What is the traditional unit of measurement of radioactivity?

Radioactivity is the spontaneous emission of radiation, such as alpha particles, beta particles and gamma rays, from the unstable nuclei of certain atoms. The activity of a radioactive sample is the number of nuclei that decay (disintegrate) per second.

The curie (Ci) is the traditional (non-SI) unit of radioactivity. One curie is defined as 3.7 × 10¹⁰ disintegrations per second. This value was chosen because it is roughly the activity of one gram of radium-226. The unit was named after Marie Curie and Pierre Curie, the pioneers of research on radioactivity, in 1910.

The SI unit of radioactivity is the becquerel (Bq), named after Henri Becquerel. One becquerel is one disintegration per second. So 1 Ci = 3.7 × 10¹⁰ Bq. The becquerel is a very small unit, so larger multiples such as megabecquerel (MBq) and gigabecquerel (GBq) are often used. Another old unit is the rutherford (Rd), equal to 10⁶ disintegrations per second.

Some history of radioactivity:

Henri Becquerel discovered radioactivity in 1896, when he found that uranium salts emitted rays that fogged photographic plates.

Marie and Pierre Curie discovered two new radioactive elements, polonium (named after Marie's homeland, Poland) and radium, in 1898. Marie Curie coined the term "radioactivity".

Becquerel and the Curies shared the Nobel Prize in Physics in 1903. Marie Curie also won the Nobel Prize in Chemistry in 1911, becoming the first person to win Nobel Prizes in two different sciences.

Other units related to radiation:

• Gray (Gy): SI unit of absorbed dose.
• Sievert (Sv): SI unit of equivalent dose, measuring biological effect.
• Roentgen (R): old unit of exposure to X-rays and gamma rays.
• A Geiger-Müller counter is used to detect and measure radiation.

Why not the other options?

• ✗Roentgen measures exposure to ionising radiation, not radioactivity itself.
• ✗Candela is the SI unit of luminous intensity.
• ✗Fermi is a unit of length (10⁻¹⁵ m), used for nuclear sizes.