Scattering of X-rays and polarization

[Gruxg]

I read once (I don't remember exactly where) that X-rays scattered perpendicularly to the direction of the incident X-rays are linearly polarized (even although the incident ones are not). I think the discussion was in the context of low energy X-rays, and the explanation used classical electromagnetism arguments: it was the same as the polarization by scattering mentioned here: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polar.html

Is it true only for very low energy X-rays (with Rayleigh or Thompson scattering, i.e, frequency of the scattering = frequency of incident radiation), or is it true also for higher energies where the scattering is mainly by Compton effect?

 

[M Quack]

This was discovered by Charles Barkla in 1905 (the paper is from 1906).

http://www.nobelprize.org/nobel_prizes/physics/laureates/1917/barkla-bio.html

The classical polarization dependence holds for Thomson scattering. There are other effects such as multiple scattering, magnetic scattering, and resonant scattering that have different polarization behavior.

http://link.springer.com/article/10.1140/epjst/e2012-01630-3I am not sure about Compton scattering. There are polarization effects that have been used for experiments, in particular to study magnetism

http://geant4.cern.ch/G4UsersDocuments/UsersGuides/PhysicsReferenceManual/html/node56.html

 

[sophiecentaur]

Isn't the Brewster Angle involved in this? At large angles of incidence, you get reflection of X Rays at metal surface. It's how X Ray telescopes work.

 

[M Quack]

Total external reflection of x-rays is indeed used for x-ray optics (terrestrial and astro). However, it is not an efficient way to control or modify polarization.

The angles up to which you get total external reflection of x-rays is typically very small. e.g. for a Gold surface and 12.4 keV x-rays (1 Angstrom wave length), the cut-off angle is 0.351 deg.

http://www.x-ray-optics.de/index.php?option=com_content&view=article&id=52&Itemid=64&lang=en

The large angles close to 90deg you get by using Bragg reflections, choosing a crystal and Bragg reflection that have the correct d-spacing (or choosing the wave length to match the crystal). With this you can get excellent polarization purity.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110.254801

The obvious disadvantage of Bragg-based optics compared to mirrors is that Bragg optics only work for a very narrow energy band, whereas mirrors can work for a wide range of energies.

 

[sardar]

The phenomenon you are describing is known as Thomson scattering, where low energy X-rays (with a frequency close to that of the incident radiation) are scattered perpendicularly to the direction of the incident X-rays and become linearly polarized. This is due to the fact that the electrons in the material being scattered off are much lighter than the incident photons, so they are easily displaced by the electric field of the X-rays. This leads to a preferred direction of scattering, resulting in linear polarization.

However, as the energy of the X-rays increases and the scattering becomes dominated by the Compton effect, the polarization of the scattered X-rays becomes more complex. This is because the Compton effect involves the transfer of energy and momentum between the photons and the electrons, resulting in a broader range of scattering angles and polarizations. In general, the polarization of the scattered X-rays will depend on the energy of the incident radiation, the angle of scattering, and the properties of the material being scattered off.

So, to answer your question, the statement you read is true for low energy X-rays (with Rayleigh or Thomson scattering), but becomes more complicated at higher energies where the Compton effect is dominant. It is important to consider the specific conditions and mechanisms of scattering when discussing the polarization of X-rays.