Definition

The becquerel (symbol Bq) is the International System of Units (SI) derived unit for radioactivity. It quantifies the activity of a radioactive sample by counting the number of spontaneous nuclear decays occurring per unit time: one becquerel equals one decay per second. The concept is fundamentally a rate; dimensionally it is expressed as inverse seconds. For the formal SI context see SI derived units and note the use of the second as the base time unit.

Physical basis and mathematical form

Activity measures how rapidly unstable nuclei transform. If N is the number of unstable nuclei and λ is the decay constant, the rate of change follows the exponential decay relation -dN/dt = λ N. The activity A is A = λ N and has units of s−1, reported in becquerels. The term "decay" refers to individual nuclear transformations that emit particles or radiation from the nucleus; for background on the microscopic origin see nucleus.

Multiples and comparisons

Practical radioactivity values span many orders of magnitude, so SI prefixes are used with the becquerel: kilobecquerel (kBq), megabecquerel (MBq), gigabecquerel (GBq), terabecquerel (TBq) and petabecquerel (PBq). Older literature often uses the curie (Ci), a non-SI unit much larger than the becquerel and primarily retained for historical or regulatory contexts. Typical sources range from small samples measured in becquerels to medical or industrial sources in the MBq–GBq range.

Uses, measurement and examples

The becquerel is used wherever the activity of radioactive materials must be reported: medical diagnostics and therapy, laboratory research, nuclear industry inventories, environmental monitoring, and regulatory limits. Instruments that detect and quantify activity include Geiger–Müller counters, scintillation detectors, semiconductor detectors and ionization chambers; each instrument type responds differently to particle type and energy and requires calibration to translate counts into becquerels. Reporting activity alone does not fully characterize hazard; it is one step in assessing radiological impact.

Distinctions and interpretation

It is important to distinguish activity from absorbed dose and biological effect. The becquerel measures the frequency of nuclear transformations (a physical rate), not the energy deposited in matter. For health, radiation protection uses units such as the gray (energy absorbed per mass) and the sievert (dose weighted by biological effect). When communicating risk or regulatory limits, activity in becquerels is combined with knowledge of radiation types, energies, exposure pathways, and time to estimate dose and potential harm. For broader discussion on radioactivity as a phenomenon see radioactivity.

History and naming

The unit is named in honor of Henri Becquerel, the French physicist who discovered natural radioactivity in the late 19th century. His experiments, together with subsequent work by Marie and Pierre Curie, established radioactivity as a new physical phenomenon. The naming recognizes Becquerel's foundational role; further biographical and historical material is available at resources about Henri Becquerel.

  • Key point: 1 Bq = 1 nuclear decay per second (rate, not energy).
  • Useful to know: Activity values are best interpreted alongside dose metrics when assessing health effects.