Mössbauer thickness

in Mössbauer spectroscopy
https://doi.org/10.1351/goldbook.M04034
The effective thickness of a source (t s) or absorber (t a) in the optical path.
Source:
PAC, 1976, 45, 211. 'Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data' on page 214 (https://doi.org/10.1351/pac197645030211)