Distance over which the transmitted X-ray intensity falls by
\(1/e\), where
\(e\) is the base of the natural logarithm.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1846 (https://doi.org/10.1515/pac-2019-0404)