614 Acta Cryst. (1996). A52, 614-620 X-ray Back-Diffraction Profiles with an Si (111) Plate C. CUSATIS,"* D. UDRON, ~ I. MAZZARO, '~ C. GILESb]" AND H. TOLENTINO' ~Departamento de Ffsica, Universidade Federal do Paran6, CP 19091, 81531-990 Curitiba (PR), Brasil, hEuropean Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble CEDEX, France, and CLaboratrrio Nacional de Luz S#ncrotron (LNLS) CNPq, CP 6192, 13081-970 Campinas (SP), Brasil. E-mail: cusatis@fisica.ufpr.br (Received 8 September 1995; accepted 15 Januao' 1996) Abstract A two- and, alternatively, a four-crystal monochroma- tor were used for simultaneous measurements of the profiles backward (h) and forward (o) diffracted by a thin Si (111) crystal plate for diffraction angles up to exactly 90 ° at DCI-LURE (Orsay). It is shown that the set-up with a four-crystal monochromator allows the characterization of the back-diffraction region for any crystal plate reflection. Asymmetry and full width at half-maximum (FWHM) of the experimental backward- diffraction profiles are analyzed. Possible simultaneous diffractions occurring near 90 ° incidence, giving extra peaks in the forward-diffracted profiles, are studied. The good contrast of the o-beam profiles suggests that the back-diffracted o beam could be used as a highly monochromatic beam. I. Introduction The X-ray near-back-diffraction regime is especially interesting for X-ray optics: it combines very large angular acceptance (the Darwin width can be orders of magnitude bigger than the usual values when the Bragg angle approaches 90 °) with high energy resolution (Kohra & Matsushita, 1972; B~mmer, Hrche & Nieber, 1979; Caticha & Caticha-Ellis, 1982; Graeff & Materlik, 1982; Hashizume & Nakahata, 1988). In this diffraction regime, the energy resolution can be as good as the intrinsic energy resolution and the goniometry is much less sensitive than for usual Bragg angles. These special characteristics make the back-diffraction regime an ex- cellent candidate for high-performance optical elements with respect to energy resolution and .intensity. X-ray back diffraction can be advantageously applied to high-resolution inelastic scattering (Dorner, Burkel & Peisl, 1986; Schtflke, 1989; Hofmann, Kalus & Schmelzer, 1992), standing waves in non-perfect crystals (Woodruff et al., 1987) and high-resolution or spin- dependent X-ray absorption spectroscopy (H~im~il~iinen, Siddons, Hastings & Berman, 1991; H~im~il~iinen et al., 1992). X-ray back diffraction was suggested for the construction of X-ray Fabry-Perot interferometers (Steyerl & Steinhauser, 1979) and cavities for X-ray t Present address: LNLS, CP 6192, 13081-970 Campinas (SP), Brasil. lasers (Denne, 1978). Nevertheless, to the best of our knowledge, few experimental studies of this diffraction regime exist until now (Graeff & Materlik, 1982; Kushnir & Suvorov, 1986; Stetsko, Kshevetskii & Mikhailyuk, 1988; Kondrashkina, Novikov & Stepanov, 1989; Kushnir & Suvorov, 1989; Nakahata, Hashizume, Oshima & Kawamura, 1989; Giles, 1991; Giles & Cusatis, 1991; Stepanov, Kondrashkina & Novikov, 1991; Giles & Cusatis, 1992), probably due to the difficulties inherent in back-diffraction geometry. An experimental set-up suitable for measurement of back-diffraction profiles using a two- and, alternatively, a four-crystal monochromator, is presented in the first section of this paper. This set-up allows simultaneous measurement of the backward-diffracted and forward- diffracted profiles for a thin crystal at diffraction angles up to exactly 90 °. First measurements made with a silicon (111) plate are presented and discussed in the second section. 2. Experiment Performing back-diffraction experiments is somewhat difficult since the backward-diffracted h beam (and, for sufficiently thin samples, the forward-diffracted o beam), which propagates very close to or in the same direction as the incident beam, has to be detected. In general, the back-diffracted h beam is detected at long distances from the crystal in order to obtain spatial separation from the incident beam. The major inconvenience is that it requires long X-ray-source-to-sample distances (Graeff & Materlik, 1982; Dorner, Burkel & Peisl, 1986; Hofmann, Kalus & Schmelzer, 1992; Sette, 1993). Moreover, such a set-up does not allow detection at exactly 90 ° . Diffraction profiles at angles closer to 90 ° can be obtained, with set-ups of a largely reduced dimension, if a first-crystal reflection is used between the X-ray source and the sample (Kushnir & Suvorov, 1986; Stetsko, Kshevetskii & Mikhailyuk, 1988; Giles & Cusatis, 1992). Another way to detect the back-diffracted h-beam profiles is to simultaneously measure the incident and back-diffracted beams, both going through the same detector but, naturally, in opposite directions, with a detector interfering weakly with the incident beam, en- @ 1996 International Union of Crystallography Printed in Great Britain - all rights reserved Acta Co,stallographica Section A ISSN 0108-7673 © 1996