Gamma radiation: range and stopping
This demonstration focuses on the properties of gamma radiation. You can show that it is much more penetrating than alpha or beta radiation and has a much longer range.
Apparatus and materials
Holder for radioactive source
Holder for Geiger-Müller tube
Scaler (if needed by Geiger-Müller tube)
Sealed "pure" gamma source, cobalt-60 (60Co), 5 μCi or sealed radium source
Download video [6.1mb]
Health & Safety and Technical notes
See guidance note on Managing radioactive materials in schools.
Note that 5μCi is equivalent to 185 kBq.
Cobalt-60 is the best gamma source. However, you may have a sealed radium source in your school. This gives out alpha, beta and gamma radiation. You can use it for this experiment by putting a thick aluminium shield in front of it. This will cut out the alpha and beta radiations.
An alternative is to try using a Geiger-Muller tube sideways. The gamma radiation will pass through the sides of the tube but alpha and beta radiation will not. Some gamma particles interact with the tube wall and knock electrons into the tube gas, where they are detected. This effect enhances the detection efficiency of the gamma particles. You can do a quick check by doubling and tripling the distance between the source and the axis of the Geiger-Muller tube and seeing if the count follows an inverse square law (by dropping to a quarter and a ninth).
Some education suppliers now stock all-in-one Geiger-Muller tubes with a counter. See e.g. www.mutr.co.uk
Education suppliers stock a set of absorbers that range from tissue paper to thick lead. This is a useful piece of equipment to have in your prep room. You can make up your own set. This should include: tissue paper, plain paper, some thin metal foil (e.g. cigarette paper, wrapping from a chocolate from an assortment box and a small piece of gold leaf).
a Set up the Geiger-Muller Tube and attach it to the scaler if needed.
b Put the source in its holder and clamp it a few centimetres from the Geiger-Muller tube.
c Show that the gamma radiation has a long range in air - at least 80 cm. You could show that the count is falling off with distance, and gets smaller and smaller rather than stopping altogether.
d Show that the gamma radiation will penetrate paper, cardboard, aluminium and thin lead, but is greatly reduced by thick lead.
The moral of this story is that in order to protect yourself from gamma radiation the best thing to do is to move a long way away.
Discuss the uses of gamma radiation in industry and for medical imaging and treatment. The applications are based on its penetrating power.
Remind students that gamma radiation is much less ionising than alpha.
This experiment was safety-tested in August 2007