Measure of the energy difference between the source (\(E_{\text{s}}\)) and the @A00034@ (\(E_{\text{a}}\)) 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.