IONIUM

Set to 2 different clues across 2 published puzzles, most often “Radioactive isotope” — The New York Times, Friday, 17 January 2003.

6 letters

Every clue on record, most used first

Radioactive isotopeonce
The New York Times — Friday, 17 January 2003
Radioactive isotope having mass 230once
The New York Times — Friday, 14 July 2000