A versatile X-ray phase retarder for lock-in XMCD measurements

Poldi, E.H.T.; Escanhoela Jr, C.A.; Fonseca Jr, J.; Eleotério, M.A.S.; dos Reis, R.D.; Lang, J.C.; Haskel, D.; Souza-Neto, N.M.

doi:10.1107/S1600577520009558

Figure 2
Top: instrumentation shown in Fig. 1

(top view, without the vacuum chamber) in situations that dispersive optics could make the beam pass or not through the QWP. In the case of this instrumentation, S ≫ d. So as the slit scans across the dispersive beam, there is a condition when the beam does not hit the QWP. This alignment limitation causes the short spectral energy bandwidth, requiring a relative slit translation to maintain the QWP centered at the beam through an entire energy scan. Middle: representation of the elements used to elaborate the correction for the QWP positioning. An X-ray beam with energy E selected by the slit at a distance S from focus, positioned at an X_S with respect to the translation axis X. The QWP ideal position is then X_d, at a distance d from the focal point. Then, equation (1)

is obtained using the relation for energy resolution in space: $[{{\Delta E}/{E}}]$ = $[{{\Delta L}/{L}}]$ , and substituting L = SX/M (because the energy bandwidth ΔL is selected at S), where X and M are variables in directions X and M. Bottom: DXAS beamline energy bandwidth (black line) at a fixed energy of 7243 eV, p = 9.75 m and q = 1.5 m; and theoretical calculation for the bandwidth available for the XMCD experiment (red line), as a function of the distance from the QWP (∼5 mm × 5 mm) to the slit. The blue line is the bandwidth calculation as a function of the QWP size, for the same energy of 7243 eV, at a fixed distance of 100 cm between the QWP and the slit.

JOURNAL OF
SYNCHROTRON
RADIATION

ISSN: 1600-5775

Volume 27| Part 5| September 2020| Pages 1240-1246

https://doi.org/10.1107/S1600577520009558

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