General aspects of nuclear hyperfine interactions in salts of the rare earths

Most of the experimental studies of recoilless nuclear resonance absorption performed until now have been using the 14.4-keV transition in Fe57. The low energy of this transition, the long lifetime of the relevant nuclear level, the long lifetime of the parent isotope, and, last but not least, the vast number of problems which could be tackled, made Fe57 the favorite isotope in the entire field covered by this conference. In increasing numbers, however, experimental and theoretical studies are progressing which involve nuclear isotopes other than Fe57