100Hz X-ray Beam Profile Measurements from a
Transmissive CVD Diamond Detector
C. Bloomer1,2, L. Bobb1, M.E. Newton2
1 Diamond Light Source, Oxfordshire, UK
2 University of Warwick, Coventry, UK

Why diamond?

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 path and is capable oftransmissively imaging the beam profile at 100 frames per second (FPS). The response of thistransmissive detector at this imaging rate is compared to synchronously acquired images using adestructive fluorescent screen. It is shown that beam position, size, and intensity measurements can beobtained with minimal disturbance to the transmitted X-ray beam. This functionality is beneficial tosynchrotron beamlines as it enables them to monitor the X-ray beam focal size and position in real-time,
during user experiments. This is a key enabling technology that would enable live beam size feedback,keeping the beamline's focusing optics optimised at all times.

Detector operating principles

A new type of pixellated
diamond X-ray beam profile
monitor has been developed. It
uses laser-written graphitic
electrodes that are buried within
the diamond instead of
traditional surface metallisation.
It also makes use of a novel
lock-in readout technique toenable all pixels to be read
simultaneously.
Shown opposite is an illustrationof the concept. An AC bias
voltage with a set frequency isapplied to the ‘bias’ electrodes,
while signal currents are readout from orthogonal‘measurement’ electrodes.

CVD diamond is
excellent for
X-ray diagnostics:
it is extremelytransparent
compared toother detector or
fluorescentscreen materials.

Detector point spread function

The signals are acquired
using commerciallyavailable electrometerswith a built in 20kHz
ADC. Modulation
frequencies are thencarefully chosen:
1.0kHz, 1.1kHz, 1.2kHz,..., 1.9kH. The plot (right)
shows the resulting
Fourier transform from1s of acquisition.
With a 20kHz sampling
rate, a 10ms
acquisition will contain
200 samples. The
discrete Fouriertransform of thesesamples will result ineach of the modulation
frequencies occupying
exactly one frequency
bin. Each frequency bincorresponds to one
column of pixels on thedetector.

On the I18 beamline at Diamondlight Source the X-ray beam was
focused down to 20µm x 24µm
FWHM in size - much smaller than
the 50µm x 50µm pixel pitch of thegraphitic wire detector.

Fluorescent screen

The detector exhibits some ‘spectral
leakage’ into neighbouring pixels.
However, this is extremely consistentacross the face of the detector.
Graphitic wire detector

The Fourier transform of themeasured signal currents builds
up a picture of the 2-dimensional
X-ray beam profile passing
through the detector.

Readout at 100 frames per second

Photon energy [keV]This makes it straightforward todeconvolve and recover an estimateof the original beam profile.

High frame rate imaging results
Fourier transform 1s acquisition

Fourier transform 10ms acquisition

To benchmark the performance of thegraphitic wire detector against a
traditional fluorescent screen
observed by a CMOS camera, bothwere installed on the I18 beamline atDiamond. The fluorescent screen wasmounted downstream of the graphitic
wire detector. The beam was focused
to 240µm x 180µm FWHM. Bothinstruments were synchronised andacquiring data at 100 FPS.

Fluorescent screen

Graphitic wire detector

A 2-dimensional Gaussian fitting
routine was used to find the beam
centroid from the obtained 100 FPS
images.
A series of vertical stepper motor
sweeps were carried out, moving thegraphitic wire detector and thefluorescent screen up and down. Two
sweeps are shown opposite:±30μm (top), and ±3μm (bottom). At
100 FPS imperfections of the motor
movement are clearly visible, eventhough the motion is much less than
one 50μm pixel size.

Conclusions

Images acquired during beamline focusing
Fluorescent screen images

450μm

The graphitic wire detector obtained 100 FPS position measurement results
are comparable to that obtained 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 theX-ray beam path, offering significant advantages over fluorescent screens or
knife-edge scans which generally cannot be used during user experiments.
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.

Graphitic wire detector images after deconvolution with point spread function

450μmThis image sequence was obtained as the beam was focused down at the sample point. The graphitic wire
detector can resolve the beam size, shape, and intensity during this operation. Though the resolution ispoorer than the fluorescent screen, it is transparent enough to remain permanently in the beam path.