Pendellösung intensity-beat measurements with 0.0392- and 0.0265-Åγradiation in silicon

The integrated reflecting power of the silicon (220) reflection has been measured with 316.5- and 468.06-keV γ radiation from ${}_{}{}^{192}{}_{}{}^{}\mathrm{Ir}$ in symmetrical Laue geometry on 〈001〉 floating-zone-grown single crystals of approximately 1 cm thickness and 10 cm diameter. By tilting the crystal around the scattering vector the effective sample thickness was increased in steps of about (1/20) of a Pendellösung length and typically five Pendellösung oscillations were observed. After an independent determination of the sample thickness the structure factor was deduced from fitting the expression for Pendellösung intensity beats from dynamical diffraction theory to the experimental data. Using the values of the Si lattice parameter and the Debye-Waller factor from an x-ray Pendellösung study by Aldred and Hart [Proc. R. Soc. London, Ser. A 332, 223 (1973); 332, 239 (1973)] we obtain $F_{220}$=69.16±0.03 for λ=0.0392 Å and 69.21±0.06 for λ=0.0265 Å, which is in excellent agreement with the x-ray value of 69.21±0.06. This result justifies the use of the Thomson cross section to describe the intrinsic interaction in Bragg diffraction experiments with short-wavelength γ radiation.