Milli-electronvolt monochromatization of hard X-rays with a sapphire backscattering monochromator

A sapphire backscattering monochromator with 1.1 (1) meV bandwidth for hard X-rays (20-40 keV) is reported. The optical quality of several sapphire crystals has been studied and the best crystal was chosen to work as the monochromator. The small energy bandwidth has been obtained by decreasing the crystal volume impinged upon by the beam and by choosing the crystal part with the best quality. The monochromator was tested at the energies of the nuclear resonances of Sb-121 at 37.13 keV, Te-125 at 35.49 keV, Sn-119 at 23.88 keV, Sm-149 at 22.50 keV and Eu-151 at 21.54 keV. For each energy, specific reflections with sapphire temperatures in the 150-300 K region were chosen. Applications to nuclear inelastic scattering with these isotopes are demonstrated.