Theoretical analysis of the background intensity distribution in X-ray Birefringence Imaging using synchrotron bending-magnet radiation

Abstract

In the recently developed technique of X-ray Birefringence Imaging, molecular orientational order in anisotropic materials is studied by exploiting the birefringence of linearly polarized X-rays with energy close to an absorption edge of an element in the material. In the experimental setup, a vertically deflecting high-resolution double-crystal monochromator is used upstream from the sample to select the appropriate photon energy, and a horizontally deflecting X-ray polarization analyzer, consisting of a perfect single crystal with a Bragg reflection at Bragg angle of approximately 45 , is placed downstream from the sample to measure the resulting rotation of the X-ray polarization. However, if the experiment is performed on a synchrotron bending-magnet beamline, then the elliptical polarization of the X-rays out of the electron orbit plane affects the shape of the output beam. Also, because the monochromator introduces a correlation between vertical position and photon energy to the X-ray beam, the polarization analyzer does not select the entire beam, but instead selects a diagonal stripe, the slope of which depends on the Bragg angles of the monochromator and the polarization analyzer. In the present work, the final background intensity distribution is calculated analytically because the phase space sampling methods normally used in ray traces are too inefficient for this setup. X-ray Birefringence Imaging data measured at the Diamond Light Source beamline B16 agree well with the theory developed here.

Spatially resolved mapping of phase transitions in liquid-crystalline materials by X-ray birefringence imaging

“…The spatial resolution in the horizontal direction is limited by the dynamical diffraction extinction depth of the polarization analyzer. The background intensity distribution in the XBI images is discussed elsewhere 31…”

Novel technique for spatially resolved imaging of molecular bond orientations using x-ray birefringence

“…The XBI data yielded many new structural insights about the materials under investigation, but the images are found to be superimposed on an instrumental background. The sources of this background are discussed at length by Sutter et al [6] and are summarized here.…”