# Nuclear Technology

1. Jun 26, 2006

when discussing the the properties of a radioisotope, what is meant by its activity, and a biological half life
thanks

2. Jun 26, 2006

### Staff: Mentor

Activity refers to the rate of decay. Specific activity is activity per unit mass. In addition to the decay of a radionuclide, the element is mobile in the body, i.e. it can be taken in (absorbed) and excreted.

Activity, which is proportional to the number of atoms of radionuclide(s) present, decreases as the atoms (nuclei) decay and as the atoms (nuclei) are excreted from the body.

This may be helpful - http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/biohalf.html

3. Jun 26, 2006

### Hootenanny

Staff Emeritus
The activity of a radioisotope is defined as the rate of decay of the nuclei. If A be the activity and N is the number of nuclei then;

$$A = -\frac{dN}{dt}$$

The radioactive half life, as you probably know, is constant an independent of any physical factors. However, if a radio isotope is ingested or injected into a living organism, the radioactive half life alone cannot be used to estimate the exposure. For example, if the organism excretes the isotope, then it is no longer exposed to the radiation. The biological half life is basically the time taken for an organism to excrete half the nuclei of the radioactive isotopes; therefore, together with the radioactive half life the biological half life can be used to give an estimate of the effective half life or exposure to radiation;

$$\frac{1}{T_{\text{effective}}} = \frac{1}{T_{\text{radioactive}}} + \frac{1}{T_{\text{biological}}}$$

Hope this helps

4. Jun 26, 2006