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100 Hz X-RAY BEAM PROFILE MEASUREMENTS FROM A
TRANSMISSIVE CVD DIAMOND DETECTOR
C. Bloomer∗ 1 , L. Bobb, Diamond Light Source, Oxfordshire, UK
M.E. Newton, University of Warwick, Coventry, UK1also at University of Warwick, Coventry, UK
Abstract
A non-destructive CVD diamond X-ray beam imaging
monitor has been developed for synchrotron beamlines. The
device can be permanently installed in the X-ray beam pathand is capable of transmissively imaging the beam profile at
100 frames per second. The response of this transmissive
detector at this imaging rate is compared to synchronously
acquired images using a destructive fluorescent screen. It isshown that beam position, size, and intensity measurements
can be obtained with minimal disturbance to the transmitted
X-ray beam. This functionality is beneficial to synchrotron
beamlines as it enables them to monitor the X-ray beam focal
size and position in real-time, during user experiments. Thisis a key enabling technology that would enable live beam size
feedback, keeping the beamline’s focusing optics optimisedat all times. Ground vibrations (10 Hz - 20 Hz) can cause
movement of focusing optics and beamline mirrors, which
disturb the X-ray beam and reduce the ultimate quality of thesample-point beam. This instrument can detect this beam
motion, enabling the source to be more easily determined
and mitigations to be put in place.

while the subsequent charge reaching a pattern of orthogonal electrodes on the opposite face of the diamond is readout using electrommeters or other sensitive ammeters. Sequentially biasing the ‘strip electrodes’ enables one row of
pixels to the read out at a time. The graphitic wire detector
improves upon this design in two respects: firstly, throughthe use of laser-written graphitic electrodes which are buried
under the surface of the diamond plate, protecting them fromdamage and resulting in a more physically robust detector;
and secondly, through a novel lock-in modulated bias technique, illustrated in Fig. 1, whereby all ‘bias’ electrodes
simultaneously have an AC bias applied to them at different frequencies using a programmable multi-channel DAC.
The resulting signal currents from each ‘measurement’ electrode are simultaneously acquired using a multi-channel
20 kS / s acquisition system. A Fourier transform of the signal currents from each measurement electrode is carried out.
The Fourier amplitude of the measured signal at a given
frequency is proportional to the flux passing through the detector at the intersection between the measurement electrodeand the bias electrode modulated by that frequency.

INTRODUCTION
Diamond detectors for synchrotron X-ray beamlines areadvantageous for a number of reasons. Firstly, diamondis highly transparent to X-rays compared to other detectormaterials, which enables it to be installed permanently on
the beamline in the beam path. Secondly, diamond has
favourable thermal properties such high melting point andheat conduction, that enable it to withstand intense X-ray
beams from synchrotrons and XFELs. Thirdly, the strong
diamond lattice is both physically robust and resistant to mechanical damage, and it is radiation tolerant. Finally, and ofparticular use for this form of detector, the diamond structure
can be modified by fast laser pulses, to ‘write’ laser conductive wires and electrodes within the bulk diamond [1]. The
results presented in this contribution are from a pixellated
single-crystal CVD diamond with laser written electrodes.
The design of the 10 x 11 pixel detector and initial resultsfrom synchrotrons and XFELs are presented in Refs. [2–5],
along with discussion of the advantages and disadvantagesof diamond for this application.
To briefly summarise the design and operation of this prototype pixellated diamond detector, it builds upon a concept
first presented in Ref. [6]. A pattern of ‘strip electrodes’ on
one face of a diamond plate could be sequentially biased,

Figure 1: A sketch of the pixel readout scheme, utilisinga different modulation frequency applied to each graphitic‘bias’ electrode. Top: A time-domain view of the modulation scheme. Bottom: A frequency-domain view, showing
how the individual modulation frequencies measured by theelectrometer build up a picture of the X-ray beam profile.

∗ chris.bloomer@diamond.ac.uk

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OBTAINING 100 FRAME PER SECOND
IMAGES
Experiments were carried out on the I18 beamline at Diamond Light Source [7]. A photon energy of 15 keV wasselected, providing a collimated photon beam with a flux of
up to 3 × 1011 ph / s. The beam size at the detector could bealtered from 20 µm up to 250 µm FWHM through the use ofadjustable focusing mirrors.
The diamond graphitic wire detector and a traditional fluorescent screen system (a CMOS camera recording the fluorescence from a cerium-doped lutetium-aluminium-garnet,
LuAG:Ce, fluorescent screen) were installed together at the
focal point, supported on a motion stage which allowed
them to be aligned to the incident beam. The transparent
graphitic wire detector (15 keV photon absorption < 5 %)was installed ∼ 10 mm upstream of the opaque LuAG:Cefluorescent screen (15 keV photon absorption > 80 %) [3, 8].
Figure 2 presents the sum of the discrete Fourier transform (DFT) from all of the 11 measurement channels, up to
6 kHz, when it was illuminated by a large, 200 µm FWHM.
The modulation frequencies used are highlighted. In thisimage one can see how the 1-dimensional beam profile isdetermined from the amplitude of the Fourier transform of
the modulation frequencies. The individual signal currents
obtained from each of the 11 readout channels from thegraphitic wire detector beam are presented in Fig. 3. This
figure shows the resulting DFT of these signals from a 1 s(top) and 10 ms acquisition period (bottom).
The acquisition goal of 100 frames per second (FPS) waschosen as this would enable any X-ray beam motion arisingfrom the most common ‘ground vibration’ frequencies at
Diamond Light Source, around 10 Hz - 20 Hz, to be clearlymeasured.
The bias modulation frequencies used were 1.0 kHz,
1.1 kHz, 1.2 kHz, ..., 1.9 kHz. This choice of modulation
frequencies has been carefully chosen:• Firstly, these frequencies easily allows for short acquisition periods of just 10 ms, enabling 100 FPS acquisitionof the images. With a 20 kHz sampling rate, a 10 ms
acquisition will contain 200 samples. The DFT of thesesamples will result in each of the modulation frequencies occupying exactly one frequency bin, simplifying
the image acquisition. Each frequency bin of the DFT
corresponds to one column of pixels on the detector.
The bottom plot in Fig. 3 presents the Fourier transformfrom one 10 ms acquisition to illustrate this.
• Secondly, it ensures that the lowest modulation frequency used, 1.0 kHz is well above the frequency of
any beating between neighbouring modulation frequencies, observed at 100 Hz, 200 Hz, 300 Hz, etc.
• Lastly, it ensures that higher harmonics of the chosen
modulation frequencies will not clash with any of themodulation frequencies themselves: the lowest modulation frequency, 1.0 kHz, has a 2nd harmonic that is
above the highest modulation frequency of 1.9 kHz.

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Frequency [kHz]Figure 2: The sum of the individual DFT results from the
11 measurement channels, showing obtained spectra from
1 Hz to 6 kHz calculated from 1 second of data acquired
at 20 kHz. These results show the appearance of lower frequency ‘beating’ and higher frequency harmonics of themain modulation frequencies.

Figure 3: The Fourier transforms from 20 kHz signals acquired from 11 measurement electrodes whilst the X-ray
beam is illuminating the detector. These plots just show
the DFT amplitudes from 0.9 kHz to 2.0 kHz, around themodulation frequencies used. Top: The Fourier transform ofa 1 second long acquisition at 20 kHz signals. Bottom: The
Fourier transform of a 10 ms acquisition.

ESTIMATED POINT SPREAD FUNCTION
AND IMAGE DECONVOLUTION
By illuminating the detector with as small an X-ray beam
as possible an estimate of the 2-dimensional point spreadfunction (PSF) of the detector can be measured. To makethis estimate, the beamline optics were used to focus the
X-ray beam down to the smallest beamsize achievable at thelocation of the detector. The resulting beamsize at the detector was measured to be 24 µm x 20 µm FWHM, as measured
by two independent means: firstly by direct X-ray camera image, and secondly by knife-edge measurement. This is muchsmaller than one 50 µm x 50 µm pixel of the graphitic wire
detector and is sufficient to make a reasonable estimate of

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Figure 4: Acquired images of the 24 µm x 20 µm FWHM
X-ray beam. Left: The fluorescent screen image of the incident beam profile. Right: The measured estimate of the
PSF of the graphitic wire detector, obtained by illuminating
the detector with the small incident beam size. Both images
depict the same field of view.
the PSF. Under this illumination the obtained 2-dimensional
image obtained using both the fluorescent screen monitor
and the graphitic wire detector is presented in Fig. 4.
It is clear that there is some ‘spread’ of the signal acrossmultiple pixels of the graphitic wire detector. Of particularnote is that this obtained PSF does not exhibit the radialsymmetry that one would anticipate from an ideal detector. There is a vertical spread of the measured beam profile, smearing the 20 µm vertical beam profile across several
50 µm sized pixels. Horizontally there is only a small amountof spread, into only the neighbouring pixels. This asymmetrical response in the vertical and horizontal directions is
not entirely unexpected, as the acquisition methods differ
in the two axes: each pixel in a horizontal ‘row’ is definedby a different modulation frequency; each pixel in a verticalvertical column is obtained from a separate measurement
channel (as indicated by the axes in Fig. 4). It becomes clearthat the frequency of the centre electrode being struck by

the incident beam, 1500 Hz in this case, is ‘leaking’ acrossthe various measurement channels.
As we have obtained a good estimate of the PSF, andconfirmed that the PSF is consistent across the face of thegraphitic wire detector, image deconvolution algorithms canbe introduced to estimate the original beam profile fromthe detector image. A commonly used method originatingfrom the fields of optics and astronomy is the RichardsonLucy algorithm [9, 10]. The deconvolution was carried out
using MathWorks MATLAB, with the built-in deconvlucy
function [11]. The default algorithm tuning parameters of
the function were used. The results of these reconstructions
are presented in Fig. 5.

IMAGING RESULTS
To demonstrate the 100 frame per second (FPS) capabilities of the detector, the motion stage was used to oscillate
both the graphitic wire detector and the fluorescent screen
back and forth relative to the incident beam. For both detectors, a 2-dimensional Gaussian fit was used to find thebeam centroid. The CMOS camera observing the fluorescent screen was configured to acquire data at 100 FPS in
free-run mode using an integration period of 1 ms, while thegraphitic wire detector signal currents were acquired continuously at 20 kHz and then divided up into 10 ms (100 FPS)
chunks for analysis. Acquisition from the two devices was
initiated by external hardware trigger. Figure 6 shows thean example of the graphitic wire detector image obtained(after deconvolution using the Richardson-Lucy algorithm)
alongside the simultaneously acquired fluorescent screen
image, both acquired as part of a 100 FPS sequence. The
combined computation time for both the deconvolution andthe 2-dimensional Gaussian fit of the graphitic wire detector image using MATLAB was < 4 ms, enabling both to be

Figure 5: Images obtained of the X-ray beam profile with 10 ms acquisition period during during a beam focusing procedure.
Top row: Images obtained from the fluorescent screen. Middle row: Raw images obtained from the graphitic wire detector.
Bottom row: The estimated reconstruction of the beam profile obtained using the Richardson-Lucy algorithm.
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Beam centre [microns]

Figure 6: Beam profile images simultaneously acquired using both the the fluorescent screen and CMOS camera (left)
and the graphitic wire detector (right). The X, Y measurements shown are units of microns.
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CONCLUSIONS

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Time [ms]Figure 7: Vertical beam position measurements acquired
from 100 FPS image sequences during stepper motor movements of the stage the detector was mounted upon. The
motor is requested to move in a sine wave with an amplitudeof ± 30 µm (top); ± 10 µm (middle); and ± 3 µm (bottom).
completed in real-time while the following image is being
acquired.
Figure 7 shows the measured variation in beam position
simultaneously obtained from the 100 FPS images from both
the graphitic wire detector and from the fluorescent screen
image for a series of 1-dimensional vertical stepper motor
sweeps. The first sweep (top) is a ± 30 µm sine wave requested from the motor; the second (middle) is a ± 10 µm
motor request; the final motion (bottom) shows the results
when a ± 3 µm sine wave was requested. This final sweep
was at the limit of the motor’s capability, and it is clear fromthe obtained position measurements that the motor cannot
reproduce a smooth sine wave, instead a series of discrete
steps is observed, with some settling time and ‘rattling’ after
each step. Both the graphitic wire detector and the fluorescent screen images observe the same effect. The use of
2-dimensional Gaussian fitting enables these motions to beobserved on the graphitic wire detector even though they areclearly significantly smaller than the 50 µm pixel size.

The graphitic wire detector presented in this work isshown to successfully obtain X-ray beam images and beam
position measurements. A novel lock-in modulation readout method has been demonstrated, enabling 100 FPS acquisition of 2-dimension X-ray beam profile and centroid
measurements. All pixels were acquired simultaneously.
The position measurement results are comparable to thatobtained from a traditional fluorescent screen, however the
X-ray absorption is significantly less than most fluorescentmaterials. The graphitic wire detector may be permanently
installed in the X-ray beam path, offering significant advantages over fluorescent screens or knife-edge scans which
generally cannot be used during user experiments as theyintercept too great a portion of the synchrotron flux or block
it entirely.
While the PSF of the detector shows ‘leakage’ into neighbouring pixels and does not exhibit radial symmetry, a software deconvolution approach has been demonstrated to reconstruct the beam profile satisfactorily, and enable submicron beam position measurement.
Possible future developments include both smaller pixel
sizes, as the laser writing steps could easily be changed to
implement a pixel pitch down to ∼ 15 µm, and higher pixel
counts, as this experiment was limited by the number ofavailable ADC channels. Using higher sample rate DACs
and ADCs would also enable an increase in the frame rate, if
required. There are commercially available 32-channel DAC
and ADC units capable of 100 kHz output/input which could
be used to increase both the pixel count and the acquisition
rate.
This detector is beneficial to synchrotron beamlines as
it enables scientists and users to monitor the X-ray beam
focal size and position in real-time, during user experiments.
This functionality would enable live feedback, keeping thebeamline focusing optics optimised at all times. It would
reduce set-up and commissioning time, as the detector canbe permanently installed in the X-ray beam path and beam
sizes can be determined without having to temporarily insert
destructive monitors into the beam path.

ACKNOWLEDGEMENTS
The authors wish to acknowledge the support of many
individuals and groups that aided in this research. The CVD
diamond plate used was provided by Element Six. We wish
to acknowledge the contributions of P. Salter for the graphitic
electrode fabrication, at the University of Oxford. The detector surface metallisation was carried out by B. Green,
and the wire bonding by F. Courtney, both at the Universityof Warwick. We are grateful for the support of G. Cook
and K. Ignatyev at Diamond Light Source for their help incarrying out these experiments.

REFERENCES[1] B. Sun et al., “High conductivity micro-wires in diamond following arbitrary paths”, Appl. Phys. Lett. , vol. 105, p. 231105,

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detector with embedded graphitic electrodes”, J. Synchrotron
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doi:10.1107/s160057752000140x[3] C. Bloomer, L. Bobb, and M. E. Newton, “Single-Crystal
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[6] D. Shu et al., “CVD-diamond-based position sensitive
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