Becquerel (Bq), the SI unit of the activity of a nuclide in a radioactive source (the activity of an isotope); 1 Bq is the activity of a nuclide at which one decay event occurs in 1 s. Named in honour of the French physicist A. H. Becquerel. 1 Bq=2.703*10-11 curie=10-6 rutherford.