recoil-free fraction

in Mössbauer spectrometry
https://doi.org/10.1351/goldbook.R05201
The fraction of all Mössbauer gamma rays of the transition which are emitted or absorbed without significant recoil energy loss.
Source:
PAC, 1976, 45, 211. (Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data) on page 214 [Terms] [Paper]