Abstract
For a few years, x-ray polarimeters have been discussed and even used as a key method for the investigation of fundamental physical questions, from quantum electrodynamics to solid state physics. However, the sensitivity of optical instruments is limited. In the case of x-ray polarimeters, this limitation is connected with the polarization purity. This article quantifies two fundamental effects which lead to a limited polarization purity and, thus, to a limited sensitivity: the divergence of the source and multiple-wave diffraction inside the polarizer crystals. A comparison shows that the current best polarization purities realized in the x-ray range are limited by these effects. The quantitative knowledge of their influence, however, can improve the purity by two orders of magnitude in future polarimetric experiments.
Highlights
Besides intensity, wavelength, and phase, the vector characteristic of an electromagnetic field is an important property in various scientific fields
A measure for this purity is the polarization purity δ which is defined as the intensity ratio of two orthogonal polarization components σ and π, both integrated over a certain angular and wavelength range in order to take into account the spectrum and the divergence of the beam, Iπ(λ, Θ) dλ dΘ
The use of multiple consecutive reflections at a Bragg angle of 45◦—the angle corresponding to Brewster’s angle in the visible range—inside the so-called channel-cut crystals led to polarization purities of the order of 10−10.10 Such exceptional degrees of linear polarization allow the detection of extremely small polarization changes
Summary
Wavelength, and phase, the vector characteristic of an electromagnetic field is an important property in various scientific fields. Beams that deviate from a Bragg angle of 45◦ are a trivial limitation for the polarization purity since they have a non-vanishing polarization component lying in the plane of diffraction.
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