Observation of nuclear diffraction from multilayers containing 57Fe

Abstract

The small resonance width Г0 of Mößbauer transitions allows monochromatization of synchrotron radiation down to a bandwidth of 10-7 - 10-9 eV by nuclear resonant diffraction. Up to now, the Mößbauer isotopes 57Fe ( E0 = 14.4 keV, Г0 = 4.7 • 10-9 eV) and 169Tm ( E0 = 8.4 keV, Г0 = 1.2•10-7 eV) have been used in this field [ 1-5 ]. In order to observe nuclear resonant diffraction, the competing contribution of electronic scattering with a bandpass of at least a few meV has to be strongly reduced. Precise determinations of hyperfine interactions can be made by observation of quantum beats in the collective decay of the nuclei excited by synchrotron pulses. Most of the experiments have been done with single crystals which have a pure nuclear reflection. In these cases the structure factor for the electronic reflection is zero because of the symmetry of the crystal. The structure factor for the nuclear reflection is nonzero, because subgroups of the Mößbauer nuclei are distinguished by a symmetry breaking hyperfine interaction or by isotopic arrangement [6]. In a layered crystal of 56Fe and 57Fe atoms obviously the electronic and nuclear lattice constant differ by a factor of two, so that a superstructure appears with a pure nuclear reflection. Such a crystal has not been grown up to now, since the preparational requirements are very hard to meet. However, an isotopic superstructure can easily be realized in a multilayer, which need not be a single crystal. Alternating layers containing 56Fe and 57Fe give rise to a multilayer Bragg peak of pure nuclear origin. Such a multilayer will be described here.