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Radiation protection for ultrashort pulse lasers

PTB has been contributing to the modification of legislation by performing radiation protection measurements on its new ultrashort pulse laser machine

PTBnews 3.2021
27.09.2021
Especially interesting for

materials processing

radiation protection

Ultrashort pulse lasers in which an individual pulse only lasts a few picoseconds are increasingly being used in materials processing. Since these devices may generate undesired X-rays, the German Radiation Protection Act has made them subject to prior authorization since 20 May 2021. PTB has contributed to this legal amendment by performing exhaustive radiation protection measurements on its new ultrashort pulse laser machine.

Example of measurement setup with a spectrometer and a dosemeter at PTB’s USP laser machine. The material used for this measurement was a tungsten rod.

Ultrashort pulse (USP) lasers are used, for example, for cutting Gorilla Glass for cell phone displays or for drilling injection nozzles for lower-emission engines. In these processes, single laser pulses are fired at a workpiece with very high intensities in the range of 1015,  W/cm2. This generates a plasma that removes material. Pulse durations in the picosecond range allow accurate material removal without heating up the surrounding material.

However, high-energy plasma electrons accelerated by laser-plasma interactions can emit X-rays with photon energies of more than 5 keV. Due to this undesired emission, USP laser machines are subject to the German Radiation Protection Act (StrlSchG). The legal regulations for USP laser machines within the scope of the Radiation Protection Act have to be specified in more detail. The first amendment makes an authorization procedure mandatory for operating a USP laser machine. Moreover, it specifies that an ambient dose rate limit of 10 μSv/h at a distance of 10 cm from the surface of the protective housing has to be observed.

PTB has contributed to this legal amendment by performing exhaustive radiation protection measurements which were carried out within the scope of the authorization application for its modern USP laser machine. For this purpose, PTB’s radiation protection experts worked in close collaboration with their colleagues from the Precision Engineering Division who are experienced in operating the machine. Measuring the strongly pulsed X-ray emissions is demanding due to typical photon energies that are in the range from less than 5 keV up to approx. 30 keV. The energy range of commercial measuring equipment starts at around 20 keV; typical X-ray devices start at around 25 keV. The radiation was measured using traceable measuring equipment, and various processing steps, materials, and laser settings were examined regarding the generated photon energy and X-ray dose rate. These detailed investigations were made necessary by the complexity and nonlinearity of the laser-plasma interaction.

At the same time, PTB, together with the Bundesanstalt für Materialforschung und -prüfung (German Federal Institute for Materials Research and Testing) (BAM) is taking part in two related departmental research projects of the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), which were assigned by the Federal Office for Radiation Protection. The objective of these projects is to gain reliable data from traceable measurements in the above-mentioned radiation fields and to develop testing concepts to ensure radiation protection when working with USP laser machines.

Based on the data obtained, PTB advises the BMU, the German Commission on Radiological Protection, as well as the Bund-Länder-Ausschuss (Committee of the Federal German Government and of the federal states) regarding this new topic. PTB is thus supporting the development of an urgently needed uniform directive on the testing of protection against ionizing radiation when working with USP laser machines. And it is therefore also supporting the corresponding adjustment of the legal and other regulations.

Contact

Ulf Stolzenberg
Department 6.3
Radiation Protection Dosimetry
Phone: +49 531 592-6226
Opens local program for sending emailulf.stolzenberg(at)ptb.de

The Radiation Protection Act (StrlSchG) (in German only)

Opens external link in new windowhttps://www.gesetze-im-internet.de/strlschg/

The Radiation Protection Ordinance (StrlSchV) (in German only)

Opens external link in new windowhttps://www.gesetze-im-internet.de/strlschv_2018/