The Radioactivity Unit Converter translates between SI Becquerel (Bq, 1 disintegration per second — the modern standard), the older Curie (Ci, defined as the activity of 1 gram of radium-226 = 3.7 × 10¹⁰ Bq exactly), the obsolete Rutherford (Rd, 10⁶ Bq), and Gigabecquerel (GBq, 10⁹ Bq, commonly used in nuclear medicine). These are units of activity — the rate at which a radioactive sample decays — not dose (which uses Sievert/Sv) or absorbed energy (Gray/Gy).
The Curie was the original radioactivity unit, established by Marie Curie and named in honor of her and Pierre. The value 3.7 × 10¹⁰ Bq was chosen because that's the activity of 1 gram of radium-226 — Marie Curie's signature element. The SI Becquerel (named after Henri Becquerel, discoverer of radioactivity) replaced the Curie as the official scientific unit in 1975 but Curie persists in medical and industrial use. 1 Ci = 37 GBq.
Typical activity ranges: natural human body contains ~6,000 Bq from internal potassium-40 (about 10 disintegrations per second per gram of tissue from natural sources). Smoke detector americium-241 source: ~30,000 Bq. Medical iodine-131 thyroid treatment: ~5 GBq (5 × 10⁹ Bq). Cobalt-60 cancer therapy source: ~50 TBq (50 × 10¹² Bq). Chernobyl reactor at meltdown: ~10²⁰ Bq. The activity scale spans roughly 20 orders of magnitude — Becquerel works well for small biological samples while Curie/GBq are more practical for medical and industrial sources.
What activity tells you: half-life × natural log of 2 × original sample atoms = total decay events that will eventually occur. Activity at any moment tells you the current decay rate. Short half-life isotopes (technetium-99m, used in medical imaging, 6 hour half-life) have very high activity briefly. Long half-life isotopes (uranium-238, 4.5 billion years) have very low activity per gram. Both can be hazardous — high-activity sources need careful shielding; long-half-life isotopes persist in environment for geological timescales.
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