X-Ray Emission From Materials Processing Lasers

Abstract

The emission of laser induced X-rays from materials processing ultra-short pulsed laser systems was measured. The absolute spectral photon fluence was determined using a thermoluminescence detector based few-channel spectrometer. The spectra at 10 cm from the laser focus were in the energy region between 2 and 25 keV with mean energies of ~4-6 keV (when weighted by fluence or directional dose equivalent) and up to 13 keV (when weighted by ambient dose equivalent). The operational quantities, H·'(0.07), H·'(3) and H·*(10), were determined to be in the order of 1600-7300, 16-71 and 1-4 mSv per hour processing time, respectively, depending on the material and condition of the workpiece. The dose contribution due to photons above 30 keV was for all quantities negligible, i.e. below 10-3.

The influence of processing parameters on X-ray emission during ultra-short pulse laser machining

“…In accordance with the dosimetric measurements a dose rate of Ḣ � (0.07) = 10.8 Sv/h could be found for tungsten as target material at a distance of 100 mm to the laser processing spot from simultaneously recorded X-ray spectra. Similar results were presented by Behrens et al [6].…”

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

“…The results clearly showed that at irradiances well above 10 14 W/cm 2 X-ray photons with energies up to about 25 keV could be observed. These results were confirmed in [24]. Furthermore, Ḣ (0.07) dose rates exceeding 10 5 µSv/h were measured at a distance of 20 cm from the plasma.…”

“…In the literature, scaling laws are found for the hot-electron temperature in keV, determined with a variety of measurement methods, typically in the form of where c T is a constant with the unit keV/(µm 2 W/cm 2 ), λ L is the laser wavelength in micrometers, and the irradiance in the center of the incident Gaussian beam averaged over the pulse duration in W/cm 2 . The exponent s is in the range of 0.3-0.5 and varies for different experimental conditions as found in [11,24,[27][28][29][30], for example. It is seen that the hot-electron temperature only depends on the laser wavelength and the irradiance.…”

“…As seen in [11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] the plasma properties k B T h , Z i , and n h are complicated functions of space and time. However, for this work it is assumed that the predominant part of the Bremsstrahlung emission in the part of the spectrum of interest is produced by the hot electrons, during the laser pulse, and at a constant hot-electron temperature in…”

“…The large scattering of the data points is typical for measurements of the hot-electron temperature as also seen in [11,24,[27][28][29][30], for example, and was not further investigated. The temperatures were determined mainly to prove the validity of the assumptions made for s and c T to experimentally verify the model.…”