Double multilayer monochromator

As an alternative to the high energy resolution delivered by the silicon crystals, a double multilayer monochromator,1 also diffracting in the vertical plane and located at 28 m from the source, can deliver a high-flux low-resolution beam. It is composed of two 300 mm long Si mirror substrates having a figure error of 0.2 mrad and a surface roughness of 0.1 nm rms. The central part of each mirror is coated with [Ru/B4C]70 multilayers to select an energy band ΔE/E is 2.10-2 between 6 keV and 30 keV. A d-spacing of 4 nm was chosen for the phase distortion of the wavefront, caused by the mirror shape errors, to be negligible.2 Talbot measurements performed at 17.5 keV with the multilayer monochromator alone show that the coatings do not degrade the surface quality and the effective source size is only slightly increased (84 µm against 80 µm). At an energy of 8 keV and at 37 m from the source the flux was measured to be ~ 4.1011 ph/s/mm2 per mA of ring current, corresponding to a gain of 86 as compared to the case of the Si (111) crystal monochromator. The estimated harmonics rejection rate is 1.8 10-4. The multilayer mirrors are placed in a vacuum chamber (10-8 hPa) and the first mirror is water-cooled by side contact. The altitude and rotation angle of each mirror are generated by two independent translations connected on the ends of the mirror holder and are performed with resolutions of 0.47 µm and 4.7 µrad, respectively. The doubly reflected beam is found below the white beam at a distance that depends on the energy: 5 mm at 22 keV, 14 mm at 8 keV. This was necessary to obtain a compact design. The double-reflection multilayer system can also be mounted in series with the double-crystal monochromators to deliver a high-resolution fixed-exit beam, with a reduction of the harmonics down to 7 orders of magnitude, which is much better than with a simple crystal detuning. Silicon PIN diodes, 300-µm and 10-µm thick, mounted at the end of the vacuum chamber are used to align the beam or to feedback loop the beam stabilization system.

 

Each monochromator can be inserted in the beam (or extracted) without breaking the vacuum. When none are used, white beam can be propagated.

 

References:      

1.          T. Bigault, E. Ziegler, C. Morawe, R. Hustache, J.Y. Massonnat, G. Rostaing, "Double multilayer monochromator to tailor bending magnet radiation spectrum", Crystals, Multilayers, and Other Synchrotron Optics, T. Ishikawa, A. T. Macrander, J. Wood, Eds, SPIE Proc. vol. 5195, pp. 12, San Diego, USA, 2003.

2.          E. Ziegler, C. Morawe, O. Hignette, P. Cloetens, R. Tucoulou, "Multilayer X-ray optics for high energy third-generation synchrotron sources: the state of the art", Ninth International Conference on Production Engineering, Precision Science and Technology for Perfect Surfaces, Y. Furukawa, Y. Mori, T. Kataoka, Eds, Japan Society for Precision Engineering Proc. vol. 3, pp. 285-91, Osaka, Japan, 1999.