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OPERATING LIQUID METALJET X-RAY SOURCES
FOR MATERIALS RESEARCH
Mirko Boin, Roland Mainz, Manuela Klaus, Daniel Apel, Paul H. Kamm, Robert C. Wimpory,Francisco García-Moreno, Guido Wagener, Christoph Genzel,
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
AbstractthEven on the 100 anniversary of the death of Wilhelm
Conrad Röntgen the demand for applications of his discovery of X-rays is not diminishing. On the contrary, both academic and industrial research and development need Xray generating devices with ever-improving properties tomeet the current challenges of science and technology. For
this reason, the development of next-generation synchrotrons is being driven forward and made available to users
worldwide. Nevertheless, the availability of synchrotron
beamtime will always remain limited, even with the mostbrilliant sources for ultra-fast and high-throughput experiments. That is why the operation of and research with decentralized laboratory equipment becomes just as important. In this context, Helmholtz-Zentrum Berlin (HZB)
has commissioned Excillum’s MetalJet X-ray devices
providing photon energies in the hard X-ray regime. Technical specifications of these sources, the HZB diffractometer lab installations and selected examples are shown. A
comparison to synchrotron measurements is made tobenchmark the performance of the available setups.

to decentralized laboratories and thus make the analysis of
microstructural properties available to a larger community.

LIQUID METALJET X-RAY SOURCES
Inside laboratory and desktop X-ray devices electrons
are extracted from a cathode and accelerated within anelectric field into the direction of an anode, where their deceleration leads to an energy conversion into heat and Xrays. In general, the X-ray power of all electron-impact Xray sources is limited by the thermal power loading of theanode. In solid-anode technology, the surface temperatureof the anode must be cooled down and stay well below its
melting point to avoid damage. Consequently, the yield interms of photon flux of conventional fixed-anode X-ray
tubes is rather low compared to synchrotron radiation. Although the X-ray output can be increased significantly withthe use of rotating-anode tubes, the melting limitation of
the solid anode material remains.

INTRODUCTION
Non-destructive testing methods utilizing conventional
laboratory (desktop) X-ray sources have become a successful tool for more and more academic and industrial research purposes, such that their physical limits, for example according to the material’s penetration depths, havebeen extended using high-energy synchrotron photons.
Furthermore, high-brilliance synchrotron photons havemade fast in-situ and in-operando experiments possible.
However, the increasing demand for measurement timeat such facilities contrasts with their availability – thebeamtime at synchrotron sources is very limited. Moreover, as a rule, the access to the beamlines cannot be obtained promptly. Waiting times of half a year or longer arequite common in this field. Synchrotron beamlines are usually overbooked many times. Hence, it seems sensible andnecessary in several ways to use suitable laboratory X-ray
sources as well. With their help, numerous questions canbe answered which do not necessarily require the use of
expensive and in their availability mostly very limited
large-scale facilities.
It is the motivation of this work to address the lack of
beamtime with the development of tailored measurement
and evaluation methods including the implementation of
appropriate experimental hardware using laboratory X-ray
sources. Here, novel X-ray sources are presented that help
to transport a number of applications from the synchrotron

Figure 1: Photographs of the LIMAX-70 (left) and
LIMAX-160 (right) laboratories.
In contrast, a liquid metal anode changes this condition
since the limitation to maintain the target at well below themelting point is inexistent – the material is already molten.
This approach is implemented by the MetalJet products
available from the Swedish company Excillum. Two of
their MetalJet D2 X-ray sources have been installed at
HZB to equip a suite of the so-called LIMAX laboratories(see Fig. 1) for microstructural, residual stress and imaging
characterizations. These sources continuously supply fresh
target material. The liquid anode, composed of a Ga-In alloy liquid already at room temperature, is provided via a
closed high-pressure circuit at 190 bars. A very fine liquid
metal jet of a diameter of about 180 µm is formed with thehelp of a nozzle. The electrons are released by a LaB6 cathode and accelerated into the direction of the liquid anode
under a voltage of 70 kV and 160 kV respectively. With thehelp of focusing optics a very well-defined electron spot of
down to 5 µm × 5 µm is created at the edge of the metal
jet. The X-ray spectrum emitted consists of a broad rangeof Bremsstrahlung (the white beam) up to 70 keV and
160 keV respectively, depending on the excitation voltage.
In addition, and due to the extremely high-power loading,
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two very intense K-emission lines occur at E = 9.2 keV
for Ga and E = 24.2 keV for In, depending on the alloy
composition (Fig. 2).
106
Intensity (counts)

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ISSN: 2673-7035

Ga K

105

In K
104
103
102

0

10

20

30
40
50
X-ray energy (keV)

60

70

Figure 2: X-ray emission spectrum of the MetalJet D2
measured with LIMAX-70.
Utilizing the so-called Ex-Alloy-I1 target alloy (Ga: 68
wt %; In: 22 wt %; Sn: 10 wt%) the absolute photon fluxes
per solid angle have been verified by photon scattering for
Ga Kα and In Kα to be 6.0(5)×1012 s−1 sr−1 and 3.8(4)×1011
s−1 sr−1, respectively [1] at 200 W emission power and
70 kV acceleration voltage. This agrees very well with thespecifications given by Excillum.
The output X-ray windows of the MetalJet devices
provide a 10° open cone beam. To shape the beam for
diffraction experiments, for example, one obvious way is
to cut out the size of your gauge volume with the help of aslit system, collimators and Soller slits. However, to makethe most use of the incident intensity, third-party primary
optics are available instead. Polycapillary optics are used
to guide a large fraction of the emitted photons up to about
40 keV onto the sample by total external reflection fromthe walls of the thin glass tubes. Higher energies are notguided by such capillary optics such that their detected
intensities are significantly lower. To provide
monochromatic X-rays so-called Montel-optics are used to
select very precisely the Ga Kα line, e.g. for angledispersive diffraction measurements. Both can be tailoredto the needs of a specific experiment setup, i.e. their focus
lengths can be either a few centimeters or some meters, forexample. At both sources, LIMAX-70 and LIMAX-160,
the primary optics – either polycapillary or Montel type –can be aligned by a 6-axis PI H811.I2 hexapod in all
directions and orientations to meet the optimum beam
conditions, e.g. the maximum intensity.
The LIMAX-70 is equipped with a diffractometer consisting of two Huber 480 goniometer circles, each with adiameter of 800 mm, to realize angle-dispersive diffraction
and the rotation of a sample setup respectively. For sample
manipulation a large x-y-z stage with a translation range of
250 mm in each direction also allows for heavier setups,
such as a stress rig, furnaces or a Huber 512 Euler cradle,
for example. In a similar manner, the LIMAX-160 laboratory is modular to change between an Euler cradle based
diffractometer setup with integrated x-y-z translation tableand other stages, such as a deposition chamber, furnaces
and stress-rigs.

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A set of detectors with a large range of specifications
provide the opportunity for a number of diffractrion, radiographic and tomographic experiments. A Dectris Pilatus3 S 1M with an X-ray detection area of 169 x 179 mm2
divided into 981 × 1043 pixels, consisting of 10 Si sensor
modules each with a size of 83.8 × 33.5 mm2 and 1000 µm
thickness, is available for angle-dispersive X-ray diffraction. Complementary, two energy-resolving LN2-cooled
Canberra Ge detectors with a sensor size of 10 ×10 mm2
are available for energy-dispersive diffraction measurements. A PCO 1200 hs CMOS camera with a 1280 × 1024
pixel resolution and short exposures times is used with
scintillators to convert X-rays into visible light. Additionally, an Advacam WidePIX detector with a CdTe sensor of
70 mm × 28 mm and an Advacam MiniPIX detector witha Si Sensor of 14 × 14 mm2 – both with a pixel size of 55
× 55 µm2, as well as a Hamamatsu flat panel detector witha 120 ×120 mm2 scintillator plate and 50 × 50 µm2 pixel
size – are available for imaging experiments. A summaryof the setup specifications and available instrument options
is given in Table 1. Both laboratories are available for external and in-house users as well as for industrial applications.
Table 1: MetalJet Experimental Setups At HZBAnode alloy
Excitation
voltage
X-ray optic
options

Samplestage

Sampleenvironment
options

Detector
options

LIMAX-70

LIMAX-160

Ga-richalloy G1

In-enrichedalloy I1

max. 70 keV

max. 160 keV

slits, collimators,
capillary optics, Montel optics
Huber-480 2-circle
goniometer of 800
mm diameter with
x-y-z stage andoptional Euler
cradleHuber-440
goniometer of 500
mm diameter with
optional Euler
cradle(Cryo-)Furnace
Load frame
PVD chamber
Extra rotation stage
Imaging PCO camera
WidePIX + MiniPIX detectors
Dectris Pilatus 1M
Hamamatsu flat panel
Canberra Ge-detectors

APPLICATIONSTaking advantage of the unique features of the X-ray
spectrum emitted by the liquid metal jet source, i.e. thecharacteristic Kα-emission lines of Ga and In, as well as thewhite beam, allows for a multitude of applications and different types of measurements, such as angle- and energydispersive diffraction and transmission imaging, for instance.

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Energy-Dispersive X-ray Stress Analysis (ED-XSA)
The basic principles of X-ray residual stress analysis relyon the correlation between lattice strain measured in any
direction with respect to the sample coordinate system andthe components of the stress tensor which is given by thefundamental equation for stress analysis using diffraction
methods [2] and the so-called sin2ψ method [3]. For the
analysis of residual stress depth gradients an extension,
taking the exponential attenuation of the X-ray intensity by
the investigated material into account, is available [4, 5]
and the application to the energy-dispersive case of diffraction has been implemented and used at synchrotron beamlines, such as EDDI@BESSY-II [6, 7], taking advantage ofthe available white and hard X-ray beam spectrum. The
same practice is transferred to the LIMAX labs to make itavailable to a broader community without the restrictionsof the limited access to a synchrotron beamline.

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The collimator, on the other hand, also guides the high energy photons without attenuation, resulting in evaluablediffraction lines up to about 125 keV, which allows to extend the information depth of the residual stress profiles
considerably. However, due to the comparably low photon
flux (no collimation effect) the counting times per spectrum are very long. The results achieved in both configurations are in good agreement with those obtained before atthe EDDI synchrotron beamline.

ED-XSA in Complex Geometries
Nevertheless, the extreme high photon flux and brilliance of modern synchrotron beamlines provides further
advantages. Diffraction measurements are possible witheven very small gauge volumes and – considering theabove-described energy-dispersive method – allow for residual stress experiments on surfaces difficult to access,e.g. on the inside of boreholes in complex-shaped industrial components. Such measurements are not feasible withconventional laboratory X-ray equipment, since the reduction of the X-ray spot size on the sample leads to a verysmall signal-to-background ratio difficult to analyze.

Figure 4: Influence of the X-ray beam size on curved surface (left) on the magnitude of a measured residual stressgradient (right) as taken from [8].
Figure 3: Diffractograms (bottom), measured with the Canberra Ge detector, of a ground steel sample measured at
LIMAX-160 with two different settings – and the corresponding residual stress evaluation (top) in comparison tosynchrotron results with significantly smaller error bars.
Here, the measurement of a ground C-80 steel sample atthe LIMAX-160 source is shown and compared with synchrotron results. The sample surface was measured under
two settings with the LIMAX-160 – with polycapillary optics (scattering angle of 2θ = 20°) as well as with collimator
optics and a scattering angle of 2θ = 10°. The measured
lattice plane reflections in the diffractograms in Fig. 3 (bottom) are indicated with the hkl Miller indices. The positionof the diffraction lines depends on the choice of the selected, but fixed, scattering angle 2θ – with smaller 2θ leading to a broader distribution in the detected energy spectrum. The diffractograms as well as the residual stress
depth profiles obtained with the two optics clearly reveal
their advantages and drawbacks. The polycapillary lensprovides sharp and intense diffraction lines within rather
short measuring times up to about 40 keV. Then the glass
becomes transparent losing the effect of total reflection.

For example, the non-destructive access of the tangential
stress component on the inside of a borehole with a large
L/D ratio (length-over-diameter) is only feasible with theenergy-dispersive diffraction method under small scattering angles, which must be chosen small enough to ‘threadthrough’ the incident and diffracted X-ray beam during asin2ψ-scan.
Despite the higher flux of a MetalJet D2, as compared toconventional X-ray sources, a reduction of the beam size is
still at the costs of the signal to be evaluated for residual
stress depth gradients in the frame of such applications. In
the below example the LIMAX-160 was used to measure
the tangential stresses at the inner wall of a borehole witha diameter of 2 mm and 10 mm length. For the comparisonof two different measurements polycapillary optics were
utilized in combination with a 2 mm and a 0.2 mm pinhole.
Counting times of 300 s per orientation were used for thelarger pinhole and 3600 s for the small pinhole. On the secondary side an equatorial Soller slit with of δ = 0.15° was
installed in front of the detector for both setups. The resulting beam sizes on the inner borehole surface are sketchedin Fig. 4 (left). Both beam diameters, as defined by the pinholes, are drawn as bold circles on the inner wall of theborehole.
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ISBN: 978-3-95450-224-0
ISSN: 2673-7035

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

The influence of the beam diameter on the residual stressevaluation under the effect of such a curved surface is discussed in detail in [8]. It leads to a systematic scaling of theevaluated stress gradient towards lower absolute stresses.
The authors have further developed a correction method byputting the geometrical constraints of the borehole andbeam diameters into the equations for strain and stressevaluation. In the example in Fig. 4 (right) a correction factor of 1.77 was derived for the measurement with the 2 mm
pinhole. Its application onto the evaluated stress gradient
finally leads to a perfect match of the evaluated stress curvemeasured with the 0.2 mm pinhole and to earlier measurements at the synchrotron beamline.

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this example has demonstrated, that fast phase diagram
studies are feasible with a lab-based setup.

White-beam Imaging
Driven by X-ray tomoscopy, time-resolved tomography,
activities at the BESSY II and SLS synchrotron sources[10-12] and the high-flux potential of the Excillum sources,
the MetalJet devices and the experimental equipment
brings further X-ray imaging opportunities to the user community. The radiographic measurements are conductedwith the original cone beam geometry provided by thesources and without primary beam optics.

Phase Transitions in Photovoltaic Materials

Figure 6: A tomographic reconstruction of an additivemanufactured steel sample.

Figure 5: Integrated diffraction pattern measured with the
Pilatus detector as a function of temperature for
CsPb(BrxI1-x)3 with x = 0.23 taken from [9].
The high X-ray flux of the MetalJet sources further enables high-throughput in-situ experiments for the investigation of phase transitions, for example in photovoltaic materials. In this example Grazing-Incidence Wide-Angle Xray Scattering (GIWAXS) measurements have been conducted to resolve all transition temperatures for a phase diagram from 300 K to 585 K of CsPb(BrxI1−x)3 with a composition range of 0 ≤ x ≤ 0.68 in a single heating process[9]. CsPb(BrxI1−x)3 is utilized as a so-called perovskite solar
cell material with a beneficial microstructure for high solar
energy conversion efficiencies. The MetalJet source wasused at 70 kV with an angle-dispersive diffraction setup,polycapillary optics, a 2D detector and a heating stage. The
setup allows for fast acquisition of the samples’ diffraction
patterns on the detector during thermal annealing duringformation of the perovskite structure. Preliminary studies
have shown that Bromine helps to stabilize a metastablegamma phase structure at room temperature, but the necessary phase diagrams were unexplored before. The measurements with the MetalJet source helped to identify thetransition temperatures for all phases. Fig. 5 shows the integrated GIWAXS patterns as a function of temperatureduring heating up and cooling down. As a result, the transition temperatures decrease with an increasing Bromium
content and are in accordance with theoretical calculations.
These findings and experimental confirmations help inthe design and the processing of solar cell materials, andThe photon energies of up to 160 keV allow for the investigation of thicker components and denser materials.
The use of the full white beam spectrum gives rise to fastmeasurements with short acquisition times. Fig. 6, for example, shows the tomographic reconstruction of an additive-manufactured test specimen made of steel in the original and in a compressed state.

Figure 7: Radiographic images of soft matter foams [13]with different contrast and exposure times measured witha conventional tungsten microfocus tube and a flat panel
detector (left) as well as with a MetalJet at 70 kV and theCMOS camera (right).
Depending on the right choice of the available detectors
fast measurements of soft matter materials are also feasible
within short time ranges – especially in the energy regimeof the very intense Ga-Ka emission line. The detectors’ energy sensitivity varies with their sensor material and leads
to different image contrasts [13] such as shown in Fig. 7.

FURTHER DEVELOPMENTS &UPGRADE ACTIVITIESThe above-mentioned applications and methods are constantly being advanced. For the imaging option a new Photonics camera with 4.5 µm pixel size and a 15 µm-thickscintillator has been purchased and will be tested with the

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aim to achieve higher spatial resolution for imaging experiments.
A setup for parallel white beam X-ray diffraction and imaging experiments has been realized at the BESSY II synchrotron beamline EDDI [14]. The potential of theMetalJet sources and the available equipment at HZB are
currently aiming at a similar approach to make such amethod available in decentralized labs.
HZB has further invested in Excillum’s latest development – the MetalJet E1+. The new X-ray source uses thesame technique of a liquid anode material, but with a total
power of 1000 W. The source will extend the HZB activities and the throughput of applications.

CONCLUSION
The MetalJet sources and the HZB equipment describedherein demonstrate a wide range of applications to be investigated with hard X-rays by academic users as well as
for industrial research in the field of material science.
Measurements of energy-resolved residual stress depthgradients are feasible within complex geometry such ascomponents with boreholes and with information depth
comparable to experiments at synchrotron beamlines.
Likewise, long-term experiments can be conducted without
the restriction of limited beamtime. The high X-ray flux in
combination with appropriate measurement and detection
methods allows for in-situ investigation which have had
been performed earlier at synchrotrons.
The installation, operation, and maintenance of theMetalJet sources are accompanied by higher complexity ascompared to conventional X-ray tubes. The need for a 24/7
setup constantly providing high-flux X-rays over a long period requires careful handling of the devices and planningof service intervals. The new equipment at HZB is available to external user and will further bridge the gap between
high-flux and high-energy synchrotron photons and conventional desktop-like low-flux X-ray tubes.

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67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035