X-ray emission

X-ray emission during ultrashort pulse laser processing

Basic principles and legal framework

X-rays from ultrashort pulse laser radiation

Due to the short pulse durations of ultrashort pulse lasers, high intensities > 10¹³ W/cm² are usually achieved in the focus. If material is processed at these intensities, X-rays may be generated. Adequate protection against this must be ensured. The Strahlenschutzverordnung, which came into force on 31 December 2018, also imposes various obligations on the system operator.

Scientific basics

The use of ultrashort pulse lasers with pulse durations <10 ps for material processing offers a variety of advantages. Some of the advantages are, for example, minimal heat input, precision in the micrometre range during processing and the possibility of processing transparent materials. With such laser beam sources, irradiance levels of more than 10¹³ W/cm² on the workpiece surface can easily be achieved with appropriate focusing. At these irradiances, a hot plasma is created during material removal. In the plasma, the electrons reach temperatures of several kiloelectron volts (keV). Interaction of these hot electrons with ablated material produces characteristic X-ray radiation (line emission) as well as bremsstrahlung with photon energies up to the keV range. From an irradiance of approximately 10¹³ W/cm², sufficient X-ray radiation with photon energies >5 keV is emitted so that a significant radiation dose can accumulate, especially at high repetition rates. This radiation can be potentially harmful to a plant operator unless adequate protection is provided. Photon energies < 5 keV are harmless in that air is not transparent to short-wave light with photon energies up to about 5 keV and thus already provides adequate protection.

The emission of X-rays during material processing with ultrashort pulse lasers is an effect that was already investigated in the 1980s within a vacuum. However, with the increase in the available average laser power of ultrashort pulse lasers in recent years, this effect has again become very relevant. Since the X-ray dose rate is linearly related to the average laser power, the X-rays emitted during processing pose a hazard, especially when high average powers are used. Accordingly, research activity in this area has also increased strongly again in recent years. An excerpt of some recent publications also referred to here:
•    Behrens et al. (2018): X-ray Emission from Materials Processing Lasers
•    Legall et al. (2018): X-ray emission as a potential hazard during ultrashort pulse laser material processing
•    Legall et al. (2019): The influence of processing parameters on X-ray emission during ultra-short pulse laser machining
•    Weber et al. (2019): Expected X-ray dose rates resulting from industrial ultrafast laser applications
•    Legall et al. (2020): X-ray radiation protection aspects during ultrashort laser processing

• Legall et al. (2021): Review of x-ray exposure and safety issues arising from ultra-short pulse laser material processing

For detailed information regarding the emission of X-rays during material processing with ultrashort pulse lasers, it is recommended to read the publications listed above. In the following, some essential basic statements of these publications are summarised:

Using a wavelength of 1030 nm of the ultrashort pulse laser beam, X-rays are produced with photon energies up to a maximum of 25 keV. The spectrum of the emission is independent of the material being processed, with tungsten, steel and aluminium being tested.

The emitted dose rate depends on the material being processed. According to the current status, the highest dose rate is emitted when processing tungsten compared to other technical materials. Similar dose rates are measured when machining steel, but when machining aluminium, the measured dose rate is already more than one order of magnitude lower. Up to 3 orders of magnitude less dose rate is emitted when processing glass.

The emitted dose rate depends on the process (ablation, drilling, turning, ...) and on the selected process strategy. According to current knowledge, the highest dose rate is emitted during scanning ablation (surface machining).

Lead and iron or steel are particularly suitable for protection against the X-rays generated during material processing with ultrashort pulse lasers. Even 2 mm stainless steel offers sufficient protection during processes with ultrashort pulse lasers with 1 kW average laser power. The use of aluminium or glass is not recommended. Stainless steel with a thickness of 1 mm has the same protective effect as 32 mm aluminium or 40 mm borosilicate glass.

Legal framework

In Germany, the operation of an ultrashort pulse laser facility may be subject to notification or authorisation according to §12 or §17 of the Strahlenschutzgesetzes (StrlSchG), or neither. At present, the framework conditions change at short intervals. If you are not sure whether your ultrashort pulse laser falls outside the notification or licensing requirement, you should contact your local competent authority. They will tell you what the facts are and what further steps are required. In both notification and licensing cases, expert radiation protection officers and an expert opinion are required. The authority will name a suitable expert.

Equivalent dose H_P(0,07) and H_P(10)

Annex 18 of the Strahlenschutzverordnung defines in Teil A: Messgrößen für äußere Strahlung. Accordingly, the measurands for external radiation for the personal dose are the depth personal dose H_P(10) and the surface personal dose H_P(0,07). The depth personal dose H_P(10) is the equivalent dose at a depth of 10 millimetres in the body at the supporting point of the dosimeter intended for the measurement. Organs with a high weighting factor tend to be located at greater depths in the body. In this measurand, mainly X-rays with photon energies around 30 keV are recorded. The surface personal dose H_P(0,07) is the equivalent dose at a depth of 0.07 millimetres in the body at the location of the dosimeter intended for the measurement. The assumption behind this is that 0.07 mm is the thinnest thickness of the cornea. In the value H_P(0,07) mainly X-rays with photon energies around 5 keV are recorded. Relating these dose values to time results in the dose rates Ḣ_P(0,07) and Ḣ_P(10). The legal limit for non-occupationally exposed persons per year is an accumulated equivalent dose of H_P(0,07) = 50 mSv and H_P(10) = 1 mSv. The legal requirement that the local dose rate should not exceed 1 µSv/h refers to the dose rate Ḣ_P(10). Information on the effect of ionising radiation on the human body can be found on the pages of the Bundesamts für Strahlenschutz.

Situation at LightPulse LASER PRECISION

A protective housing test of the LightPulse LASER PRECISION processing system was carried out. Two dosimeters were placed inside the processing startion near the process zone. Five additional dosimeters were placed on the outside of the equipment at critical locations such as viewing windows, location of least distance to the process zone and door seams. The measurement was carried out during an ablation process. During ablation, the plasma is always on the top of the workpiece (unlike drilling, for example) and the complete laser power hits the workpiece (unlike laser turning or tangential processing, for example). Ablative processes are, for example, surface structuring or micro engraving. Ultra-fine contours can be produced with high quality using the ultrashort pulse laser. Micro-engraving is used, for example, in the production of injection moulding tools for microfluidic applications or the engraving of stamp tools. During processing, the maximum available laser power was implemented. The maximum irradiance is 4,7⋅10¹⁴ W/cm².

The protective housing of the system at LightPulse LASER PRECISION is made of 2 mm stainless steel and has overlapping folds at the door openings. The evaluation of the dosimeters attached to the outside of the unit showed that all dosimeters always measured H_P(0,07) = 0 mSv and H_P(10) = 0 µSv. The protective housing of the system in operation at LightPulse LASER PRECISION therefore provides sufficient protection against the X-rays emitted during processing.

With the successful housing radiation test, the application for approval for the operation of the ultrashort pulse laser processing system could be submitted. The permit was granted by the Regierungspräsidium Stuttgart. LightPulse LASER PRECISION is the first company in Baden-Württemberg and probably one of the first in Germany with the approval of an ultrashort pulse laser processing machine according to § 12 para. 1 No.1 StrlSchG.