Electron ejected from a radioactive nucleus.
Sources:
Green Book, 2nd ed., p. 93 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1536 (https://doi.org/10.1351/pac198254081533)