Figure - available from: Applied Physics A
This content is subject to copyright. Terms and conditions apply.
Experimental setup used for X-ray dose measurements. Shown is the optical beam path, the scanner head, the sample stage and the arrangement of the detector systems. In all measurements the CdTe spectrometer and the dosimeters (OD-02 and DIS-1) were positioned at the same angle vertical to the target surface. In the horizontal plane, the dosimeters were placed under an angle of about 12° to the scan direction, whereas the CdTe spectrometer was oriented parallel to the scan direction. The setup is enclosed by a radiation protection housing shielded with 1 mm Pb. To prevent secondary emissions, the wall of the housing and all other potential sources of secondary emission were covered with PMMA and aluminum

Experimental setup used for X-ray dose measurements. Shown is the optical beam path, the scanner head, the sample stage and the arrangement of the detector systems. In all measurements the CdTe spectrometer and the dosimeters (OD-02 and DIS-1) were positioned at the same angle vertical to the target surface. In the horizontal plane, the dosimeters were placed under an angle of about 12° to the scan direction, whereas the CdTe spectrometer was oriented parallel to the scan direction. The setup is enclosed by a radiation protection housing shielded with 1 mm Pb. To prevent secondary emissions, the wall of the housing and all other potential sources of secondary emission were covered with PMMA and aluminum

Source publication
Article
Full-text available
In laser machining with ultrashort laser pulses unwanted X-ray radiation in the keV range can be generated when a critical laser intensity is exceeded. Even if the emitted X-ray dose per pulse is low, high laser repetition rates can lead to an accumulation of X-ray doses beyond exposure safety limits. For 925 fs pulse duration at a center wavelengt...

Citations

... Impact on X-ray Emission References Peak intensity -Doubling the peak intensity increases the X-ray dose rate by around ten times [3,[12][13][14] Pulse energy -With higher pulse energies, higher X-ray dose rates tend to be expected, - ...
... Angle of incidence -Oblique laser beams with increased angle of incident with respect to the plasma flank enhance resonance absorption that tends to cause higher X-ray emissions [7,16,21] Material -Higher X-ray dose rates occur with elements with a higher atomic number -Highest X-ray dose rate was determined on tungsten, the X-ray dose rate is comparably high on steel and stainless steel materials [2,12,14] Suface roughness -A higher surface roughness leads to lower X-ray dose rates due to the shielding of the X-rays on microscopic substructures, similar to the shielding effect of boreholes or trenches [11,16] ...
... Accordingly, the maximum peak intensity of the ultrashort pulses was I 0/UFFL = 2.7 × 10 13 W/cm 2 and I 0/FX = 1.5 × 10 13 W/cm 2 for the UFFL 100 and FX 200 lasers, respectively. Notably, these peak intensity values were only slightly above an intensity of 1 × 10 13 W/cm 2 , where the X-ray emission dose rate can exceed 1 µSv/h [3,7,12]. In fact, according to the German Radiation Protection Regulations, both the values 1 × 10 13 W/cm 2 and 1 µSv/h at a 100 mm distance from the accessible area represent the legal limits for approval-free laser operations [22]. ...
Article
Full-text available
The ongoing trend in the development of powerful ultrashort pulse lasers has attracted increasing attention for this technology to be applied in large-scale surface engineering and modern microfabrication. However, the emission of undesired X-ray photon radiation was recently reported even for industrially relevant laser irradiation regimes, causing serious health risks for laser operators. In the meantime, more than twenty influencing factors have been identified with substantial effects on X-ray photon emission released by ultrashort pulse laser processes. The presented study on enhanced X-ray emission arising from high pulse repetition frequency ultrashort pulse laser processing provides new insights into the interrelation of the highest-contributing parameters. It is verified by the example of AISI 304 substrates that X-ray photon emission can considerably exceed the legal dose rate limit when ultrashort laser pulses with peak intensities below 1 × 1013 W/cm² irradiate at a 0.5 MHz pulse repetition frequency. The peak intensity threshold value for X-ray emissions decreases with larger laser spot sizes and longer pulse durations. Another key finding of this study is that the suction flow conditions in the laser processing area can affect the released X-ray emission dose rate. The presented results support the development of effective X-ray protection strategies for safe and risk-free ultrashort pulse laser operation in industrial and academic research applications.
... Irradiances of up to several 10 15 W/cm 2 can be achieved when the laser beam is tightly focused on a plane surface. Such high irradiances lead to strong plasma generation resulting in the emission of bremsstrahlung, recombination radiation, and line emission, with photon energies exceeding 5 keV and even up to tens of keV [9]. This X-ray emission might lead to possibly hazardous dose rates for an operator. ...
... During the interaction of a laser pulse with material, free electrons are created by the leading edge of the pulse [17]. The free electrons are further excited by the remainder of the pulse due to different mechanisms, such as inverse bremsstrahlung and resonance absorption [18], resulting in a fraction of electrons with high kinetic energy and temperatures of several keV [9], which are commonly referred to as hot electrons [17]. This fraction of hot electrons is responsible for the X-ray emission considered in this paper. ...
... The scaling of T h as a function of the wavelength-corrected laser irradiance λ 2 L I 0 was experimentally determined for the industrial processing conditions [19] considered here for irradiances ranging from I 0 ≈ 10 12 W/cm 2 to I 0 ≈ 10 15 W/cm 2 , yielding [9,[20][21][22][23][24] ...
Article
Full-text available
Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 1014 W/cm2, a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.
... Mit den mittlerweile zur Industriereife gebrachten hochrepetierenden Ultrakurzpulslasern wurden von Legall et al. [12] [24]: ...
... Analog wird für den AFS folgendes Ergebnis bestimmt: [10], [12], [22] und Weber et al. [8]. ...
Thesis
Full-text available
In this thesis, the formation of X-rays during laser material processing using ultrashort pulsed laser radiation is investigated. For this purpose, two laser beam sources are used, which can realize different pulse durations and pulse repetition frequencies. To carry out the experiments, an industry-oriented process regime is used by marking rectangular fields by means of a scanner system and the resulting X-ray emissions are measured. Peak intensities between 10^12 and 〖3∙10〗^13 W/cm² and high pulse repetition frequencies up to 2 MHz are used to investigate the effects of different irradiation regimes on the level of measurable X-ray emissions. For this purpose, the influence of pulse repetition frequency, geometric pulse-to-pulse distance, air flow regime, surface roughness and polarization is investigated. To compare some results, two different materials are used, stainless steel X2CrNi18-9 and the titanium-molybdenum alloy rematitan®SPECIAL. The investigations showed that the measurable X-ray emissions strongly depend on the peak intensity used. The larger this was, the more X-ray emission could be measured. Under certain process conditions, already at intensities below 10^13 W/cm², high local dose rates above 1 (µSv)/h could be measured. Furthermore, a non-linear relationship between pulse repetition rate and dose rate was found, for which a plasma-follow-up pulse interaction is assumed to be the cause due to the short times between two pulses. The geometric pulse-to-pulse distance turned out to be another important influencing factor. For each pulse repetition rate examined, there was a certain pulse-to-pulse distance, at which the dose rate become maximum. In addition, a completely new influence of the prevailing air flow on the X-ray emission was determined, which in this certain form does not yet emerge from the known literature. It was also shown that the surface roughness of the samples used also had a strong influence on the measurable X-ray emissions. Furthermore, there was a dependence of the amount of X-ray emissions on the polarization direction of the laser radiation, especially at high pulse repetition frequencies from 1 MHz, which suggests plasma resonance absorption as the underlying mechanism of formation.
... This laser-induced secondary non-coherent generation of x-ray might be either useful or undesirable depending on applications [9,10,11]. ...
... Usually radiation safety and monitoring is not addressed in material processing laboratories which use femtosecond laser. According to Legall et al. [11] in an intensity range of 10 13 -10 14 W/cm 2 , the generation of dose relevant x-ray radiation is very inefficient and the emitted dose per pulse is low. The other factor limiting dose is that air is opaque for ionizing radiation with photon energies up to about 5 keV [14]. ...
... In case of the lasers with the high power and high repetition rate dose can accumulate over time. This issue should be addressed while evaluating radiological impact to personel in material processing and laboratories using high repetition rate lasers [11,17]. The need for a characterization of the potential radiological impact on personnel might be necessary while evaluating safety of the activity. ...
Article
The ionizing radiation created by high intensity and high repetition rate lasers can cause significant radiological hazard. Earlier defined electron temperature scalings are used for dose characterization and prediction using Monte Carlo modeling. Dosimetric implications of different electron temperature scalings are investigated and the resulting equivalent doses are compared. It was found that scaling defined by Beg et al.(1997) predicts the highest electron temperatures for given intensities, and subsequently the highest doses. The atomic number of the target, x-ray generation efficiency and interaction volume are the other parameters necessary for the dose evaluation. The set of these operational parameters should be sufficient to characterize radiological characteristics of ultrashort laser pulse based x-ray generators and evaluate radiological hazards of the laser processing facilities.
... The use of laser peak intensities above 10 13 W/cm 2 in combination with laser pulse repetition rates in the few 100 kHz range can already lead to X-ray dose rates clearly exceeding the permitted limits for members of the public. Tungsten and steel in particular show significant X-ray emission [11][12][13][14][15][16][17][18][19]. In [11], it was demonstrated that other materials such as aluminum and glass show significantly lower X-ray emissions. ...
... Tungsten and steel in particular show significant X-ray emission [11][12][13][14][15][16][17][18][19]. In [11], it was demonstrated that other materials such as aluminum and glass show significantly lower X-ray emissions. The measured dose rate for an aluminum target was approximately two orders of magnitude lower than the dose rate of steel and tungsten. ...
... Such a reference material processing scenario has the advantage that the direction of the radiation field is well known. Due to intrinsic absorption in the target material, the highest dose rates of the laser-induced radiation are measured in the opposite direction to the movement of the laser, i.e., parallel to the laser ablated grooves [11,13,14]. ...
Article
Full-text available
Interactions between ultrashort laser pulses with intensities larger than 10 13 W/cm 2 and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 10 15 W/cm 2 at pulse durations of~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.
... IX. X-ray emission: X-ray measurements by Legall et al. [18] during materials processing with ultrafast lasers at a wavelength of 1 µm, a pulse duration of 1 ps, an irradiance of about 3·10 14 W/cm 2 , and an average power of 40 W yielded skin-dose ratesḢ (0.07) which exceed 100 mSv/h at the distance of 42 cm from the plasma, which is far above the legal annual limit. These high dose rates led to strict regulations for radiation protection, which involve a considerable additional effort for ultrafast materials processing. ...
Article
Full-text available
Materials processing with ultrafast lasers with pulse durations in the range between about 100 fs and 10 ps enable very promising and emerging high-tech applications. Moreover, the average power of such lasers is steadily increasing; multi kilowatt systems have been demonstrated in laboratories and will be ready for the market in the next few years, allowing a significantly increase in productivity. However, the implementation of ultrafast laser processes in applications is very challenging due to fundamental physical limitations. In this paper, the main limitations will be discussed. These include limitations resulting from the physical material properties such as the ablation depth and the optimal fluence, from processing parameters such as air-breakdown and heat accumulation, from the processing system such as thermal focus shift, and from legal regulations due to the potential emission of soft X-rays.
... 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 portionally with higher average laser power, and actual X-ray emission dose rates c cause serious health risks for the laser operators [9][10][11][12][13][14]. In particular, this is the case wh high-pulse repetition frequency lasers will be used in materials ablation as the X-ray em sion per pulse can accumulate to harmful X-ray photon dose over time. ...
... This was in order to identify the optimum measuring conditions for the assessment of X-ray emissions in the accomplished study. On the one hand, this was encouraged, as the authors literature review revealed a dependence between the X-ray emission dose and the detection angle [9]. So far, a maximum X-ray dose rate level was reported at a 30 • detection angle, as investigated in the range between 10 • and 40 • . ...
... 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 ...
Article
Full-text available
The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 < I0 < 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as H˙′ (0.07) > 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.
... In recent publications, it has been shown that during ultrafast laser machining at high repetition rates of several 100 kHz, the amount of emitted X-ray radiation can exceed the regulatory exposure limits for members of the public [8]. Low X-ray doses per pulse can accumulate through high repetition rates and exceed radiation protection limits. ...
... 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. ...
Article
Full-text available
Abstract: Ultrashort pulse laser machining is subject to increase the processing speeds by scaling average power and pulse repetition rate, accompanied with higher dose rates of X-ray emission generated during laser–matter interaction. In particular, the X-ray energy range below 10 keV is rarely studied in a quantitative approach. We present measurements with a novel calibrated X-ray detector in the detection range of 2–20 keV and show the dependence of X-ray radiation dose rates and the spectral emissions for different laser parameters from frequently used metals, alloys, and ceramics for ultrafast laser machining. Our investigations include the dose rate dependence on various laser parameters available in ultrafast laser laboratories as well as on industrial laser systems. The measured X-ray dose rates for high repetition rate lasers with different materials definitely exceed the legal limitations in the absence of radiation shielding.
... 26,27 Additionally, the realizable high intensities of the ultrafast laser radiation with intensities above 10 14 W=cm 2 lead to very high electron energies in the plasma, resulting in bremsstrahlung, recombination, and line emission of x-rays with photon energies up to the keV range. 28 Of special interest in the field of laser safety with regard to radiation hazard are the measured dose rates _ H 0 (0.07) and _ H 0 (10), ...
... 29 In some studies, the dose rates that occur with ultrafast laser radiation have been investigated. 28,[30][31][32][33][34][35] The results indicate that x-ray photons with energies up to 30 keV were measured at peak intensities of the laser radiation above 10 14 W=cm 2 , and _ H 0 (0.07) dose rates of more than 100 mS/h were measured at a distance of 20 and 42 cm of the interaction zone of the laser radiation to the material surface. The scaling of the dose rates using conventional ultrashort pulses with pulse repetition rates up to 400 kHz has been investigated in other studies but not in the burst mode with intra-burst pulse repetition rates reaching up to several GHz. ...
... The accuracy of the OD-02 is established up to a single-pulse x-ray dose of the directional dose equivalent H 0 (0:07) ¼ 0:1 nSv. 28 In order to consider the influence of the ambient pressure and temperature, a correction factor 43 given by ...
Article
Ultrashort pulsed laser sources generating pulse trains (bursts) with intra-burst repetition rates in the MHz and the GHz regime enable an efficient production of microstructures with a high surface quality. However, x-ray radiation can be generated during the laser micromachining using large intensities of the laser radiation and its interaction with the ablation cloud or high-density plasma. Therefore, the authors report on the interaction of bursts with a wavelength of 1030 nm and pulse durations of 0.24 and 10 ps with intra-burst repetition rates of 65 MHz (MHz-burst mode) and 2.5 GHz (GHz-burst mode) as well as a combination of both burst modes, called BiBurst mode, with stainless steel, and the x-rays are generated. The x-ray dose rates determined in the respective burst modes are compared and discussed with those of conventional ultrafast laser radiation (single-pulse mode). Furthermore, a theoretical model is used to calculate the expected x-ray dose rates. In the investigated parameter range, the highest dose rates of more than 10 5 μSv/h are determined at a specific burst setting. Compared to the single-pulse mode, significantly higher dose rates are determined using the burst mode with the same total intensity. Based on the results of this study, it can be stated that the interaction of ultrafast laser radiation in the burst mode with a generated ablation cloud or high-density plasma plays a major role in x-ray generation and the resulting x-ray dose rates.
... With the availability of laser sources suitable for industrial use having repetition rates in the several 100 kHz to MHz range, new investigations on harmful x-ray emissions became more and more urgent. In recent years several groups started to work on this topic and published first results [12][13][14][15][16][17]. The status of the work is presented here in an overview. ...
... Overview of state-of-the-art ultrafast laser systems, based on different technologies (colour coded: disk, fibre, slab laser; see insert), illustrating the trend of current ultrafast laser technology towards multi-kilowatt average power [21]. In addition, the upper limit of laser pulse energy and repetition rate (covering the green area of laser parameters) were marked (white filled circles with collared outlines: black [12], red [13], blue [16], green [17]) for which the x-ray emission was investigated in laser material processing. Copyright 2019 under Creative Commons BY 4.0 license [21]. ...
... These instruments are characterized by high detection sensitivity and can be equipped with a sufficiently large chamber volume to prevent saturation losses due to high peak doses. In [12] an ionization chamber was proven for x-ray measurements in ultra-short pulse laser material processing. The verification was performed by comparing the simultaneously accumulated x-ray doses collected with a passive dosimeter with direct ion storage (DIS-1, Mirion Technologies GmbH) and an air-open ionization chamber (OD-02, STEP GmbH). ...
Article
Full-text available
Laser processing with ultra-short laser pulses enables machining of materials with high accuracy and throughput. The development of novel laser technologies with laser pulse repetition rates up to the MHz range opened the way for industrial manufacturing processes. From a radiological point of view this evolution is important, because X-ray radiation can be generated as an unwanted side effect in laser material processing. Even if the emitted X-ray dose per pulse is comparably low, the X-ray dose can become hazardous to health at high laser repetition rates. Therefore, radiation protection must be considered. This article provides an overview on the generation and detection of X-rays in laser material processing, as well as on the handling of this radiation risk in the framework of radiological protection.