Becquerel

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.

SI uses the becquerel rather than the second for the unit of activity measure to avoid dangerous mistakes: a measurement in becquerels is proportional to activity, and thus a more dangerous source of radiation gives a higher reading. A measurement in seconds is inversely proportional.

Contents

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.[1] The nuclear explosion in Hiroshima (14 kt or 59 TJ) is estimated to have produced 8×1024 Bq (8 YBq, 8 yottabecquerel).[2]

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

This SI unit is named after Henri Becquerel. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Bq). When an SI unit is spelled out in English, it should always begin with a lower case letter (becquerel), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

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=39.38388×1015 seconds, and has an atomic mass of 39.96399848 g/mol, so the radioactivity is 31.825 kBq.

See also

References

  1. ^ Radioactive human body — Harvard University Natural Science Lecture Demonstrations
  2. ^ Michael J. Kennish, Pollution Impacts on Marine Biotic Communities , CRC Press, 1998, p. 74. ISBN 9780849384288.

External links

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Accepted for use
with SI
See also
Book:International System of Units  · Category:SI base units