Radioactive decay#Radioactive decay rates

The activity or rate of decay of a radioactive substance quantity is the number of nuclear disintegrations per unit time. The SI unit of activity is the becquerel (Bq). 1 Bq corresponds to one nuclear disintegration per second. An obsolete unit is the Curie ( Ci). The following applies: 1 Ci = 3.7 x 1010 Bq. Usual formula signs of activity, for example, A or R.

The ratio of the activity to the mass of the sample is called specific activity. The SI unit of specific activity is therefore Bq / kg. In the specific activity must always be specified, in which mass is related: to the mass

• The pure radionuclide,
• The chemical element including the remaining isotopes,
• The chemical compound
• Or of the entire sample, which may be a mixture of substances.

In nuclear medicine, the activity of a specimen is measured prior to its use in a activimeter.

Activity and decay constant

Each radionuclide has a decay constant (lambda), which describes the "speed" of the disintegration. And between the half-life, there is a simple relationship

Is the probability per unit time for the decay of individual atomic nucleus. Why can express the activity of a sample of N atoms at time t as

Multiplying the decay law

( Is the number of atoms at the time t = 0) and, following the type of the specimen at a given time

I.e. the type followed by the same exponential decay law, such as the number of radioactive atoms in the preparation.

Calculation example

Polonium -210 has a half -life of 138 days. One mole of polonium -210 weighs 209.98 g and comprises approximately atoms ( Avogadro constant NA). One gram of polonium -210 therefore has an activity of

Since the amount of one gram were adopted, the specific activity ( activity per kilogram) in this case has the value: 1.7 × 1017 Bq / kg.

Examples of specific activities

• Nuclear physics
• Physical size style