Cooknell Electronics Ltd, Dorset - Radioactive Half Life Demonstrator

Using a Gas Mantle to demonstrate half-life.

The national curriculum double science key stage 4 programme of study includes radioactivity and the meaning of the term half-life. The concept of half-life is also required in most A2 level physics courses, including its determination from a decay curve.

In the 1960s, school science apparatus was available that used radon gas generated from a fine thorium compound powder in a bottle. This apparatus is no longer available, and some employers have prohibited its use owing to concerns about the radiological risks. The CP3-IC system uses gas mantles to generate radon so it does not present a risk of thorium powder release, and the quantities of radon generated are much less than in the 1960’s design.

The CP3-IC is a development by Cooknell Electronics Ltd, of a prototype ionisation chamber built by Ralph Whitcher to investigate the use of thoriated gas mantles for demonstrating and measuring the half life of radon-220 in school science. Ralph Whitcher is a Health and Safety Manager and Radiation Protection Adviser for several Local Authorities including West Sussex. He was formerly an advisory teacher for science. Cooknell Electronics Ltd is an R & D company with many years experience in developing and manufacturing specialist Instruments for the nuclear and other industries.

The gas radon-220 (also called thoron) lends itself well for demonstrating and measuring the half-life of a radioactive substance. Radon-220 has a half-life of approximately 56 seconds and occurs naturally from the radioactive decay of thorium-232. Thorium-232 decays through a series of radionuclides, including radon-220, ending in lead-208.

Some brands of gas mantle use a small quantity of thorium oxide because when hot it is incandescent and gives a brilliant white light. Although thorium is radioactive, it has a low radiological risk because of its relatively low specific activity. The low risk is reflected in that thoriated gas mantles can be purchased from hardware stores.

If a few gas mantles are put into a bottle, a small quantity of radon-220 will collect in it, reaching equilibrium - as much decays as escapes from the gas mantles. If the radon gas is puffed into an ionisation chamber, the air will be ionised and a very small current will flow. The current will be proportional to the radon-220 activity. If the current is plotted against time, it will demonstrate the decay curve.

Note Typical graph shown on "News" page.