The beamline we are using is I07 which specialises in surface and interface diffraction and I'll talk about the detector and sample set up for conducting the experiments later.
So we have three main methods of controlling the beam shape and flux; slits, lenses and attenuators. First things first is to put a camera in the sample position to image the X-ray beam shape and alter the lensing to give a uniform symmetrical gaussian beam profile in both horizontal and vertical directions. I won't go into too much detail here on the X-ray optics as others have done a far better job elsewhere, such as this presentation from the European Synchrotron Radiation Facility.Then for our uses we then draw in the slits to box off the centre of the beam to give a nice neat square beam footprint. Note there is still some gradient due to the divergence of the beam in the distance between the slits and the samples. That's the shape then we move on to the flux.
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| Image of the flux detector in the middle of the beam |
Replacing the camera with a silicon photodiode detector allows direct and quantitative measurement of the photon flux of the X-rays. This however can only be used for calibration, rather than ion chamber detectors which can be used during the experiment showing relative flux. If you are really interested have a look here for more information on the detectors. This allows us to measure the flux while we change the attenuation using filters, which when it comes to X-rays means big lumps of material. In this case predominantly different thicknesses of aluminium and molybdenum.
So put all these pieces of information together and it means we know what beam profile and what flux of photons are hitting our sample, which is key for our calculations at the end converting observed intensity on the detector into information about the illuminated volume.
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