Radioactive Decay

Radioactive materials decay at exponential rates unique to each radioisotope. The half-life of a radioisotope is the time that it takes for one half of the atoms of that substance to disintegrate into another nuclear form. These can range from mere fractions of a second, to many billions of years. In addition, the half-life of a particular radionuclide is unique to that radionuclide, meaning that knowledge of the half-life leads to the identity of the radionuclide. The following is a listing of the half-lives of radionuclides that are commonly used at KUMC, with the units of each as shown.

Radioisotope

Half-Life

Hydrogen-3

12.3 years

Carbon-14

5730 years

Phosphorus-32

14.3 days

Phosphorus-33

25.3 days

Sulfur-35

87.6 days

Calcium-45

162.7 days

Iron-59

44.5 days

Rubidium-86

18.7 days

Iodine-125

60.1 days

Cesium-137

30.1 years

The amount of radioactive material remaining after a particular amount of decay time can be accurately calculated using the following equation:

Activity = Initial Activity x exp (-.693 x Elapsed time / Half-life)

Note:  The Elapsed Time and the half-life must be in the same units.  There are many websites including this one that will do this calculation for you -- just search for "decay calculator".

***Please contact the Radiation Safety Officer if you would like to learn to do this calculation using your calculator.***

Last modified: Jul 10, 2014
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