Becquerel

"Bq" redirects here. For other uses, see BQ and Becquerel (disambiguation).

The becquerel (symbol Bq) (pronounced: 'be-kə-rel) is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s⁻¹. The becquerel is named for Henri Becquerel, who shared a Nobel Prize with Pierre and Marie Curie in 1903 for their work in discovering radioactivity.

In a fixed mass of radioactive material, the number of becquerels changes with time. Therefore, a sample radioactive decay rate is stated with a timestamp for short-lived isotopes, sometimes after adjustment to some specific date of interest (in the past or in the future). For example, one might quote a ten-day adjusted figure, that is, the amount of radioactivity that will still be present ten days in the future. This can de-emphasize short-lived isotopes. The average human body has 4400 becquerels from decaying potassium-40, which is a naturally-occurring isotope of potassium.

Expressing the number of decays per second rather than the mean time between decays avoids the confusing situation that a lower number represents more radioactivity.

Prefixes

As any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 10³ Bq), MBq (megabecquerel, 10⁶ Bq), GBq (gigabecquerel, 10⁹ Bq), TBq (terabecquerel, 10¹² Bq), and PBq (petabecquerel, 10¹⁵ Bq). For practical application, 1 Bq is a small unit; therefore, the prefixes are common. For example, natural potassium (⁴⁰K) in a typical human body produces 4,000 disintegrations per second, 4 kBq of activity. The nuclear explosion in Hiroshima (14 kt or 59 TJ) is estimated to have produced 8×10²⁴ Bq (8 YBq, 8 yottabecquerel).

Bq versus counts per second

When measuring radioactivity of a sample with a detector, a unit of "counts per second" (cps) or "counts per minute" (cpm) is often used. Some radiation detectors are calibrated in "disintegrations per second" or "decays per second." All of these units can be converted to the absolute activity of the sample in Bq if one applies a number of significant conversions that take into account the radiation background, the detector efficiency, the counting geometry, the sample size, and the self-absorption of the radiation by the sample.

Relationship to the curie

The curie (Ci) is an older, non-SI unit of radioactivity equal to the activity of 1 gram of radium-226.

The conversion factors are:

1 Ci = 3.7×10¹⁰ Bq

1 Ci = 37 GBq

1 μCi = 37,000 Bq

1 Bq = 2.70×10⁻¹¹ Ci

1 Bq = 2.70×10⁻⁵ μCi

1 GBq = 0.0270 Ci

Definition

1 Bq = 1 s⁻¹

The becquerel is named after Henri Becquerel. As with every SI unit named for a person, its symbol starts with an upper case letter (Bq), but when written in full, it follows the rules for capitalisation of a common noun; i.e., becquerel becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

Calculation of radioactivity

For a given mass $m$ (in grams) of an isotope with atomic mass $m_{a}$ (in g/mol) and a half-life of $t_{1/2}$ (in s), the amount of radioactivity can be calculated using:

radioactivity(in Bq) = ${\frac {m}{m_{a}}}N_{A}{\frac {\ln(2)}{t_{1/2}}}$

With $N_{A}$=6.022 141 79(30)×10²³ mol⁻¹ the Avogadro constant.

For instance, one kilogram of potassium contains 0.12 gram of ⁴⁰K (all other isotopes are stable) that has a $t_{1/2}$ of 1.248×10⁹years=39.38×10¹⁵ seconds, and has an atomic mass of 39.963 g/mol, so the radioactivity is 32 kBq.

See also

• Sievert (biological dose equivalent of radiation).
• Counts per minute
• Ionizing radiation units
• Curie (unit)
• Gray (unit)
• Ionizing radiation level examples - Example exposure scenarios
• Rad (unit)
• Rem (unit)
• Roentgen (unit)
• Rutherford (unit)
• Background radiation
• Relative Biological Effectiveness
• Radiation poisoning
• Orders of magnitude (radiation)
• Banana equivalent dose

External links

• Derived units on the International Bureau of Weights and Measures (BIPM) web site