The SI unit for radiation is the Bq (Becquerel) and equals the number of atoms inside a source that decay per second. Often, source strength is given in Bq per mass unit (Bq/kg). For natural radiation, other units like ppm, % or even pCi/g are used. One should be careful translating from Bq/kg to ppm's. Often people overlook whether the conversion was done assuming pure radionuclides or oxides.
Conversion from Bq/kg to ppm and %
The following table lists the conversion between Bq/kg to ppms
|Nuclide||To %/ppm||To %/ppm oxides|
|40-K||1 Bq/kg = 1/316%||1 Bq/kg = 1/262% K2O|
|238-U||1 Bq/kg = 1/12.3 ppm||1 Bq/kg = 1/10.4 ppm U3O8|
|232-Th||1 Bq/kg = 1/4.1 ppm|
1 Bq/kg = 1/3.55 ppm ThO2
For conversion between becquerel (Bq) and the (much older) curie (Ci) the following holds:
1 Bq = 27 pCi = 2.7 x 10-11 Ci
1 pCi = 0.037 Bq
Interestingly, the Bq has the same SI unit (s-1) as the Hertz (Hz). However, the difference being that 1 Hz describes a truly periodic phenomenon whereas the Bq is purely stochastic.
To gray or sievert
There is a difference between radioactivity of a radioactive source and the radiation dose which may result from this source. The radiation dose depends on the following factors:
- Activity of the source, in units Bq or Bq/kg;
- Type of radiation;
Converting becquerel (Bq) or becquerel per kilogram (Bq/kg) to gray (Gy) or sievert (Sv) is therefore not straightforward and often not possible. Becquerel is a unit for radiation, while gray and sievert are both units for absorbed radiation dose. The gray is a physical quantity, where 1 Gy is the deposit of 1 joule of radiation energy in 1 kg of matter or tissue. The sievert represents the equivalent biological effect of the deposit of 1 joule of radiation energy in 1 kilogram of human tissue.