Design and performance of a γ-ray diffractometer at 0.12 Å

At the University of Missouri Research Reactor, a γ-ray diffractometer has been constructed which uses 0.12 Å radiation produced from the decay of ¹⁵³Sm. Use of this source rather than ¹⁹⁸Au (λ = 0.03 Å), which is used at several instruments in Europe, has important advantages which are discussed. Details of the design necessary to work with 1000 Ci (37 TBq) sources at this wavelength are presented. In addition, the results of several experiments intended to test the capability of the instrument for measuring extinction-free structure factors at low angles and for measuring to high sin θ/λ (~ 2 Å⁻¹) are reported. These results show that it will be relatively straightforward to make absolute structure-factor determinations for light elements and to determine electron density distributions more accurately than has been done in the past with X-ray diffraction. Furthermore, determination of anharmonic thermal motion and form factors will be possible. Crystal perfection studies on small samples have also been performed and can be done routinely.