Radiation Units Explained — Becquerel, Gray, Sievert, Curie

Radiation measurement has a confusing zoo of units, partly because older CGS units (rad, rem, Curie) coexist with newer SI units (Gray, Sievert, Becquerel). Here's what each actually measures.

Activity: how fast something decays

Activity measures disintegrations per second. The Becquerel (Bq) equals 1 decay/second. The Curie (Ci) was originally the activity of 1 gram of Radium-226: 3.7 × 10¹⁰ decays/second. So 1 Ci = 37 billion Bq. A smoke detector has about 37,000 Bq of Americium-241. Our decay calculator converts between these.

Absorbed dose: energy deposited

Absorbed dose measures energy deposited per kilogram: 1 Gray (Gy) = 1 J/kg. The older rad equals 0.01 Gy. A chest X-ray: about 0.0001 Gy. A lethal whole-body dose: around 4-5 Gy.

Equivalent dose: biological damage

Alpha particles cause 20x more biological damage per Gray than gamma rays. The equivalent dose accounts for this: H = D × w_R, in Sieverts (Sv). For gammas, 1 Gy = 1 Sv. For alphas, 1 Gy = 20 Sv. The older rem equals 0.01 Sv.

Effective dose: organ sensitivity

Different organs have different sensitivity. The thyroid and bone marrow are more radiation-sensitive than skin. Annual public limit: 1 mSv. Radiation workers: 20 mSv/year averaged over 5 years. Our dose and shielding calculator estimates dose rates and required shielding.