The becquerel (/ˌbɛkəˈrɛl/; symbol: Bq) is the unit of radioactivity in the International System of Units (SI). One becquerel is defined as an activity of one decay per second. For applications relating to human health this is a small quantity,^[1] and SI multiples of the unit are commonly used.^[2]

The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre and Marie Curie in 1903 for their work in discovering radioactivity.^[3]

1 Bq = 1 s⁻¹

A special name was introduced for the reciprocal second (s⁻¹) to represent radioactivity to avoid potentially dangerous mistakes with prefixes. For example, 1 μs⁻¹ would mean 10⁶ disintegrations per second: (10⁻⁶ s)⁻¹ = 10⁶ s⁻¹,^[4] whereas 1 μBq would mean 1 disintegration per 1 million seconds. Other names considered were hertz (Hz), a special name already in use for the reciprocal second, and fourier (Fr; after Joseph Fourier).^[4] The hertz is now only used for periodic phenomena.^[5] While 1 Hz is one cycle per second, 1 Bq is one event per second on average for aperiodic radioactive decays.

The gray (Gy) and the becquerel (Bq) were introduced in 1975.^[6] Between 1953 and 1975, absorbed dose was often measured in rads. Decay activity was measured in curies before 1946 and often in rutherfords between 1946^[7] and 1975.

As with every International System of Units (SI) unit named after a person, the first letter of its symbol is uppercase (Bq). However, when an SI unit is spelled out in English, it should always begin with a lowercase letter (becquerel)—except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.^[8]

Like any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 10³ Bq), MBq (megabecquerel, 10⁶ Bq, equivalent to 1 rutherford), GBq (gigabecquerel, 10⁹ Bq), TBq (terabecquerel, 10¹² Bq), and PBq (petabecquerel, 10¹⁵ Bq). Large prefixes are common for practical uses of the unit.

For practical applications, 1 Bq is a small unit. For example, there is roughly 0.017 g of potassium-40 in a typical human body, producing about 4,400 decays per second (Bq).^[9]

The activity of radioactive americium in a home smoke detector is about 37 kBq (1 μCi).^[10]

The global inventory of carbon-14 is estimated to be 8.5×10¹⁸ Bq (8.5 EBq, 8.5 exabecquerel).^[11]

These examples are useful for comparing the amount of activity of these radioactive materials, but should not be confused with the amount of exposure to ionizing radiation that these materials represent. The level of exposure and thus the absorbed dose received are what should be considered when assessing the effects of ionizing radiation on humans.

The becquerel succeeded the curie (Ci),^[12] an older, non-SI unit of radioactivity based on the activity of 1 gram of radium-226. The curie is defined as 3.7×10¹⁰ s⁻¹, or 37 GBq.^[4][13]

Conversion factors:

• 1 Ci = 3.7×10¹⁰ Bq = 37 GBq
• 1 μCi = 37,000 Bq = 37 kBq
• 1 Bq = 2.7×10⁻¹¹ Ci = 2.7×10⁻⁵ μCi
• 1 MBq = 0.027 mCi

The following table shows radiation quantities in SI and non-SI units. W_R (formerly 'Q' factor) is a factor that scales the biological effect for different types of radiation, relative to x-rays (e.g. 1 for beta radiation, 20 for alpha radiation, and a complicated function of energy for neutrons). In general, conversion between rates of emission, the density of radiation, the fraction absorbed, and the biological effects, requires knowledge of the geometry between source and target, the energy and the type of the radiation emitted, among other factors.^{[14][not specific enough to verify]}

• Background radiation
• Banana equivalent dose
• Counts per minute
• Ionizing radiation
• Orders of magnitude (radiation)
• Radiation poisoning
• Relative biological effectiveness
• Rem (unit)
• Rutherford (unit)
• Sievert (biological dose equivalent of radiation)