Becquerel, gray and sievert measure different quantities related to radiation. Using these units interchangeably leads to an incorrect comparison, like trying to express the speed of a vehicle in kilograms.
- Becquerel, how fast the radioactive source decays;
- Gray, how much energy a substance absorbs;
- sievertis used to express the dose associated with the biological effect in terms of radiation protection.
What does Becquerel measure?
Becquerel (Bq) is the unit of activity. One becquerel means one radioactive decay per second. Activity tells how fast nuclei turnover is occurring within the source; It does not directly indicate the dose a person has received.
Two sources with the same activity may emit different types of radiation, be located in different geometries, or be at different distances from the person. Therefore, the health effect cannot be deduced from the Becquerel value alone.
What does Gray measure?
Gray (Gy) is the unit of absorbed dose. One joule of energy absorbed by one kilogram of matter is defined as one gray. The substance here may be human tissue, an industrial product, or another material.
Gray describes physically absorbed energy. Additional considerations are required in radiation protection as the biological effects of different types of radiation may differ for the same absorbed energy.
What does Sievert measure?
Sievert (Sv) is the unit of protection quantities that takes into account the type of radiation and the relevant biological weightings. Concepts such as equivalent dose and effective dose are expressed in sieverts.
In daily and professional evaluations, the smaller subunits of the sievert, millisievert and microsievert, are frequently used. However, the meaning of a dose value should not be interpreted without specifying the duration and context of the measurement.
Why isn't there a single radiation unit?
There are three different questions in the radiation process:
- How much degradation occurs in the source?
- How much energy does the target substance absorb?
- How is this energy evaluated in terms of radiation protection?
Becquerel, gray and sievert answer different parts of these questions. When reading a measurement report, the size, unit, time interval and measurement conditions used should be examined together.
Common mistakes
- Automatically count high becquerel value as high personal dose
- Accepting gray and sievert as equal in all circumstances
- Confusing hourly dose rate with total dose
- Overlooking micro, milli, and base unit prefixes
Resources
- TENMAK, What is Radiation?: https://www.tenmak.gov.tr/nuken/ogrenci/bolum4_03.html
- IAEA, Radiation, People and the Environment: https://www.iaea.org/sites/default/files/radiation0204.pdf