Curie and becquerel — measuring how radioactive something is
1 becquerel = 1 decay/s. 1 curie = 37 billion decays/s — the activity of 1 gram of radium-226. The becquerel is SI; the curie is history.
The activity of a radioactive source is how many nuclei decay per second. The becquerel is the SI unit — trivially small. The curie is enormous. Most real sources sit in the megabecquerel-to-gigabecquerel range, or equivalently the microcurie-to-millicurie range. Neither unit produces friendly numbers alone, so both remain in use.
The exact factors
1 Bq = 1 decay/second (SI) > 1 Ci = 3.7 × 10¹⁰ Bq = 37 GBq (exact) > 1 mCi = 37 MBq > 1 µCi = 37 kBq > Natural K-40 in an adult: ~4,400 Bq (0.12 µCi) > Smoke detector Am-241: ~30 kBq (0.8 µCi)
Why 37 billion?
The curie was defined as the activity of one gram of radium-226 — historically, the most-studied radioisotope. When the numerical value was pinned down (3.7 × 10¹⁰ decays/s), it was frozen as a defined constant. The becquerel replaced it as the SI unit in 1975, but nuclear medicine, radiography and health physics kept the curie for continuity with decades of dose records.
Regional convention
US medical isotopes: mCi and µCi still dominate prescriptions and shipping labels. European medical isotopes: MBq. Radiography sources for industrial NDT: Ci (US), TBq (EU). All modern instruments display both, and safety data sheets list both.
Worked example — a thyroid I-131 treatment
A hyperthyroidism ablation dose is typically 5–15 mCi = 185–555 MBq of iodine-131. The patient's thyroid concentrates most of it within 24 hours. I-131 half-life is 8.02 days, so after 8 days the activity halves; after 40 days (5 half-lives) it is 3% of the original — one reason iodine ablation isolation is 3–7 days for hospital treatment.
Activity vs dose
Activity (Bq) tells you how many decays. Dose (Sv) tells you what those decays do to tissue. A 1 GBq source outside your body is much less harmful than a 1 kBq source inside your lung, because dose depends on the type of radiation, the exposure time and the tissue distance. Do not conflate them.
The five-second recap
1 Bq = 1 decay/s. 1 Ci = 37 GBq. Medical isotopes: mCi (US) or MBq (EU). Activity is not dose — Bq is not Sv.
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FAQ
How radioactive is a banana?+
About 15 Bq from natural K-40 — negligible dose (~0.1 µSv from eating one).
What is a 'critical mass'?+
The minimum mass of a fissile isotope needed for a self-sustaining chain reaction. About 52 kg of pure U-235 or 10 kg of Pu-239 for a bare sphere; less with a reflector.
How long does a radioisotope stay radioactive?+
Depends entirely on half-life. Technetium-99m: 6 hours. Iodine-131: 8 days. Cesium-137: 30 years. Plutonium-239: 24,100 years.
This article was written by The Turbo Unit Converter engineering desk (Nuclear medicine & health physics) and last reviewed on 2026-06-30 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.