray Detector

“…The mechanism generating X-rays during ultrafast laser interaction with materials has been described elsewhere [ 3 , 4 , 8 ]. Under experimental conditions, high-kinetic-energy electrons would directly collide against electrons in atoms inner shell orbitals, resulting in the emission of characteristic lines for e.g., k-α, k-β line emissions along with a bremsstrahlung mechanism.…”

“…Recently, it has been demonstrated that ultrashort laser machining can be accompanied with an X-ray radiation at high repetition rates, if the peak intensity exceeds 10 13 W/cm 2 [ 3 , 4 , 5 , 6 ]. Industrial laser sources used for materials processing typically have pulse durations in the range of pico- to femtoseconds.…”

“…These results were confirmed by Behrens et al (2019) under similar experimental conditions [ 10 ]. Also, an analytical model was recently presented by Weber et al (2019) to estimate the expected X-ray dose when laser machining with ultrashort-pulse lasers under industrial conditions up to an intensity 7 × 10 14 W/cm 2 [ 4 ].…”