See: effective thermal cross-section
Sources:
Orange Book, 2nd ed., p. 218 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2517 (https://doi.org/10.1351/pac199466122513)