What are the potential sources/causes of noise/artifacts in white beam synchrotron X-ray imaging/tomography? The SNR appears to be poor in white beams as compared to monochromatic beam. Is this because of some kind of radiation streaming? Would detector shielding and collimation of incident beam help?
Dear Ashish Kumar Agrawal ,
I think you are on the right track;
in case of non-sufficient collimation and non-sufficient detector shielding the scattered radiation affects the detector signal, especially via an offset (background) and its associated noise onto its output.
Monochromatic and white beams are produced by different means and will thus cause different ambient x-ray scatter levels in the lab (x-ray hut), which for example in the case of strong absorbing samples will contribute a non-negligible offset to the detector output.
In the case of my former x-ray set-ups I always checked this scatter offset in the detector signal by
a) putting a beam stop just behind the sample
and
b) just in front of the detector window.
In case (a) you will see how much x-ray scatter will enter in total the detector; and in case (a+b) you will see how much scatter enters the detector passing the housing/shielding. The difference of these signals [ (a) - (a+b) ] will give you the scatter contributions passing through the detector window.
From the strength of that value*) you might decide to improve your collimation in front of the detector, and from the signal level of (a + b) you may decide to improve your detector shielding...
There is no necessity here to do a complete tomographic scan. These data can be taken from a simple static set-up.
I think the high energy part of your white beam will make the trouble, because of the increase of Compton scatter cross-section with increasing x-ray photon energy and due to the decrease of x-ray attenuation of the detector housing/shield with increasing photon energy.
With respect to 'artifacts' in white beam tomographic images you certainly know, that there are artifacts showing up due to beam hardening. The low energy part of the x-ray spectrum is preferably attenuated by the sample, giving rise to a change of effective attenuation in the CT projections by the structures in the sample. Please 'google' for 'beam hardening artifacts' to get more familiar with this topic...
(See for example: https://radiopaedia.org/articles/beam-hardening )
*) compared to the detector output without any beam stop
Good luck and
best regards
G.M.
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