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Dosimetry on an ultrashort pulse laser processing machine to ensure radiation protection and to support the authorization procedure of a USP laser at PTB

23.12.2020

Key innovations in the field of ultrashort pulse (USP) lasers have led to a growing number of USP laser machines being used in fields such as material processing. The legal regulations for radiation protection for such machines are currently under revision. The measurement results obtained within the scope of the authorization procedure of PTB’s USP laser machine also provide the data used as a basis for counseling the legislator on how to revise and implement radiation protection legislation in practice.

Due to key innovations in the field of ultrashort pulse (USP) laser technologies (the Nobel prize in physics 2018 was awarded for inventions in this field), a constantly growing number of USP laser machines are now being used in industry. We find them in high‑precision material processing such as structuring, cutting, and the laser turning of workpieces). Cutting so‑called Gorilla Glass for the displays of mobile phones and drilling fine injection nozzles for low‑emission combustion engines are examples of such processes. Single laser pulses are fired at the workpiece at very high intensities in the range of 1015 W/cm2 and with kHz to MHz repetition rates. The advantage of this type of processing is that material is removed very accurately without causing the material near it to melt.

The laser pulses generate a plasma at the surface of the workpiece to remove material. Laser‑plasma interactions produce high‑energy plasma electrons which emit X‑rays (above 5 keV) [1,2]. Due to this unintentional emission of pulsed ionizing X‑rays, USP laser processing machines fall under the Radiation Protection Act (StrlSchG) to prevent hazards to operators’ health. However, there is currently no practicable implementation of the legal regulation for these types of machines. This is due to the fact that the measurements required for this purpose are very complex and time‑consuming due to the properties of the emitted radiation and the number of different laser machines available. There is a lack of experience and concepts to make these tests applicable in practice.

Since November 2019, PTB (Working Group 5.56, Manufacturing Technology) has also had its own USP laser machine. Within the scope of applying for the authorization to operate this machine, Working Group 6.33 (Radiation Protection in X-ray Installations) has performed dosimetric measurements on it. Various processing steps, materials, and laser settings have been tested with regard to the generated photon energy and X‑ray dose rate and to the shielding effectiveness of the housing. These tests require the internationally recognized measurement competence of Department 6.3 in the field of dosimetry for radiation protection. The results measured on PTB’s machine are currently being documented. They will subsequently be transferred to the section dealing with Operational Radiation Protection (Section 6.71) to prepare for the authorization procedure. At the same time, Working Group 6.33 has been working on two thematically related projects of the Federal Office for Radiation Protection (BfS). Based on the measured data, PTB is counseling the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) and the German Commission on Radiological Protection (SSK) about revising the Radiation Protection Act with regard to USP laser machines.

 

Fig.: Overview of PTB’s USP laser machine. The small insert (top right) shows the inside of the machine. In this case, the workpiece (a short tungsten rod) is located in the center of the insert. Above it, we can see the laser head with the focusing optics. In the bottom half of the insert, the extraction system used to remove the material debris and particles is visible. The measuring devices are arranged on either side of the focal point directed on the specimen: on the left (scintillation dosemeter) and on the right (spectrometer).

Literature

[1]        H. Legall, C. Schwanke, S. Pentzien, G. Dittmar, J. Bonse, J. Krüger, X-ray emission as a potential hazard during ultrashort pulse laser material processing, Applied Physics A 124 (2018) 407

[2]        R. Behrens, B. Pullner, M. Reginatto, X-ray emission from materials processing lasers, Radiation Protection Dosimetry 183 (2019) 361–374

Contact persons

Opens local program for sending emailU. Stolzenberg, Department 6.3, Working Group 6.33

Opens local program for sending emailB. Pullner, Department 6.3, Working Group 6.33