Mossbauer

The Mössbauer effect is the recoil-free emission and absorption of gamma rays by atomic nuclei bound in a solid. Discovered in 1958 by Rudolf Mössbauer, it occurs when a nucleus at lattice sites emits or absorbs a gamma photon without transferring recoil momentum to the lattice, allowing resonant nuclear transitions to occur with extremely small energy shifts.

The underlying physics relies on the fact that in a crystal, the nucleus can couple to the whole lattice, reducing the recoil energy by the Lamb-Mössbauer factor, which describes the fraction emitted recoil-free. The factor depends on temperature, Debye temperature, and the gamma transition energy. Because the recoil is suppressed, the emitted and absorbed gamma rays present extremely narrow resonances, enabling high-resolution spectroscopy.

Mössbauer spectroscopy uses a source of a radioactive isotope that decays to the nucleus of interest and an absorber containing the same or another isotope. A gamma-ray source and absorber are mounted on a velocity transducer; scanning the relative energy by Doppler shifting the gamma ray yields a spectrum. Analysis reveals hyperfine interactions: isomer shift (chemical environment), quadrupole splitting (electric field gradient and site symmetry), and magnetic hyperfine splitting (magnetic field at the nucleus).

Common isotope: 57Fe via 57Co source; other isotopes used include 119Sn, 121Sb, 149Sm, 151Eu and 129Xe. Applications span metallurgy, chemistry, solid-state physics, geology, catalysis, and biology, where information about oxidation state, electronic structure, magnetism, and dynamics can be obtained.

History and significance: Mössbauer's discovery earned him the Nobel Prize in Physics in 1961. Since then, Mössbauer spectroscopy has become a standard tool in material characterization.