Beta radiation: deflection in a magnetic field

Demonstration

This demonstration focuses on the properties of beta particles. It follows closely from

Identifying the three types of ionizing radiation

You can show that beta radiation is deflected in a magnetic field; this is an impressive and striking demonstration.

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
• Holder for radioactive source
• Retort stands, bosses, and clamps, at least 3
• G-clamps, 2
• Lead block
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)

Health & Safety and Technical Notes

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.

Read our standard health & safety guidance

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.

You need to be especially careful handling the Geiger-Müller tube near the Eclipse magnet, which is extremely strong. The strong magnet can pull the Geiger-Müller tube out of a loose holder or even your fingers. Make sure that the Geiger-Müller tube is firmly fixed in a retort stand which is clamped to the bench before you start setting up the magnet.

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

Mindsets

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 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

1. Use a G-clamp to secure one of the retort stands to a bench. Fix the Geiger-Müller tube in its clamp. Point it up at an angle of about 30°.
2. Secure a second retort stand to the bench and clamp the holder for the radioactive source in it. Again, face it up at an angle of about 30°.

3. Place the large eclipse magnet on the lead block between the source and the Geiger-Müller tube. Arrange it so that the source and the Geiger-Müller tube are pointing into the middle of the space between its two poles. Take great care when handling the magnet near the Geiger-Müller tube - it is very strong and can dislodge the tube if it's not secure.
4. Check that you can detect beta particles with the magnet in place (in one orientation). If the magnet is removed or turned around, you will not be able to detect beta particles. Make a note of which orientation works.
5. Remove the magnet and return the beta source to the safe.
Carrying out
6. Remove the magnet and place the sealed source in its holder and show that the lead sheet blocks all the radiation. You can slide the lead in and out to show that beta radiation is being emitted and will reach the Geiger-Müller tube.
7. Put the magnet in place (the correct way) and show that the Geiger-Müller tube is now detecting beta radiation. You can show this by using various shields next to the source and the tube.
8. Rotate the poles of the magnet through 180° and show that this stops the radiation reaching the Geiger-Müller tube.

Teaching Notes

• The beta radiation is deflected by the magnetic field. This suggests that it is made of moving charges.
• With advanced students, you may want to use Fleming's Left Hand Motor Rule to identify the sign of the charge as negative. Or you can refer to the experiment

  Force on a wire carrying a current in a magnetic field

• The fact that the beta radiation is deflected only a finite amount means that it must have mass. This suggests that it is a stream of (negative) particles. Students might suggest that it is made of electrons. You can say that further studies show this to be the case.
• You might mention that alpha radiation is also deflected by a magnetic field, but not enough to measure with this equipment. It is deflected the other way, showing that it has a positive charge.
• 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.
• Beta particles are formed when a neutron changes into a proton in the nucleus and the atom rises one place in the periodic table.

This experiment was safety-tested in August 2007