Historical Remarks on “Gamma Resonance Spectroscopy” (Mössbauer-Effect)

Abstract

Gamma resonance spectroscopy, synonymous with recoilless nuclear resonance absorption of gamma radiation or with the Mössbauer effect, forms part of the general field of resonance fluorescence. In optical spectroscopy, this phenomenon was already well known at the beginning of this century, while in nuclear spectroscopy, the phenomenon remained unobserved for a very long time. It was already in the late twenties, when W. Kuhn [1] suggested the existence of nuclear resonance fluorescence and outlined the reasons for the existing complications, as shown in Fig.1. In both atomic and nuclear resonance fluorescence, emission and absorption is confined to identical atoms or nuclei. Re-emission by an excited level, which proceeds in all directions, is called resonance fluorescence. The excited state is characterized by a finite lifetime x and a corresponding natural level width Г =ħ/τ. The energy E₀= E_exc - E_g released in the transition is essentially given to the emitted photon, with a small fraction providing the kinetic energy of the recoiling nucleus, shifting the emission line to somewhat smaller energies. Likewise, absorption of a photon necessitates the procurement of the transition energy E₀ on top of the recoil energy of the absorbing nucleus, E_R causing a shift of the absorption line towards somewhat higher energies. The resulting energy shift 2E_R of the emission and absorption lines relative to each other rapidly exceeds the width of the lines in the case of gamma-transitions and this in spite of the fact that the lines are usually substantially broadened beyond their natural line widths by temperature broadening due to the Doppler effect.

A historical outline is given of the discovery of recoilless nuclear resonant absorption of gamma radiation, also called the Mössbauer effect.