specific activity, a

in radiochemistry
For a specified isotope, or mixture of isotopes, the activity of a material divided by the mass of the material.
PAC, 1994, 66, 2513 (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2515
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 993