Measuring radioactivity-Becquerel, gray and sievert

Measuring radioactivity is not that easy. The units of bequerel, gray and sievert measure activity, absorbed dose and dose equivalent but the meaning of each and how they relate is often confusing.

Taking a precise numerical measurement of the radioactivity of a source is difficult and understanding the various units of measurement is far from straightforward. However, after that downbeat start, let’s have a try. The first is fairly straightforward.

The becquerel Bq

is a SI unit used to measure the activity of a radioactive source.

The formal definition is that the activity is one Becquerel when one nucleus decays per second. For most purposes this is a very low activity and multiple units are usually used.

The gray (symbol: Gy)

is a unit in the International System of Units (SI). It is a derived unit used to measure the dose of radiation. One gray is defined as the absorption of one joule of radiation energy per kilogram of matter.

The measurement of absorbed dose is difficult. Some radiation to which the body is exposed is not absorbed and some scattered. The average radiation dose from an abdominal X-ray is 0.7 milligrays (that is 0.0007 Gy)

A whole-body exposure to 5 grays or more, tiny in terms of energy, (5 joules of energy per kilogram would raise the temperature by about 1/1000 of a degree) of high-energy radiation is likely to cause death within a couple of weeks. Cheerful eh!

The sievert (Sv)

is a SI unit and is a measure of the health effect of low levels of ionising radiation on the human body. It aims to quantify the effect of a low value dose on the risk of disease.

It is directly related to the gray by a factor Q which varies dependant on the type of radiation and the part of the body effected.

Sv = Gy x Q

We use the equivalent dose to consider the danger of local exposure to different radiations on different parts of the body.

For instance, 1 Gy (1J/kg) of alpha is 20 times more dangerous biologically than 1Gy of gamma.

So 1Gy of alpha absorbed locally in the ovaries is likely to carry a biological risk of 20 x 20 that is 400 times greater than the same dose of gamma to a patch of skin.

The video below explains the definitions of the units and the methods and how they are used.