Periodic table of the elements. Marked in red are the elements (more than 40) for which the Mössbauer effect has been observed. The most prominent “Mössbauer nuclide” is ⁵⁷Fe. More than 90 % of the nearby 50 000 publications which have appeared so far refer to ⁵⁷Fe isotope. Nuclides suitable for Mössbauer spectroscopy should possess excited nuclear states with lifetimes in the range of ca. 10^-6 to ca. 10^-11 s, and transition energies between ca. 5 and 180 keV. Longer (shorter) lifetimes than indicated lead, according to the Heisenberg Uncertainty Principle, to too narrow (broad) emission and absorption lines, which no longer effectively overlap. Transition energies beyond ca. 180 keV cause too large recoil effects which destroy the resonance (see next figure). Gamma quanta with energies smaller than ca. 5 keV will mostly be absorbed in the source and absorber material.