X-ray emission as a potential hazard during ultrashort pulse laser material processing [Invited Paper]

Abstract

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 wavelength of 1030 nm, the X-ray emission was investigated up to an intensity of 2.6 × 1e14 W/cm2. The experiments were performed in air with a thin disk laser at a repetition rate of 400 kHz. X-ray spectra and doses were measured for various planar target materials covering a wide range of the periodic table from aluminum to tungsten. Without radiation shielding, the measured radiation doses at this high repetition rate clearly exceed the regulatory limits. Estimations for an adequate radiation shielding are provided.

Full text

Vol.:(0123456789)
1 3
Applied Physics A (2018) 124:407
https://doi.org/10.1007/s00339-018-1828-6
INVITED PAPER
X-ray emission asapotential hazard duringultrashort pulse laser
material processing
HerbertLegall1 · ChristophSchwanke1· SimonePentzien1· GünterDittmar2· JörnBonse1· JörgKrüger1
Received: 13 April 2018 / Accepted: 27 April 2018 / Published online: 3 May 2018
© The Author(s) 2018
Abstract
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 accumu-
lation of X-ray doses beyond exposure safety limits. For 925fs pulse duration at a center wavelength of 1030nm, the X-ray
emission was investigated up to an intensity of 2.6 × 1014W/cm2. The experiments were performed in air with a thin disk
laser at a repetition rate of 400kHz. X-ray spectra and doses were measured for various planar target materials covering a
wide range of the periodic table from aluminum to tungsten. Without radiation shielding, the measured radiation doses at
this high repetition rate clearly exceed the regulatory limits. Estimations for an adequate radiation shielding are provided.
1 Introduction
During the past years material processing using ultra-short
laser pulses has undergone a considerable development. This
development was initiated by the invention of novel laser
technologies, as e.g. the thin disk lasers [1]. The thin disk
laser technology delivers high stable output power in the
range of several 10W up to several 100W, with pulse dura-
tions below 1ps at repetition rates of several 100kHz. These
high repetition rates make laser processing quick, cost-effec-
tive, and thereby more attractive for industrial applications.
For that different ultra-fast laser processing strategies are
employed. For an optimum laser ablation efficiency along
with high machining precision, fluences of about ten times
the ablation threshold are used [2–4]. An optimum process-
ing window around 1J/cm2 was found for steel. Glasses
require higher fluences ranging up to ∼ 30J/cm2 [5]. How-
ever, in alternative sequential processing strategies it might
be beneficial to increase the removal rate via much higher
fluences in a first roughing step. In the subsequent finishing
step at lower fluence the requiredsurface quality is realized
[6]. Hence, to regard the full hazard potential the highest
achievable fluence with the used laser system must be con-
sidered as realistic scenario. In this work, a maximum laser
fluence of 255J/cm2 was reached, corresponding to a peak
intensity of 2.6 × 1014W/cm2 taking into account the tem-
porally Gaussian pulse shape.
At high laser intensities energy can be transferred to
electrons by means of laser plasma interaction. The energy,
which can be transferred to a plasma electron, depends on
the mechanism dominating the laser plasma interaction. If
the kinetic energy of the plasma electrons becomes high
enough, characteristic X-ray radiation and Bremsstrahlung
can be generated by interaction with the target material. In
an intensity range of 1013–1014W/cm2, the generation of
dose relevant X-ray radiation is very inefficient and the emit-
ted dose per pulse is low. Therefore, laser safety in terms of
radiation hazard in the intensity region from 1013 to 1014W/
cm2 was addressed only in rudimentary form so far. Bunte
etal. pointed out a possible danger when using a 30-fs laser
system operated at 1kHz repetition rate with an intensity
above 1014W/cm2 for processing of copper [7]. However,
in laser material processing with high repetition rates, the
low X-ray radiation dose per pulse can accumulate over time,
leading to a dose high enough to be hazardous for the oper-
ating staff. For safety precautions, a detailed knowledge of
the emitted radiation dose and its spectral distribution is
required. For this purpose, the spectral X-ray emission and
the generated dose in laser material processing at intensities
up to 2.6 × 1014W/cm2 was investigated in the present work.
* Herbert Legall
herbert.legall@bam.de
1 Bundesanstalt für Materialforschung und -prüfung (BAM),
Unter den Eichen 87, 12205Berlin, Germany
2 Steinbeis-Transferzentrum Technische Beratung und
Entwicklung, Albrecht-Erhardt-Str. 17, 73433Aalen,
Germany
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H.Legall et al.
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407 Page 2 of 8
It will be shown that the dose relevant X-ray emission in
laser material processing at intensities up to 2.6 × 1014W/
cm2 with a pulse duration of 925fs and at a wavelength of
1030nm is restricted to an energy range below 30keV.
2 Experimental
2.1 Optical setup
The experiments were performed using a TRUMPF laser
system (TruMicro 5050 femto edition). The laser delivers
high-contrast pulses (no pedestal, no pre-pulse) with 925fs
pulse duration at a laser center wavelength of 1030nm. The
maximum repetition rate of the laser system was 400kHz,
the maximum single pulse energy was 100µJ, resulting in
an average laser power of 40W. The collimated output beam
of the laser with a diameter of about 5mm was expanded by
a factor of 2 in diameter and directed onto a Galvanometer
scanner head (hurrySCAN II 14, SCANLAB GmbH), which
was connected to the controller unit of the TRUMPF laser
system (cp. Fig.1). The laser beam was focused in air onto
the sample by means of an F-Theta lens with a focal length
of 56mm. The angle of incidence of the laser beam onto the
sample plane was 0°. The focal beam diameter (1/e2) was
determined by the D2-method [8] and set to 2w0 = 10 ± 1µm,
resulting in a maximum intensity of 2.6 × 1014W/cm2.
Technical grade planar samples of pure tungsten, carbon
steel (S235JR), an aluminum alloy (AlMgSi), and alumo-
silicate glass (Gorilla glass) were investigated. The sam-
ples were mounted on a 5-axis stage. This stage allows the
alignment of the sample horizontally and to position it in
height with an accuracy of < 5µm. For dose measurements
the samples were irradiated over 5s using a scan speed of
1000mm/s. The X-ray radiation dose was accumulated in
the experiment. From the accumulated dose, the dose rate
was calculated. Debris emitted during the laser ablation was
continuously removed by an exhaust system. The selected
scan field area was 10mm × 10mm. Within the scan fields,
lines with a spacing of 20µm were written in a direction,
away from the X-ray detectors, with the linear polarization
of the laser beam parallel to the lines. In this arrangement,
the highest dose values were obtained. Since the planarity
of the samples (rolled metal sheets) was rather low, meas-
urements were performed at different sample positions. The
highest dose was finally selected from the entire set of dose
measurements. This highest dose can be regarded as “worst
case” scenario. In case of tungsten, the highest dose values
were obtained after repeating the scan process several times
at the same area. In case of steel, the aluminum alloy, and
Gorilla glass, the highest dose values were obtained in the
first scan.
2.2 X‑Ray dose measurements
X-Ray radiation was measured using different detection sys-
tems. For dose measurements, a highly sensitive ionization
chamber (OD-02, STEP GmbH), an electronic Direct Ion
Storage (DIS) memory cell (DIS-1, Mirion Technologies
GmbH) and thermoluminescence dosimeters (MCP-N, Rad-
Pro International GmbH) were applied. The thermolumines-
cence dosimeters were read out manually (TLDcube, RadPro
International GmbH).
Different operational quantities are in use to define
human-body protection quantities in an external radiation
field. With the ionization chamber used in this work, the
ambient dose equivalent H*(10) and the directional dose
equivalent H′(0.07) can be measured. The ambient dose
Fig. 1 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 emis-
sion were covered with PMMA and aluminum
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X-ray emission asapotential hazard duringultrashort pulse laser material processing
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equivalent H*(10) is the operational quantity for area moni-
toring. It is the dose equivalent at a depth of 10mm in a
30-cm diameter sphere of unit density tissue (ICRU-sphere,
International Commission on Radiation Units and Measure-
ments). The ICRU-sphere is a sphere with a density of 1g/
cm3 and a mass composition of 76.2% oxygen, 11.1% car-
bon, 10.1% hydrogen, and 2.6% nitrogen. The directional
dose equivalent H′ (0.07, Ω) is the operational quantity for
the determination of the dose to the human skin and is meas-
ured at a depth of 0.07mm in the ICRU-sphere. The quantity
Ω denotes hereby the angle of incidence of the X-ray radia-
tion to the sphere.
The DIS-1 dosimeter is a personal dosimeter, which
yields the personal dose equivalents Hp(10) and Hp(0.07).
These operational quantities are used for monitoring the
exposure dose of an individual. For X-ray photon ener-
gies below 50keV, the measured personal radiation dose
(with a dosimeter calibrated on a slab phantom to simulate
the human torso) and the ambient radiation dose deliver
almost identical values and a ratio of H*(10)/Hp(10) = 1
and H′(0.07,0°)/Hp(0.07) = 1 can be used to compare these
operational quantities [9, 10]. For dose limits, most coun-
tries adopt the recommendations of the ICRP (International
Commission on Radiological Protection). Dose limits for
members of the public are 1mSv/a effective dose, controlled
by the dose equivalents at a depth of 10mm, and 50mSv/a
for the exposure of the skin [11]. For working personal, the
dose limits must be related to the occupational exposure time
per year. Note that the regulatory radiation limits for work-
ing personal can differ from country to country.
The DIS-1 dosimeter and the TLDs are both passive
dosimeters, which accumulate the radiation dose. Passive
dosimeters are recommended for pulsed X-ray radiation,
since a saturation, which would lead to an underestimation
of the radiation dose, can be ruled out using this dosim-
eter type. However, the latter does not mean, that active
dosimeters, as the OD-02 are not suited for measurements
of pulsed radiation, as will be shown below. If the X-ray dose
per pulse is low enough to not saturate the dosimeter, even
active dosimeters can yield reliable results. Since passive
dosimeters must be read out manually after each measure-
ment, they are not convenient in practical use, e.g. if meas-
urements must be repeated many times, to align a radiation
source. For that reasons, all measurements were performed
using the OD-02 in an “accumulation mode”, which delivers
a resolution of 0.01µSv in H′(0.07). The resolution deliv-
ered by the DIS-1 is 10µSv in Hp(0.07). The reliability of
the OD-02 dosimeter was ensured in all measurements by
comparing the accumulated doses collected by the OD-02
over an extended period of time with the simultaneously
accumulated doses collected by the passive dosimeter sys-
tems. In Fig.2 the comparison of the doses accumulated by
the passive dosimeter DIS-1 and the active dosimeter OD-02
is shown. In addition, selective accumulated doses collected
with the TLDs are also displayed. A linear dependence
between the accumulated doses collected by the OD-02 and
the accumulated doses collected by the passive dosimeters
was observed. In this way, the reliability of the OD-02 up to
the highest measured dose rates was verified in the experi-
ment. All detectors were calibrated by the manufacturer and
by using radiation standards of the Physikalisch-Technische
Bundesanstalt (PTB, Braunschweig, Germany).
2.3 Measurement ofspectral X‑ray emission
For the selection of an instrument for spectral X-ray emis-
sion measurements, the specific experimental conditions in
laser material processing must be considered. Appropriate
instruments for spectral measurements of pulsed plasma
radiation in a dose relevant photon energy range above 2
up to 50keV are efficient broadband X-ray crystal spec-
trometers [12, 13], single photon counting semiconductor
detectors [14] and TLD based spectrometers [15]. Consid-
ering the specific requirements in laser material processing,
a single photon counting CdTe spectrometer (X-123CdTe
Spectrometer, 3 × 3 × 1mm3, 100µm Be window, Amptek
Inc.) was selected for the spectral measurements in this
work. The CdTe spectrometer tolerates changes of the posi-
tion of the radiation source during the scan process, it allows
to collect a complete spectrum within the scanning time of
several seconds, and the high stopping power of CdTe pro-
vides a detection efficiency of nearly 100% over a broad
energy range up to photon energies of 50keV. Even though
these detectors are ideally suited for the requirements in this
Fig. 2 Comparison of the accumulated doses, which were simulta-
neously collected by the active dosimeter OD-02 and by the passive
dosimeter DIS-1. In selected measurements additionally TLDs were
applied. The linear least-squares fit gives a slope of 1, which confirms
that, regardless of its active measurement principle, the measure-
ments with the OD-02 are reliable here
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H.Legall et al.
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407 Page 4 of 8
work, the detector type has some disadvantages, e.g. its vul-
nerability to pile-up. This effect arises, if multiple X-ray
photons hit the single photon detector in the time frame of
the processing time and are registered as one single photon
with higher energy. Because of the moving plasma spot in
the scan process, no X-ray collimator could be employed
in the measurements to reduce the pile-up, since a colli-
mator would lead to an underestimation of the emitted
photon flux. Therefore, to minimize the pile-up, the CdTe
spectrometer was operated at a large distance of 64.5cm to
the laser-generated plasma. In addition, the radiation per
pulse was attenuated by aluminum foils placed in front of
the X-ray detector. The thickness of the aluminum foils was
determined from the attenuationusing a 55Fe and a 109Cd
radiation standard (PTB). Measurements were performed
with aluminum foil thicknesses up to 362µm. Another
characteristic feature of CdTe spectrometers, which must
be considered in quantitative X-ray spectroscopy, is an una-
voidable background continuum appearing in the spectrum
with a step close to intense emission lines. This step-like
background with increased count rates at the low-energy side
of the emission lines arises from energy degrading effects,
e.g. by Compton scattering and incomplete charge collection
[16]. The X-ray emission lines itself can be approximated
by Gaussian shaped peaks in an energy range below 15keV
[17]. As will be shown in Sect.3.3, the Bremsstrahlung con-
tinuum can be approximated by a Maxwellian distribution.
If these contributions to the spectrum are known, the back-
ground continuum can be estimated and eliminated from the
measured spectrum. The latter was done in the present work.
Since the evaluated background continuum was vanishingly
low in the spectral region of interest, it was neglected in the
evaluation process described in Sect.3.3. Calibration of the
energy scale of the detector was done by radiation stand-
ards of PTB and by the known emission line energies of the
investigated target materials.
3 Results anddiscussion
3.1 Dependence ofdose rates ondetection angle
In a first step, the angular distribution of the dose was
determined in the experiment. The distance from the X-ray
emitting plasma source to the OD-02 dosimeter was set
to 420mm in all measurements. This is a typical distance
between the laser-plasma source and the torso of an opera-
tor. The range of detection angles for which the detector
volume of the OD-02 was completely exposed to X-rays was
geometrically restricted by the scanner head and the size
of the target. In Fig.3 the collected dose rates Ḣ′(0.07) in
dependence on the detection angle vertical to the target plane
are shown for steel S235JR and tungsten at an intensity of
2.6 × 1014W/cm2.
As can been seen in Fig.3, the emitted X-ray dose
strongly depends on the detection angle. The angle at which
the highest X-ray dose rates can be found was determined to
about 29° with respect to the sample plane.
3.2 Dependence ofdose rates onthematerial
Tungsten (Z = 74), steel (Z = 26), aluminum alloy (Z = 13),
and Gorilla glass were investigated. The atomic number Z
of these materials covers a wide range of the periodic table.
The selection represents common used materials in the
laser-based industrial fabrication process [18]. Doses of the
directional dose equivalent H′(0.07) were measured with the
ionization chamber dosimeter OD-02 at a fixed distance of
420mm and a fixed detection angle of 29°. The related dose
rates Ḣ′(0.07) are displayed in Fig.4 as function of the laser
peak intensity. The intensity was varied in the experiment
by tuning the single pulse energy of the laser.
Figure4 shows an increase of the dose rate with rising
laser intensity. Furthermore, a tendency of increasing dose
rates with increasing atomic number Z can be observed at
a fixed intensity. The lower dose rates measured for Gorilla
glass in comparison to the aluminum alloy can be explained
by the lower concentration of aluminum in Gorilla glass.
The measured dose rates in Fig.4 clearly exceed the
radiation safety limit for skin dose, which is 50mSv/a for
members of the public (25µSv/h @ 2000h operation time
per year) [11]. The maximum dose rate Ḣ′(0.07) measured at
the highest intensity was 163mSv/h. This dose rate exceeds
the allowed regulatory radiation limit by a factor of about
Fig. 3 Dose rates in dependence on the detection angle vertical to the
target surface measured in air at the distance of 420mm to the laser
ablation spot. The laser beam wasincident normal to the target sur-
face. Measurements of Ḣ′(0.07) were performed with an intensity of
2.6 × 1014W/cm2
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X-ray emission asapotential hazard duringultrashort pulse laser material processing
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6.6 × 103. This highest measured dose rate corresponds to a
single pulse dose of 0.1nSv.
3.3 Dependence ofspectral X‑ray emission
onthematerial
A comparison of the spectral X-ray emission for the inves-
tigated target materials at the highest laser intensity of
2.6 × 1014W/cm2 is shown in Fig.5. The spectra in Fig.5
were collected at the distance of 64.5cm from the laser
ablation spot, with a 362µm thick aluminum foil placed in
front of the spectrometer. In the lowphoton energy range,
the spectra are strongly attenuated by the filter absorption
and the absorption in air. The accumulated counts visible
at lowest photon energies can be therefore attributed to the
step-like background continuum mentioned in Sect.2.3.
The spectral distribution of the plasma emission consists
of two parts: the characteristic lines (visible for tungsten,
discriminated by the aluminum filter and the absorption
in air for steel S235JR) and a Bremsstrahlung continuum.
The Bremsstrahlung continuum can be approximated with
a Maxwellian distribution of the form [19]:
where
kB
=8.6 ×10
−5
eV∕
K
and 1 eV ≙ 1.2 × 104K, E
denotes the X-ray photon energy, kB is the Boltzmann con-
stant, and Te is the electron temperature, by convention
expressed in keV. It can be seen in Fig.5, that for the dif-
ferent investigated materials the spectral distribution (curve
shape) varies much less than the number of emitted X-ray
photons (area below the curves). While the area below the
measured curves clearly increases with the atomic num-
ber Z, the shape of the measured photon energy distribu-
tion remains almost constant. The Maxwellian distribution
describes well the measured curves. Only at the high-energy
tail a deviation from the calculated Maxwellian distribution
is observed, which can be explained by a pile-up. For tung-
sten, the deviation of the calculated Bremsstrahlung spec-
trum from measured data is most pronounced.
A simple approach was used to estimate the spectral
photon flux emitted by the laser plasma from the measured
spectra. The latter was done by combining two spectra,
which were recorded at nearly the same dose rate, but with
different aluminum filter thickness and, therefore, differ-
ent pile-up contributions. As a result, a spectrum with low
pile-up was obtained, without losing the spectral informa-
tion in the low-energy channels of the CdTe spectrometer
due to the strong attenuation by filters or the absorption
in air at large distances. In a first step, the spectra were
corrected for the detector efficiency given by the manu-
facturer, and the attenuation of the spectral emission due
to filters and absorption in air was eliminated from the
two spectra using tabulated mass attenuation factors [20].
The high-energy tail of the spectrum, which was less
attenuated and, therefore, more affected by pile-up, was
replaced by the high-energy tail of the stronger attenu-
ated spectrum with lower pile-up contribution. To combine
both spectra, the less attenuated spectrum was scaled in
amplitude until both spectra exhibit the same count rates
(1)
fMaxwell(E)dE=

4E
𝜋

k
B
T
e
3
⋅exp

−E
kBTe

dE
,
Fig. 4 Measured dose rates Ḣ′(0.07) in dependence on the target
material and the incident laser peak intensity. Different target materi-
als were investigated (tungsten, steel S235JR, an aluminum alloy, and
Gorilla glass) using the ionization chamber dosimeter OD-02at a dis-
tance of 420mm in air. The intensity was varied between 2.6 × 1013
and 2.6 × 1014W/cm2 by tuning the laser pulse energy
Fig. 5 X-Ray spectra in air measured with the CdTe spectrometer at a
laser intensity of 2.6 × 1014W/cm2 at a distance of 64.5cm from the
laser ablation spot. In the measurement a 362µm thick aluminum foil
was placed in front of the detector. Shown are spectra recorded with
tungsten, steel S235JR, and an aluminum alloy as target material. The
black curves are calculated Maxwellian distributions
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H.Legall et al.
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in the overlapping region. By this procedure, losses due
to pile-up could be compensated in the less attenuated
spectrum. In the energy range, where no counts were accu-
mulated in the spectra (absorption in air and by filters), the
spectrum was filled up with an appropriate Maxwellian
distribution fitting the whole spectrum. Consequently, the
combined spectrum represents the energetic distribution
of the emitted X-ray photons in vacuum extrapolated to
low photon energies. In Fig.6 the two tungsten spectra
with different filter attenuation and the combined spec-
trum are shown. The upper spectrum in the right of Fig.6
was taken with a 45µm thick aluminum filter in front of
the CdTe spectrometer, the lower spectrum with a 362µm
thick aluminum filter. The grey dashedcurve represents
the calculated Bremsstrahlung continuum estimated by a
Maxwellian distribution with an electron temperature Te
of 1.76keV. As seen in Fig.6, no counts were collected by
the CdTe spectrometer at photon energies above 30keV.
The spectral X-ray photon flux can be estimated from the
combined spectrum assuming, that the accumulated counts
in the spectrum represent the number of photons emitted
into the solid angle, which is covered by the CdTe spectrom-
eter area at the distance of 64.5cm. In Fig.7 the calculated
photon flux ΦΕ for tungsten is displayed for two different
propagation distances in air (100 and 420mm). The dis-
tance of 100mm was chosen because of its relevance for
radiation-protection, the distance of 420mm was chosen to
compare the results with the performed dose measurements
presented in this work.
Using the spectral photon flux ΦΕ the dose rates as func-
tion of photon energy can be calculated [21, 22]. For the
calculation of the spectral dose rates for tungsten shown
in Figs.8 and 9 conversion factors in vacuum were taken
from [21]. Integration over the curves in Figs.8 and 9 yields
the dose rates Ḣp(0.07) and Ḣp(10) (cp. Table1). The cal-
culated dose rate Ḣp(0.07) in air at a distance of 420mm
was determined to 156mSv/h and corresponds well to the
dose rate of 158mSv/h in H′(0.07), which was simultane-
ously measured with the OD-02 dosimeter for the spectra
displayed in Fig.6. Since in an energy range < 50keV the
personal radiation dose and the ambient radiation dose can
Fig. 6 Displayed is the com-
bined spectrum in vacuum
(left), which was constructed
from the two measured emission
spectra at the right. The meas-
urements were performed in air
with tungsten as target material
at the highest intensity of
2.6 × 1014W/cm2 at the distance
of 64.5cm. The upper spectrum
at the right is attenuated by
45µm thick aluminum foil, the
lower spectrum by 362µm thick
aluminum foil. The combined
spectrum was extrapolated to
lower X-ray photon energies by
a Maxwellian distribution (grey
dashed line)
Fig. 7 Spectral X-ray photon flux ΦΕ for tungsten calculated in air at
the highest intensity of 2.6 × 1014 W/cm2 at two distances (100 and
420 mm). Additionally, the photon flux calculated from the Max-
wellian distribution in Fig.6 is shown (grey dashed line)
Table 1 From the spectral photon flux in Fig.7 calculated dose rates
Ḣp(0.07) and Ḣp(10) for different propagation distances in air
Conversion factors were taken from [21]. The dose rates clearly
exceed the dose limits for members of the public, which are 25µSv/h
for the exposure of the skin and 0.5 µSv/h for the effective dose,
assuming an exposure time of 2000h per year
Distance in mm Ḣp(0.07) in mSv/h Ḣp(10) in mSv/h
100 10.8 × 10313.4
420 156 0.68
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X-ray emission asapotential hazard duringultrashort pulse laser material processing
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be set equal [9, 10], the dose rates in Table1 also present the
quantities Ḣ′(0.07) and Ḣ*(10). All the dose rates in Table1
clearly exceed the regulatory limits.
Based on the spectral dose rates in Figs.8 and 9,
the estimation of dose rates at photon energies above
30keV can be refined. For both operational dose quanti-
ties H′(0.07) and H*(10) dose rates in air of 0.7 µSv/h at
100mm and 0.04µSv/h at 420mm can be estimated for
the photon energy range above 30keV by integration over
the spectral dose rates calculated from the Maxwellian
distribution.
3.4 Radiation protection
In Sect.3.2 it was shown that the measured radiation dose
at high-laser repetition rates can exceed the regulatory
exposure limits for members of the public (25µSv/h for
the exposure of the skin and 0.5µSv/h effective dose @ an
operation time of 2000h per year) even at relative low laser
peak intensities between 1013 and 1014W/cm2. Furthermore,
it was shown in Sect.3.3, that for the used laser system the
dose relevant X-ray emission in the investigated intensity
range is limited to photon energies below 30keV. Since the
radiation dose exceeds the regulatory limits, an adequate
radiation shielding should be used. To estimate an adequate
shielding, the attenuation factor of the shielding material for
the incident radiation must be known. That attenuation fac-
tor can be evaluated by dividing the dose obtained from the
non-attenuated photon spectrum by the dose obtained from
the calculated photon spectrum behind a specific shielding
material of a certain thickness.
In Fig.10 attenuation factors for three different shield-
ing materials (Fe, Al, SiO2) are displayed. These attenua-
tion factors were calculated in air for a distance of 420mm
to the ablation plasma spot. It can be seen that a steel plate
of 0.5mm thickness can reduce the X-ray radiation dose of
H′(0.07) in air by a factor of 106 and in H*(10) by a factor of
104. Consequently, a steel plate of 0.5mm thickness placed at
the distance of 420mm to the plasma can reduce effectively
the highest measured X-ray radiation dose rate Ḣ′(0.07) of
163mSv/h and the calculated dose rate Ḣ*(10) of 680µSv/h
Fig. 8 Spectral dose rate Ḣp(0.07) for tungsten at the highest intensity
of 2.6 × 1014W/cm2 at two distances (100 and 420mm) calculated in
air from the spectral X-ray photon flux ΦΕ in Fig.7. Additionally, the
spectral dose rate Ḣp(0.07) calculated from the Maxwellian distribu-
tion in Fig.7 is shown (grey dashed line)
Fig. 9 Spectral dose rate Ḣp(10) for tungsten at the highest intensity
of 2.6 × 1014W/cm2 at two distances (100 and 420mm) calculated in
air from the spectral X-ray photon flux ΦΕ in Fig.7. Additionally, the
spectral dose rate Ḣp(10) calculated from the Maxwellian distribution
in Fig.7 is shown (grey dashed line)
Fig. 10 Calculated shielding attenuation factors for H′(0.07) and
H*(10) in air. The calculations were performed at the distances
of 420 mm from the laser ablation spot for the highest intensity of
2.6 × 1014 W/cm2 with tungsten as target material and for different
thickness of shielding materials (iron, aluminum, and SiO2)
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

H.Legall et al.
1 3
407 Page 8 of 8
to a level below the regulatory radiation limits, assuming an
exposure time of 2000h per year. Here, it must be noted that
other laser wavelengths or pulse durations at the same laser
intensity (e.g. also in the ps range), another than a perpendicu-
lar laser incidence and pre-pulses can lead to higher dose rates
and that the calculated radiation shielding must be modified in
these cases. Furthermore, it must be underlined that the lim-
ited planarity and the surface quality of our targets (as being
present during industrial machining processes), i.e. the “worst
case” in the presented measurements cannot be adopted to
highly planar samples with deviating surface quality. There-
fore, the thicknesses of the radiation shielding material should
be chosen rather generous, e.g. in the range of 1mm for steel.
4 Conclusions
In summary, we have presented X-ray dose measurements in
air at a distance of 420mm to the laser ablation spot using
a 400kHz repetition rate 925-fs laser system at 1030nm
wavelength. The measurements were performed on tungsten,
carbon steel (S235JR), an aluminum alloy (AlMgSi), and alu-
mosilicate glass (Gorilla glass) as target materials. Up to laser
intensities of 2.6 × 1014W/cm2, the dose relevant X-ray emis-
sion is limited to photon energies below 30keV. The highest
measured X-ray dose rate Ḣ′(0.07) of 163mSv/h obtained
with tungsten as target material corresponds to a single pulse
dose of 0.1nSv. A radiation protection shielding of 1mm
thick steel is suggested here for the used laser system.
Acknowledgements The authors gratefully acknowledge financial
support by the German Federal Ministry for Education and Research
(BMBF) in the funding program Photonics Research Germany under
contract number 13N14249. We would like to thank Simone Russ and
Marc Sailer (both TRUMPF Laser GmbH) for experimental support
and Dr. David Heisenberg (TRUMPF GmbH) for valuable discussions.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creat iveco
mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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