X-RAY PHASE PLATE TO CHARACTERIZE THE POLARISATION OF AN HELICAL UNDULATOR SOURCE AT 2.8 keV. C. Malgrange1, J. Goulon2, C. Giles2,3, C. Neumann2, A. Rogalev2, E. Moguiline2, F. de Bergevin4, 1Laboratoire de Minéralogie-Cristallographie, 4 place Jussieu, 75252 Paris cedex 05, France, 2ESRF, BP 220, 38043 Grenoble cedex, France, 3LNLS, C.P. 6192, 13081, Campinas, SP, Brazil, 4Laboratoire de Cristallographie, BP 166, 38042 Grenoble cedex, France
A quarter-wave plate made of a ca. 16um thick silicon single crystal was used at an energy as low as 2.8 keV to convert circularly polarised photons into linearly polarised ones.
The principle of x-ray phase plate based on the use of the forward-diffracted beam outside the reflexion domain is now well-established[1,2,3]. The originality of the approach presented here stems from the following points:
-the quarter-wave plate was inserted upstream with respect to the monochromator.
-the double crystal monochromator which is equipped with a pair of Si 111 crystals is operated at 2.8 KeV where the Bragg angle is 45deg.. The monochromator acts then also as a linear analyser. This experimental configuration makes it possible to analyse the polarisation state of the undulator beam without any alteration by the monochromator.
-instead of analysing the linear polarisation of the beam after the quarter-wave plate (as most frequently done), we have analysed the change of the electric field component normal to the diffraction plane of the Si 111 monochoromator (which is kept fixed) on scanning the angular offset of the quarter-wave plate. In other terms, what we scan is the phase-shift induced by the phase plate for a fixed orientation of the linear analyser.
-at such a low energy, the operation of the phase plate becomes more tricky due to the high absorption coefficient.
The result of the measurement is a circular polarisation rate P3 such that 0.95< P3 <1 in good agreement with theoretical predictions.
[1] Dmitrienko V. E., and Belyakov V. A. (1980), Sov. Techn. Phys. Lett., 6, 621-622.
[2] Hirano K., Izumi K., Ishikawa T., Annaka S. and Kikuta S., (1991), Jpn. J. Appl. Phys. Lett., 30, L407-L410.
[3] Giles C., Malgrange C., Goulon J., De Bergevin F., Vettier C., Dartyge E., Fontaine A., Giorgetti C., and Pizzini S. (1994), J.Appl. Cryst., 27, 232-240.