Alpha radiation: range and stopping
This demonstration focuses on the properties of alpha particles. It follows on closely from the experiment Identifying the three types of ionising radiation.
Apparatus and materials
Power supply, EHT, 0-5 kV (with option to bypass safety resistor)
Spark counter (or Geiger-Müller tube and counter
Sealed pure alpha source, plutonium-239 (239Pu), 5 μCi (if available) or sealed (semi-pure) alpha source, americium-241 (241Am), 5μCi
Holder for radioactive source (e.g. forceps)
Download the video [2.5mb]
Health & Safety and Technical Notes
See guidance note on Managing radioactive materials in schools.
A school EHT supply is limited to a maximum current of 5 mA, which is regarded as safe. For use with a spark counter, the 50 MΩ safety resistor can be left in the circuit. This reduces the maximum shock current to less than 0.1 mA.
Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors. Use female 4 mm connectors where required.
Read our standard health & safety guidance
Note that 5μCi is equivalent to 185 kBq.
Sealed sources for radium and plutonium are no longer available (see Radioactive sources – isotopes, radiation and availability guidance note). However, if you have them in your school, you can use them as long as you follow your school safety policy and local rules.
If you do not have a pure alpha source (239Pu), 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 241Am can penetrate aluminium and has a long range. This is because it gives out gamma as well as alpha radiation (see guidance note Radioactive sources - isotopes, radiation and availability).
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 radium or americium-241), this is the recommended method because the spark counter does not respond to beta or gamma radiation. See The spark counter 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 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 Connect the positive, high voltage terminal of the spark counter to the positive terminal of the EHT supply without the 50 MΩ safety resistor. (The spark counter’s high voltage terminal is joined to the wire that runs under the gauze.)
b Connect the other terminal on the spark counter to the negative terminal of the power supply and connect this terminal to earth.
c Turn the voltage up slowly until it is just below the point of spontaneous discharge. This is usually between 3,000 V and 4,500 V.
Carrying out the demonstration
d Use forceps to hold a radioactive source over the gauze. You should see and hear sparks jumping between the gauze and the high voltage wire underneath.
e Move the source slowly away from the gauze and note the distance at which it stops causing sparks.
f Move the source back towards the gauze so that sparks reappear. Try putting a very thin piece of paper between the source and the gauze. Try a thicker piece of paper. Note the effect in each case.
g Taking care that the foil does not blow onto the spark counter, try putting a thin piece of foil between the source and the gauze. With gold leaf or aluminium, you may still get some sparks. Try moving the source (and the gold leaf) away from the spark counter. You should find that the range has been reduced by the gold leaf.
1 This experiment can be done in conjunction with Beta radiation: range, and stopping and Gamma radiation: range and stopping. You might decide to merge these three experiments with Identifying the three types of ionizing radiation so that you do the range of all three types of radiation. You can then show the effects of a magnet on beta radiation separately.
2 You should find that the range of the alpha particles is between 3 and 10 cm. The alphas from americium have a range of about 3 cm, from plutonium 5 cm, and the most energetic ones from radium, 7 cm. Refer to the Diffusion cloud chamber experiment to reinforce this evidence.
3 You should find that the alpha particles are stopped by anything except the very thinnest of paper or foil leaf. The gold leaf reduces the range of the alpha particles, because they lose energy getting through the gold leaf.
4 Remind students that this is alpha radiation, which is the most ionizing of the three main ionizing radiations. Link this with the observations that you have made. Alpha radiation collides with and ionizes a lot of particles in the material through which it passes. Because of this, it loses its energy quickly and is slowed down and absorbed.
5 Refer to cloud chamber photographs of alpha particle tracks, showing them being deflected in a magnetic field (see Alpha particle tracks bent in a very strong magnetic field). The deflection is too small to measure in the school laboratory, but shows that they have a positive charge. The small deflection shows that they have a relatively large mass. Collisions with helium produce 90° forks showing that they have the same mass as a helium (nucleus). You can say that alpha particles are thought to be a doubly ionized helium atom (see Alpha particle tracks including a collision with a helium nucleus). If the students have already met the idea of nuclei, then you can call alpha particles a helium nucleus.
6 An alpha particle is a helium nucleus with two positively charged protons and two neutral neutrons. The atomic mass of the radiating atom falls by four units when an alpha particle is emitted. The speed of an alpha particle can be up to 15 x 106 m/s.
7 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 ionizing the radiation, the more harmful it is. This makes sources of alpha radiation very hazardous – especially if they are ingested.
8 Relate the hazard to the safety precautions that you are taking during the demonstration.
This experiment was safety-checked in August 2006