The distance into the sample measured from the physical surface from which all but
of the particles or radiation detected have originated.
PAC, 1979, 51, 2243
(General aspects of trace analytical methods - IV. Recommendations for nomenclature,
standard procedures and reporting of experimental data for surface analysis techniques)
on page 2247
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook