femtobecquerel
An SI unit of radioactive activity equal to 10⁻¹⁵ becquerels. Symbol: f
femtobecquerel: one quadrillionth of a decay per second
A femtobecquerel (fBq) is a unit of radioactive activity in the SI system, equal to 10-15 becquerels or one decay event per quadrillion seconds. It represents an extraordinarily small rate of nuclear disintegration, used almost exclusively in high-precision radiation metrology where background levels or trace contamination must be quantified with extreme sensitivity.
The becquerel itself defines activity as decays per second; a femtobecquerel is therefore one decay per roughly 32 million years. To reach this scale, laboratories typically employ gamma spectrometry, alpha counters, or mass spectrometry coupled to accelerators. At these levels, measurement uncertainty often exceeds the signal itself, so femtobecquerel activity is rarely stated as a direct result but rather as a detection limit or background correction.
Femtobecquerel measurements appear most often in environmental and health physics work: long-lived isotopes in drinking water analysis, ultra-trace plutonium or uranium in soil samples, or radiostrontium in food chains. The SI prefix femto (10-15) is uniform across all SI units, so a femtobecquerel obeys the same scaling rules as a femtometer or femtosecond, making it logically consistent within the standard but practically extreme.
Confusion arises because the older unit, the curie, does not scale evenly with SI prefixes. A femtocurie equals 0.037 becquerels (a much larger amount), so direct conversion is essential when reading older health physics literature. Regulatory limits for radioactive substances in the United States are often stated in picocuries per liter, which sits roughly 1000 times above the femtobecquerel scale.
In practice, claiming a measurement at the femtobecquerel level requires rigorous blank controls, detector calibration with certified sources, and peer-reviewed methodology. Commercial instruments rarely reach this sensitivity without specialized preparation and extended counting times. The femtobecquerel exists in standards and regulatory frameworks more as a theoretical floor than a routine measurement outcome.