Beta radiation: range and stopping

Demonstration

This demonstration focuses on the properties of beta particles. It follows on closely from Identifying the three types of ionising radiation.

Apparatus and Materials

• Geiger-Müller tube
• Holder for Geiger-Müller tube
• Scaler (if needed by Geiger-Müller tube)
• Sealed pure beta source, strontium-90 (⁹⁰Sr), 5 μCi
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)
• Holder for radioactive sources

Health & Safety and Technical Notes

See guidance note on Managing radioactive materials in schools...

Managing radioactive materials in schools

This experiment puts the demonstrator at a small risk of receiving a dose of β radiation. The demonstrator should avoid leaning over the source and, if it cannot be avoided, should reduce the exposure time as far as possible. There are safer versions of doing this experiment which use a collimated beam and much smaller magnets.

Note that 5 μCi is equivalent to 185 kBq.

Geiger-Müller tubes are very delicate, especially if they are designed to measure alpha particles. The thin, mica window needs a protective cover so that it is not accidentally damaged by being touched.

Some education suppliers now stock all-in-one Geiger-Müller tubes with a counter.

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).

To cut off the direct path in step 4, the lead block from the absorbers kit is just adequate but a block with a bigger area is better.

Procedure

Absorption of beta radiation

1. Set up the Geiger-Müller tube in a clamp and connect it to a scaler if needed.
2. Fix the beta-source in its holder and clamp it near to the G-M tube.
3. Take 30-second counts of the beta particles at equal distances from the G-M tube until the count rate falls to the background count rate.
4. A graph of count rate against separation distance could be plotted.
5. Move the beta source and G-M tube so that a reasonable count rate is achieved (about 5 cm) and place paper, cardboard, thin aluminium sheet and lead sheet between the source and the G-M tube.

Teaching Notes

• The absorption properties of beta radiation make it useful in industrial and some medical applications.
• Experiments which deflect beta particles can measure their speed, which is about 98% of the speed of light. Hence relativistic effects cause an increase in the electrons mass.

This experiment was safety-tested in April 2006.