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PhD Research

At Warwick University I am working within the Diamond Science and Technology group (named after the carbon-based material, not the Diamond Light Source!) in order to make better X-ray detectors for synchrotron beamlines.

My PhD supervisor is Prof. Mark Newton, and I am studying at Warwick part-time. I started my PhD in January 2016, and will hopefully finish within about six years!

Synchrotron Radiation Instrumentation

X-ray instrumentation plays a vital role in the work of scientists and users at Diamond Light Source and other synchrotrons worldwide. Synchrotron light sources are paticle accelerators used to produce beams of light and X-rays. These X-ray beams can be focused down onto sample just a few microns big, and are often billions of times brighter than even the most powerful hospital X-ray. A typical synchrotron looks like the picture below, courtesy of Wikimedia Commons.

Copyright © EPSIM 3D/JF Santarelli, Synchrotron Soleil

Copyright © EPSIM 3D/JF Santarelli, Synchrotron Soleil

The ring is the synchrotron, a particle accelerator that circulates electrons at nearly the speed of light. The red shapes are magnets that bend the electron beam, causing it to emit synchrotron light, mostly X-rays. This light travels into the various beamlines shown branching out of the synchrotron. Each beamline contains different scientific instruments with which to perform experiments using the intense beam of X-rays that is produced.

As an example, shown below is the layout of one of these beamlines at Diamond Light Source. This is the B16 Test Beamline:

A schematic of the B16 Test Beamline at Diamond Light Source

Synchrotron X-rays can be focused down to a micrometer in size to examine microscopic crystals, or to enable high resolution mapping of larger samples. These X-rays are used in scientific fields from archaeology to protein biology, and everything in-between. Common to all of the experiments conducted at synchrotrons is the requirement to maintain the stability of the X-ray beam on the sample over the whole range of timescales exploited by the synchrotron users, nanoseconds up to months!

It is of paramount importance to the users of synchrotron light sources that the X-ray beam remains stable, and pointint at the sample. Being able to accurately measure the intensity and the position of these intense, focused beams is extremely important.

Using diamond as an X-ray detector

The use of diamond as a detector of ionizing radiation is nothing new: it has been studied since the 1940s! Its exceptional mechanical, thermal, optical and electrical properties make it very well suited to the monitoring of highly intense beams of X-rays and other ionizing particles. The unique properties of single-crystal diamond make it an excellent choice for a “transparent” detector material for on-line synchrotron X-ray monitoring:

  • high thermal conductivity and resistance to both thermal and radiation damage make it an ideal choice for insertion into intense X-ray beams;
  • high carrier-mobility produces a detector with an extremely fast response to changes in the X-ray beam position or intensity;
  • low atomic mass make it nearly transparent to X-ray light, whilst being just absorbing enough to detect the incident photons;
  • and a (nearly) perfect diamond lattice produces very little X-ray scattering that could interfere with a user’s experiment.

The current state-of-the-art X-ray beam monitoring instruments are single-crystal diamond detectors, capable of measuring the synchrotron X-ray beam position with resolutions of a few 10s of nanometres, or better than 0.1% of the X-ray beam size, at kHz bandwidths. However, there are outstanding questions regarding how to best extract signal currents from the diamond, and how best to arrange surface electrodes to maximise the detector sensitivity to the incident beam motion and to measure the beam shape.

The high cost of the diamond has prevented its widespread use for all but the most extreme of applications, but single-crystal diamond, mostly dislocation-free and of exceptional purity is now available at a palatable cost. Innovations in CVD fabrication and polishing techniques mean that diamond windows a few millimeters in size and suitable for use as X-ray detectors can now be produced and used by the scientific community.

Synchrotron beamline instruments will typically use detectors made from diamond windows that are around 0.1mm thick, with electrodes deposited on opposite surfaces of the window and wire-bonded to a PCB frame or holder for the diamond. Standard lithography techniques allow the size and shape of the electrodes to be controlled: dots, quadrants, strips, and pixels can all be applied to the diamond surface.

Diamond X-ray sensor

Sufficiently energetic X-rays absorbed by the diamond will kick electrons from the valence band into the conduction band, forming electron-hole pairs. Under the influence of a bias voltage these charge-carriers travel to one of the electrodes where this current can be measured.

Some of these X-ray windows manufactured by different companies can be seen below, made by Dectris and Cividec Instrumentation. They both have four quadrants on the surface (with the wiring for these visible at the edge of the images), and this allows the position of the X-ray beam passing through the sensor to be found.

Dectris XBPM Cividec XBPM

As the X-ray beam passes from one quadrant into the neighbouring quadrant the signals measured will change. This is what allows the X-ray beam position to be determined.

Future research

This PhD is exploring how to best measure the perfection of the raw diamond, searching for defects and dislocations in a cost-effective way. Ensuring that the quality of the diamond and the detector are consistent from device to device will result in better, more reliable diagnostic instruments, at lower cost to the end user.

Following this, I want to learn how best to process the raw diamond into suitable thin, uniform plates, and finally, I want to establish the best ways to apply surface electrodes to the diamond itself.

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Chris Bloomer

PhD in physics

 

c dot bloomer at warwick dot ac dot uk

Diamond Science &
Technology Centre
University of Warwick
Gibbet Hill Road
Coventry
CV4 7AL
UK
 

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chris dot bloomer at diamond dot ac dot uk

Diamond Light Source Ltd
Diamond House
Harwell Science &
Innovation Campus
Didcot
OX11 0DE
UK
 

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