becquerel
Radioactivityexact by definition
The becquerel is the SI unit of radioactivity, defined as one nuclear decay per second. It is dimensionally a reciprocal second but carries its own name to make clear that the events counted are spontaneous nuclear transformations. The definition is exact, the becquerel being a coherent derived SI unit.
Watch out: One becquerel is an extremely small activity. An ordinary adult human body is naturally about 4000 to 5000 Bq from potassium-40 and carbon-14 alone, so a bare becquerel count says nothing about hazard without also knowing which isotope, what radiation it emits, and how it reaches the body.
| 1 Bq | 1 Bq |
Named for Henri Becquerel, who discovered radioactivity in 1896 when uranium salts fogged a photographic plate in the dark. He shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie.
About the becquerel
Activity counts decays, not dose. One becquerel is one disintegration per second, and it tells you how busy a sample is without telling you what that means for anything nearby. The step from activity to biological risk needs three more pieces: the energy and type of radiation emitted, the fraction that actually reaches tissue, and the time over which exposure happens. Those are handled by the gray for absorbed dose and the sievert for equivalent dose, and a gigabecquerel of a shielded, short-range beta emitter can be far less consequential than a much smaller activity of an inhaled alpha emitter.
Activity falls off exponentially with the isotope's half-life: \(A = A_0 e^{-\lambda t}\) where \(\lambda = \ln 2 / t_{1/2}\). That is why medical isotopes are ordered by delivery time. Technetium-99m, the workhorse of nuclear imaging, has a six hour half-life, so a dose loses half its activity between morning and afternoon and is undetectable within a couple of days. At the other end, uranium-238 has a half-life of 4.5 billion years, which is exactly why it is still here and also why its specific activity is low enough to hold a sample in a gloved hand.