isomer shift

in Mössbauer spectroscopy
https://doi.org/10.1351/goldbook.I03296
Measure of the energy difference between the source (\(E_{\text{s}}\)) and the @[email protected] (\(E_{\text{a}}\)) transition. The measured Doppler velocity shift, \(\delta \), is related to the energy difference by \[E_{\text{a}}- E_{\text{s}}=\frac{\delta \ E_{\gamma }}{c}\] where \(E_{\gamma }\) is the Mössbauer gamma energy and \(c\) is the speed of light in vacuum.
Source:
PAC, 1976, 45, 211. (Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data) on page 214 [Terms] [Paper]