Identifying three types of ionizing radiation

Demonstration

In this demonstration, students can get an overview of different types of radiation and can label them as alpha, beta and gamma. They can also see that there are different ways of detecting the different types of radiation.

Apparatus and Materials

• Geiger-Müller tube
• Holder for Geiger-Müller tube
• Scaler (if needed by Geiger-Müller tube)
• Solid state detector and pre-amplifier (if available)
• Power supply, EHT, 0–5 kV (with option to bypass safety resistor)
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)
• Sealed pure gamma source, cobalt-60 (60Co), 5 μCi (semi-pure: some were sold without β filters)
• Sealed pure beta source, strontium-90 (90Sr), 5 μCi
• Sealed pure alpha source, plutonium-239 (239Pu), 5 μCi (if available)
• or sealed (semi-pure) alpha source, americium-241 (241Am), 5μCi
• Sealed radium source, 5 μCi (if available)
• Holder for radioactive sources
• Connecting leads

Health & Safety and Technical Notes

See guidance note on...

Managing radioactive materials in schools

NB Return each source to its box before removing another one so that only one source is in use at any one time. This is necessary because the dose rate calculations have been done for single sources only.

Read our standard health & safety guidance

Note that 5 μCi is equivalent to 185 kBq.

Sealed sources for radium and plutonium are no longer available (see the guidance note...

Radioactive sources – isotopes, radiation and availability

...However, if you have them in your school, you can use them as long as you follow your school safety policy and local rules.

Solid state (semiconductor) detectors are no longer available.

Nevertheless, you may have one in your school. They are effective at detecting alpha radiation. However, they are often very sensitive to light so you get a large background count. They use the energy of the incoming particle without a flash of light playing a part. The particle pushes some electrons into upper energy levels in the semiconductor, leaving holes which act as positive charges. A strong electric field makes a pulse of current which can be counted.

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

www.mindsetsonline.co.uk

If you do not have a pure alpha source ( ²³⁹ Pu), you need to be careful about trying to show the properties of alpha using a Geiger-Müller tube. The radiation from a mixed source like ²⁴¹ Am can penetrate aluminium and has a long range. This is because it gives out gamma as well as alpha radiation.

The most effective way of demonstrating the properties of alpha radiation is to use the spark counter. If you do not have a pure alpha source (i.e. you are using americium-241), this is the recommended method because the spark counter does not respond to beta or gamma. See...

Spark counter

...experiment for technical notes.

The 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. Education suppliers stock a set of absorbers that range from tissue paper to thick lead. This is a useful piece of kit to have in your prep room.

Procedure

1. Point each of the available sources in turn to the Geiger-Müller tube, the spark counter, and the solid state detector (connected to the scaler). The scaler responds by counting the ionizing events which occur.
2. You can show that only one type of source produces sparks. The others register a count on the Geiger-Müller tube, showing they are producing some kind of radiation but they do not produce sparks. Tell students "the radiation that produces sparks is the most ionising and is known as alpha."
3. In each case, put absorbers between the source and the detecting device. You can quickly show that paper stops alpha radiation. Of the remaining two types of radiation, one is stopped by a sheet of Perspex, an exercise book, or thin aluminium: "we call this beta radiation". The remaining type of radiation is very penetrating and needs thick lead to reduce it to a low level: "we call this gamma radiation".

4. Having identified the three types of radiation, try moving each one away from the detector. You can quickly show that alpha is very short range, beta has a range of about 10 cm in air, and gamma gets weaker with distance but doesn’t come to a stop at any particular distance.

Teaching Notes

• At this stage, you can use this practical in two ways.
• As a quick introduction to the three types of radiation. You may not want to dwell too much on the different properties. You can investigate these in more detail in later experiments.
• As a more detailed round-up of the range and penetrating properties of the three types of radiation. In effect, you can merge this experiment with...

Alpha radiation: range and stopping

Beta radiation: range, stopping and deflecting

Gamma radiation: range and stopping

• Relate the range and poor penetration of alpha to its strong ionisation. You can also refer to cloud chamber tracks if students have seen the experiment or photographs of the tracks.
• You can discuss the dangers of radioactivity in general. Radiation harms people by making ions in our flesh and thereby upsetting or killing cells. The more ionising the radiation, the more harmful it is.
• Relate the hazard to the safety precautions that you are taking during the demonstration.
• Unstable radioactive atoms send out particles. The remaining atom is different from the initial atom, and it becomes an atom of a different chemical element.

This experiment was safety-tested in April 2006