4. Activity and Dose (gray, becquerel, sievert)

1. Activity in Becquerel. 2. Specific Activity. 3. Radiation Dose. 3.1. Dose Units and their History. 4. Equivalent Dose. 5. Effective Equivalent Dose. 6. Other Dose Units. 7. Dose Measurements.

1. Activity in Becquerel

When an atom disintegrates, radiation is emitted. If the rate of disintegrations is large, a radioactive source is considered to have a high activity.

The unit for the activity of a radioactive source was named after Becquerel (abbreviated Bq) and is defined as:

1 Bq - 1 disintegration per sec.

In a number of countries, the old unit, the curie (abbreviated Ci and named after Marie Curie) is still used. The curie-unit was defined as the activity in one gram of radium. The number of disintegrations per second in one gram of ra­dium is 37 billion. The relation between the curie and the becquerel is given by:

1 Ci = 3.7 1010 Bq

The accepted practice is to give the activity of a radioactive source in becqu­erel. This is because Bq is the unit chosen for the system of international units (SI units). But one problem is that the numbers in becquerel are always very large. The opposite holds true when a source is given in curies. For example, when talking about radioactivity in food products, 3,700 Bq per kilogram of meat is a large number. The same activity given in Ci is a really small number, 0.0000001 curie per kilogram.

Most people are used to measuring the amount of a substance in kilograms or liters, not in becquerels or curies. Using the best balances in the world one can accurately measure down to about one microgram of substance (10^-6 gram). By using our knowledge of radioactive detection, amounts more than a million times smaller than this can be measured. Radioactive sources as small as 10 Bq to 100 Bq can be readily measured; this corresponds to only about 10^-l4 gram.

2. Specific Activity

Specific activity is the activity per mass or volume unit. For example, the radioactivity in meat is given as Bq/kg. For liquids the specific activity is given in Bq/l and for air and gases the activity is given as Bq/m³.

In the case of fallout from a nuclear test or accident, the activity on surfaces can be given either as Bq/m² or as Сі/km². Both are used to describe radioactive pollution. The conversion between them is:

1 Ci/km2 = 37,000 Bq/m2

A great deal of information must be considered to calculate radiation doses and risk factors associated with these specific activities. The information must include the specific activity along with the various types of isotopes, their energies, physical and biological half-lives and methods of entry into the body. After considering all of these factors and calculating the dose, a determination of medical risk can be calculated.