Adrian Jonas, Steffen Staeck, Birgit Kanngießer, Holger Stiel, Ioanna
Mantouvalou
Laboratory quick near edge x-ray absorption fine
structure spectroscopy in the soft x-ray range with
100 Hz frame rate using CMOS technology
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Jonas, A., Staeck, S., Kanngießer, B., Stiel, H., & Mantouvalou, I. (2021). Laboratory quick near edge x-ray
absorption fine structure spectroscopy in the soft x-ray range with 100 Hz frame rate using CMOS technology.
In Review of Scientific Instruments (Vol. 92, Issue 2, p. 023102). AIP Publishing.
and may be found at https://doi.org/10.1063/5.0032628.
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Rev. Sci. Instrum. 92, 023102 (2021); https://doi.org/10.1063/5.0032628 92, 023102
© 2021 Author(s).
Laboratory quick near edge x-ray
absorption fine structure spectroscopy in
the soft x-ray range with 100Hz frame rate
using CMOS technology
Cite as: Rev. Sci. Instrum. 92, 023102 (2021); https://doi.org/10.1063/5.0032628
Submitted: 12 October 2020 • Accepted: 24 January 2021 • Published Online: 16 February 2021
Adrian Jonas, Steffen Staeck, Birgit Kanngießer, et al.
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Laboratory quick near edge x-ray absorption fine
structure spectroscopy in the soft x-ray range
with 100Hz frame rate using CMOS technology
Cite as: Rev. Sci. Instrum. 92, 023102 (2021); doi: 10.1063/5.0032628
Submitted: 12 October 2020 •Accepted: 24 January 2021 •
Published Online: 16 February 2021
Adrian Jonas,1,2,a) Steffen Staeck,1,2 Birgit Kanngießer,1,2 Holger Stiel,1,3 and Ioanna Mantouvalou1,2,b)
AFFILIATIONS
1Berlin Laboratory for Innovative X-ray Technologies (BLiX), D-10623 Berlin, Germany
2TU Berlin, Analytical X-Ray Physics, D-10623 Berlin, Germany
3Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany
a)Author to whom correspondence should be addressed: [email protected]
b)Current address: Helmholtz Zentrum Berlin, D-12489, Germany.
ABSTRACT
In laboratory based x-ray absorption fine structure (XAFS) spectroscopy, the slow readout speed of conventional CCD cameras can prolong
the measuring times by multiple orders of magnitude. Using pulsed sources, e.g., laser-based x-ray sources, the pulse repetition rate often
exceeds the frame rate of the CCD camera. We report the use of a scientific CMOS (sCMOS) camera for XAFS spectroscopy with a laser-
produced plasma source facilitating measurements at 100Hz. With this technological improvement, a new class of experiments becomes
possible, starting from the time consuming analysis of samples with small absorption to pump-probe investigations. Furthermore, laboratory
quicksoftx-rayabsorptionfinestructure(QXAFS)measurementswith10mstime resolution are rendered feasible. We present the character-
ization of the sCMOS camera concerning noise characteristics and a comparison to conventional CCD camera performance. The feasibility
of time resolved QXAFS measurements is shown by analyzing the statistical uncertainty of single shot spectra. Finally, XAFS spectroscopy on
a complex sandwich structure with minute amounts of NiO exemplifies the additional merits of fast detectors.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0032628
I. INTRODUCTION
X-ray absorption fine structure (XAFS) spectroscopy is an
analytical tool for the investigation of the element specific elec-
tronic structure of atoms or molecules inside a sample. In quick
x-ray absorption fine structure (QXAFS) measurements, absorption
spectra are taken in quick succession to resolve dynamical pro-
cesses in the millisecond to minute range.1–10 The technique is a
powerful tool for investigating in situ reactions and slow dynamic
processes. XAFS is currently most commonly performed at syn-
chrotron radiation facilities due to the available high flux and effi-
cient instrumentation. For measurements, the excitation energy is
scanned in the region of the probed absorption edge and absorption
is measured via transmission, fluorescence, or electron detection.
For QXAFS measurements, only synchrotron radiation experiments
have been presented up until now, with a focus on hard x rays and
the extended XAFS (EXAFS) region and limited applications in the
soft x-ray regime.11,12
In the laboratory, x-ray sources are invariably less brilliant,
which triggered the use of scan-free approaches for XAFS, where
the polychromatic excitation radiation is first transmitted through
a sample and then detected with wavelength dispersive instrumen-
tation. Today, soft XAFS setups in the laboratory use light from
high harmonic generation sources13–18 or laser-produced plasma
(LPP) sources19–21 together with dispersive optics and position
sensitive detectors. In all published cases, a large area CCD sensor
is used to collect the spatially separated photons. CCD sensors
show a high sensitivity in the soft x-ray region, a very low noise
level, and a large active sensor area, which is crucial for a high
spectral resolving power and bandwidth. The downside of these
large CCD sensors is the readout time, which ranges from hundreds
of milliseconds to seconds. These readout times can prolong
Rev. Sci. Instrum. 92, 023102 (2021); doi: 10.1063/5.0032628 92, 023102-1
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the measuring times by multiple orders of magnitude if sin-
gle shot measurements are advantageous or mandatory and
would result in a time resolution of seconds for QXAFS
investigations.22 Advances of laser-based laboratory soft x-ray
sources with repetition rates of 100Hz to MHz call for detec-
tors that can reach equal frame rates. Electron-multiplying
CCDs or pn-CCD cameras manage the high repetition
rates but show an inadequate image area or pixel size, limiting
either the energy range or energy resolution of the measured XAFS
spectra.
CMOS sensors are used in numerous scientific and non-
scientific applications ranging from consumer electronics to space
missions.23 For most applications, front-illuminated cameras are
used, which are equipped with microlens arrays to improve the
sensitivity in the visible spectral range. For the detection of soft
x rays, CCD cameras still find widespread use and only recently
suitable CMOS camera chips are becoming available.24–26 One of
the key differences between common CMOS and CCD cameras is
that a CCD sensor has only one readout unit, while in a CMOS
detector each pixel or each pixel row converts the electrons into
voltageand amplifiesitindividually. Thisleadsto muchhigherread-
out rates at the cost of additional noise contributions in CMOS
detectors.Duetomultipleamplifiers,thedarksignalnon-uniformity
(DSNU), defined as the temporal noise adjusted standard devia-
tion of the pixel base values, and the pixel response non-uniformity
(PRNU), which describes the standard deviation of the pixel gain,
must be taken into account for CMOS detectors. Very recently,
back-illuminated 16-bit scientific CMOS (sCMOS) sensors without
on-chip microlenses became commercially available and were char-
acterized regarding energy and spatial resolution, noise level, and
soft x-ray quantum efficiency24–27 with results showing the potential
of sCMOS cameras for soft x-ray applications.
In this article, we present measurements regarding the noise
and gain values of a modified Tucsen Dhyana 95 sCMOS camera,
which are crucial when measuring small signals. Using a laboratory
XAFS setup based on a laser-produced plasma source operating at
a repetition rate of 100Hz, a reduced acquisition time compared
to the use of a standard CCD camera is presented. The ability to
perform QXAFS measurements with up to 10ms time resolution is
demonstrated and discussed. Finally, a sandwich sample with very
low absorption at the Ni L edge is investigated demonstrating the
advantages of high speed soft x-ray cameras.
II. EXPERIMENTAL
A. Scientific CMOS and CCD camera
The camera used is the commercially available Tucsen Dhyana
95 V1 with a 4 MPixel (2048 ×2048) resolution, a chip size of
22.5 ×22.5mm2, and a pixel size of 11 ×11 μm2. The sCMOS is
based on the back-illuminated GSENSE400BSI sensor by GPIXEL
Inc., with the quantum efficiency increasing from 0.3eV at 284eV
(CK-edge)to0.8at1303eV(MgK-edge).25 Itcanbecooleddownto
−20○C using a Peltier element and air cooling. Using the full frame
and 16-bit pixel values, the camera can collect 24 frames per second
(fps). The camera has readout electronics for each pixel row, so by
choosing a fragmentary number of rows to be read out, the frame
rate can be increased. If a cutout with 460 rows or less is selected,
100fps can be reached. The camera offers two measurement modes,
the High Gain (HG) mode and High Dynamic Range (HDR) mode,
the former using a higher gain and lower dynamic range compared
to the latter.
ThesCMOS cameraisdesigned forvisiblelightandis equipped
with an entrance window and commonly with an anti-reflection
coating on the chip. To be used for soft x-ray detection, the entrance
window was removed and an ISO–K 160 flange was attached on the
front side of the camera. The modified camera is depicted in Fig. 1.
The used CCD camera is the GE 2048 2048 from Greateyes
GmbH.28 Its back-illuminated chip is 27.6 ×27.6mm2large and has
a pixel size of 13.5 ×13.5 μm2. The quantum efficiency ranges from
0.65 at 284eV (C K-edge) to 0.8 at 1303eV (Mg K-edge). The gain
is 1 count/e−, and the full well capacity is 100 ke−. At −20○C and
1MHz readout speed (0.25fps), the camera features a readout noise
of 8.5 e−and a dark noise of 1 e−/pixel/s.
B. Experimental setup
In this work, a laser-produced plasma (LPP) was used as the
source of soft x-ray radiation. The source is explained in detail in
Ref. 29. Briefly, the LPP consists of a Yb:YAG thin disk laser with a
repetition rate of 100Hz and a solid state metal target. A laser pulse
with a wavelength of 1030nm and a pulse energy of up to 200 mJ is
focused onto a rotating cylinder with a changeable target material
(typically Cu or W), which forms a hot dense plasma with a
size in the range of 50 μm (FWHM). The pulse duration can be
continuously varied between 0.5ns and 30ns. The plasma
emits polychromatic and isotropic radiation in the range of
100 eV–1600eV. A photograph of the setup is shown in Fig. 2.
The XAFS measurements were performed using a twin-arm
reflection zone plate spectrometer previously described in Ref. 22.
The sample is measured in transmission mode, and the incident
x-ray spectrum is collected simultaneously using a pair of off-
axis reflection zone plate optics. A 200nm thick Al filter blocks
FIG. 1. Photograph of the Tucsen Dhyana 95 sCMOS camera that was modified
for the use in the soft x-ray range. The entrance window was removed and an
ISO–K 160 vacuum flange was attached.
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FIG. 2. Photograph of the setup including the Yb:YAG thin disk laser, the LPP
chamber, and the two beamlines used for the characterization measurements
(front) and the XAFS measurements (back).
residuallaser light, thevisiblepartof theemissionspectrumand sec-
ond order diffraction. The dispersed light is collected using either
the sCMOS or the CCD detector. The CCD is cooled down to
−30○C, and the sCMOS chip is operated at T =−15○C for all pre-
sentedmeasurements. Theabsorption ofthesampleisthen obtained
by the natural logarithm of the incident x-ray spectra divided by
the sample spectra. Through the use of multiple pairs of zone
plates, all existing absorption edges between 284eV (C K-edge) and
1303eV (Mg K-edge) can be measured with a resolving power
of E/ΔE≥900. The resolving power of the spectrometer at the
Ni L-edges amounts to roughly 1000. The whole spectrometer is
under high vacuum conditions (10−6mbar).
For the characterization of the sCMOS, the camera was
mounted to the chamber in 110cm distance to the target. The gain
was determined using monochromatized radiation and the single
photon counting procedure. To attenuate the produced radiation
to a photon flux suitable for single photon counting, four layers of
aluminum foil of a total thickness of 52 μm were inserted between
the target and the detector. The copper plasma emission spectrum
convoluted with the transmission of the Al filters results in a mean
photon energy of Emean =(1440 ±30) eV.
The gain was measured via single photon counting. Frames
with 10ms exposure time were recorded continuously with the LPP
at a repetition rate of 100Hz so that one shot was recorded on each
frame on average. 200 frames and 300 dark frames were recorded
for each measurement. The median of all dark frames was sub-
tracted, and for each pixel, the standard deviation was calculated.
Thestandarddeviation values were then usedtodeterminethenoise
thresholds for each pixel. The algorithm used to detect photon
events on the frames and to assign an intensity value in analog-to-
digitalunits (ADU)wastheclusteringalgorithm describedinRef.30
with threshold values of σ1=6 and σ2=3. To determine an accurate
result for the camera gain, only one-pixel events were evaluated.
The sCMOS dark current was measured by recording dark
frames for exposure times between 1s and 10s, with 20 frames for
each measuring point. Since the detector shows some regions with
unusually high dark current at the edges of the chip, only a section
of the total frames was evaluated to avoid distortion of the results.
This value can be seen as a lower limit for the dark current, while
an upper limit is provided by evaluating the whole frame. The mean
values of the selected region were plotted over exposure time and
the dark current was obtained from the slope of the linear fit. The
actual fit error was slightly raised to one significant digit to account
for possible small temperature fluctuations.
The readout noise was measured by taking five times 500 dark
frames with 20 μs exposure time each, so that the dark noise is neg-
ligible. The standard deviation from 500 dark frames was calculated
for each pixel and averaged for the whole frame. For the five mea-
surement series, the mean value was calculated. Since the standard
errors for both HG and HDR modes are smaller than the difference
of both mean values, the errors were estimated on the basis of this
difference.
The noise contribution originating from differing offset values
ofthepixels,thedarksignal non-uniformity (DSNU), was measured
with the same set of dark frames. For each set of 500 dark frames,
the mean value for each pixel was calculated and from those val-
ues the DSNU was determined by calculating the standard deviation
of the mean pixel values and subtracting the temporal noise. This
step was repeated for each set of 500 frames, and the mean value
and thestandard error were calculated from the five sets. Prior mea-
surements have shown differing values outside of the calculated
error range, so a greater error was estimated based on the difference
of the results.
For measuring the gain dependent part of the overall noise
originating from the different gain characteristics of the readout
amplifiers (pixel response non-uniformity, PRNU), flat field frames
are necessary, where the sCMOS chip is homogeneously illuminated
to investigate the different pixel responses. As a simple alternative
to an Ulbricht sphere, a gooseneck lamp combined with a pinhole
as the source of homogeneous illumination in the visible range was
used for the flat field frames. Frames were recorded for exposure
times between 100ms (10% saturation) and 250ms (50% satura-
tion) in 50ms steps in the HDR mode. For every measuring point,
500flatfieldframesinadditionto250darkframesweretaken,which
were averaged and subtracted from the flat field frames to mitigate
the DSNU. The relative PRNU was estimated by dividing the frame
standard deviation by the mean frame value. A homogeneous part
of the frame was chosen to obtain a lower estimate for the PRNU,
while the whole frame was evaluated for an upper estimate (assum-
ing some degree of non-uniformity in the illumination). The mean
values of the four measuring points were calculated. For the opera-
tionofthesCMOScamera,themanufacturer’ssoftwareMosaic1.6.1
was used.
C. Samples
For the XAFS measurements, a Ni sample has been fabri-
cated by resistive thermal evaporation at the nanoscale lab of the
Max–Born-Institute (MBI). The sample consists of a thin Ni layer
with a thickness of 100nm on top of a 150nm thick Si3N4window
with a size of 3 ×3 mm2.
As another example, a sandwich sample consisting of three lay-
ers (TiO2on Au-nanoparticles on NiO) was measured at the Ni
L-edges. The exact layer thicknesses are unknown but it is expected
that they amount to some tens of nanometers. The sample was
prepared on a 150nm thick, 1 ×2mm2Si3N4window. The sam-
ple system finds application as a plasmonic solar cell prototype
device.31,32
Rev. Sci. Instrum. 92, 023102 (2021); doi: 10.1063/5.0032628 92, 023102-3
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TABLE I. Measured noise values and dark current for the Tucsen Dhyana 95 sCMOS camera compared with the manufacturer’s values.
Measured value Manufacturer’s value
Dark current/e−/s Lower limit: 1.5 @ T =−10○C34
0.9 ±0.1@T=−15○C, HDR 1.0 @ T =−20○C (CCD)
0.9 ±0.1@T=−15○C, HG
Upper limit:
1.1 ±0.1@T=−15○C, HDR
1.1 ±0.1@T=−15○C, HG
Readout noise/e−1.81 ±0.02, HDR 1.4534
1.79 ±0.02, HG 8.5 @ 1MHz (CCD)
Dark signal non-uniformity (DSNU)/e−0.38 ±0.05 N/A
Pixel response non-uniformity (PRNU)/% Lower limit: 0.7 (x: 220–1460,y: 593–1508) N/A
Upper limit: 2.7 (full frame)
III. RESULTS
A. Characterization measurements
The sCMOS detector was characterized regarding its dark cur-
rent, readout noise, DSNU, and the PRNU with visible light. For
both gain modes (HDR and HG), the results of these measure-
ments are shown in Table I. The results are in accordance with the
manufacturer’s values and show that the dark and readout noise lev-
els of the sCMOS detector are superior to a conventional soft x-ray
CCD detector.28 The gain for the HDR mode was calculated to be
gHDR =(0.52 ±0.01) e−/ADU and for the HG mode, gHG =(0.03
±0.01) e−/ADU. The errors are estimated by assuming an error for
the determination of the mean photon energy of ΔEph =30eV. As
the average number of electrons per eV, the value of 3.65 eV/e−33
was used.
B. XAFS measurements
For the XAFS measurement, the sCMOS camera was triggered
bytheLPPsourcewith100Hz.20000imagesweretaken.Toachieve
the desired 100Hz frame rate, only a region of interest (ROI) con-
sisting of 460 out of the 2048 rows was read out. The limited size
of the ROI has no effect on the XAFS spectra other than reducing
the measured energy range to ∼50eV, which is more than suffi-
cient for the investigation of the Ni L3 and L2 edges simultaneously.
To minimize the effect of the DSNU on the XAFS spectrum, the
camera was oriented so that the directions of the readout and dis-
persion are equal. The camera is connected to a personal computer
(PC) via universal serial bus (USB) 3.0. The data can be stored on a
hard disk drive (HDD) or temporarily in the random-access mem-
ory (RAM). Each tiff image with 460 ×1344 pixels2has a size of
1.21 MB. Due to the use of a conventional non-optimized personal
computer with 4 GB RAM and a HDD, the data could not be saved
at 100fps and the 20000 images had to be taken in 20 stacks of
1000 images. Each 1000 image stack was acquired with a frame rate
of 100fps. After recording a stack, the 1.21 GB of data were trans-
ferred from the RAM to the HDD, which took about 20s. There-
fore, the actual acquisition time for the 20000 images was about
10min, which translates to 33Hz acquisition rate. Using dedicated
hardware, data accumulation with 100Hz is possible continuously.
For the XAFS measurement with the CCD detector, 1000 single
shot images were collected with 8×binning, resulting in a 0.5Hz
frame rate. The total measuring time was 33min. The data process-
ing procedure to evaluate the XAFS spectrum is explained in detail
in Ref. 22.
Figure3shows the XAFS spectra ofthenickelsamplemeasured
withthe CCD(black)and sCMOSdetector.For bothmeasurements,
the LPP is operated with 100 mJ pulse energy and 1ns pulse dura-
tion. Tungsten was used as the plasma target material. The spectrum
was energy calibrated using a reference spectrum.35 The resolving
power has been estimated to be around 1000. The spectra show four
distinct features. Starting from low energies, the first peak corre-
sponds to the Ni L3-edge (2p3/2 to 3d). The third peak corresponds
to the Ni L2-edge absorption (2p1/2 to 3d). The second (858eV)
FIG. 3. XAFS spectra of a 100nm thick Ni foil at the Ni L3and L2edges measured
with a CCD and sCMOS camera for direct comparison. Measurement times were
10min (20000 images) and 30s (1000 images) for the sCMOS and 33min for the
CCD detector (for 1000 images). The spectra are displayed with an offset.
Rev. Sci. Instrum. 92, 023102 (2021); doi: 10.1063/5.0032628 92, 023102-4
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andfourth(877eV)weakerfeatures arerelated tothedensityofstate
effects discussed in Refs. 35 and 36.
To investigate the feasibility of QXAFS measurements, the
spectraderived fromdifferentnumbers of averagedshotsare shown.
Using 100Hz, one XAFS spectrum is generated every 10ms. If a
lower statistical error is needed for a given application, multiple
imagescanbeaveragedatthecostoftimeresolution.Averagingover
multiple images increases the quality of the XAFS by the square root
of the number of images taken. Table II summarizes the values for
selected parameters. If the full frame of the CMOS would be neces-
sary, a time resolution of about 40ms can be reached, resulting in a
standard deviation of 0.128.
To compare the absolute statistical error of the sCMOS and
the CCD measurement, the standard deviation of the signal in the
region between 862eV and 868eV was used, which corresponds to
29 pixels and a channel width of 0.21eV for the CCD and 34 px and
0.18eVforthesCMOS.Theregionismarkedwithgraydashedverti-
cal lines in Fig. 3 and was flattened using a linear fit prior calculating
the standard deviation. The standard deviation of one single shot
image is 0.280 for the CCD and 0.248 for the sCMOS camera. After
averaging over 1000 images, the statistical error is reduced to 0.009
for the CCD measurement and 0.009 for the sCMOS measurement.
After20000 images,thestatisticalerror of the sCMOSmeasurement
is reduced to 0.004. While for the CCD measurement the statisti-
cal error is in good agreement with the Poisson limit,22 the sCMOS
measurement shows an offset of >0.002. This is presumably due to
PRNU.TheeffectofthePRNUintheXAFSspectrumissmallerthan
the value given in Table I because of averaging processes. At 1000
images, using the full repetition rate and high readout speed of the
sCMOS camera, the quality of the XAFS spectrum is very similar to
the CCD but at only 1/66 of measurement time.
As an example for the merits of fast acquisition possibilities, in
general, a more complex layered sample was measured. The XAFS
spectrum is depicted in Fig. 4.
The TiO2/Au-nanoparticle/NiO sample contains approxi-
mately one order of magnitude less nickel than the pure Ni sample
on a 4.5 times smaller sample area. While the exact NiO layer thick-
ness is unknown, the absolute absorption values suggest a thick-
ness of about 30nm, of which only half is nickel. 50000 images
were taken with the sCMOS in 25min and evaluated. The statis-
tics follow the same pattern as the Ni measurement. The stan-
dard deviation of one single shot image is about 0.45. After 50
000 images, a standard deviation of 0.004 was determined. Again,
an offset of 0.002 larger than pure Poisson statistics can be seen.
TABLE II. Summary of the different possible QXAFS scenarios with the correspond-
ing time resolution and standard deviation of the XAFS spectrum using the sCMOS
detector.
Number of avg. shots Time resolution Standard deviation
1 10ms (0.5s CCD) 0.248, (0.280 CCD)
4 40 ms 0.128
10 100 ms 0.070
100 1 s 0.022
1000 10s (33min CCD) 0.009, (0.009 CCD)
20000 200 s 0.004
FIG. 4. XAFS spectrum at the Ni L-edges of a sample containing a TiO2, an Au
nanoparticle, and a NiO layer.
The spectrum shows a 5 times higher relative error than the Ni spec-
trum(cp. Fig.3), butmostfeaturescanbediscerned.The L2-andL3-
edge each split into t2g and egorbitals, but only the transitions into
the egorbital can be seen because the t2g orbital is fully occupied in
the ground state. The L3-edge is further split due to multiplet effects.
A detailed description of the features and NiO reference spectra can
be found in Refs. 37 and 38.
IV. DISCUSSION AND OUTLOOK
A commercially available sCMOS camera intended for the vis-
ible light range was successfully adapted to investigations using soft
x-ray radiation. As an application, XAFS measurements of a 100nm
thin nickel film with a laboratory LPP source were selected, where
the readout time of the detector, a commercial x-ray CCD cam-
era, was up until now the limiting factor for fast data acquisition.
The measurement time could be reduced drastically by a factor of
66, possibly 200, when using dedicated hardware for data acquisi-
tion in the future. Successive XAS measurements with 100Hz rep-
etition rate could be realized. The fast data acquisition rate of the
sCMOS renders laboratory QXAFS measurements with a time res-
olution of down to 10ms possible, which is comparable to the state
of the art QXAFS beamlines at synchrotron radiation facilities.3,4,7,9
Depending on the time scale and the requirement for statistical cer-
tainty of the measurement, the number of averaged images can be
adjusted. A statistical analysis of the XAFS spectra shows that an
offset of the statistical error of about 0.002 exists in each XAFS spec-
trum taken with the sCMOS, which is most probably due to the
pixel response non-uniformity. With a good calibration using more
elaborate flat field measurements than those used in this work, this
noise source can possibly be eliminated. With the current sample
chamber,QXAFS measurements ofthermalprocesses byheatingthe
sample can be performed.11 Using dedicated sample chambers, thin
film growth processes can be investigated.10 By adapting gas or liq-
uid transmission flow cells to this setup, gas–solid or liquid–solid
catalyticreactionscan be studied in thelaboratory.12 Regardingtime
consuming pump-probe XAFS, changes in absorption of <10−3can
be detected, rendering measurements feasible in less than one day
compared to weeks using CCD technology. A XAFS spectrum, con-
sistingof50000averagedsingleshotspectra,onacomplexsandwich
structure with minute amounts of NiO was shown. This presents
an example, where a high frame rate is mandatory for adequate
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statistics in reasonable measurement time scales. Taking 50000
images with the CCD detector would take 28 h.
sCMOS technology shows great potential for replacing CCDs
in ultrafast x-ray science, especially due to their superior frame rate.
However, the absence of dedicated sCMOS cameras for the (soft)
x-ray range makes adaption such as removal of windows and
vacuum integration necessary for now. For future experiments,
improved software and detector functionality may enable the simul-
taneous readout of specific parts of the frame or the reduction of the
pixeldepthto 12 bit, thus improving theframerateandreducing the
frame file size. The used version of the Dhyana does not allow mea-
surements with 12 bit pixel depth, although it is in principle possible
withthe GSENSE400BSI.26,27 sCMOS sensorswithreadout electron-
ics in every pixel could improve frame rates and readout flexibility
even further, but the complexity of the electronics leading to high
noise values combined with a lacking fill factor and quantum effi-
ciency renders the use of such detectors in the x-ray range difficult
for now.
ACKNOWLEDGMENTS
This work was conducted in the framework of DFG Project
No. 313838950. We thank Dieter Engel and Denny Sommer (MBI)
for producing the nickel sample. We thank the working group of
Jacinto Sá for providing us with the TiO2/Au-NP/NiO sample. This
work was funded by Deutsche Forschungsgemeinschaft (Grant No.
313838950) and LASERLAB Europe (Grant No. 654148).
The authors declare no conflicts of interest.
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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