Demonstration

A radioactive source produces radiation that will ionise the air. The conducting air completes a circuit to charge an electroscope. Use the circuit to show the ionising effect of the radiation and present it as a means of detecting ionising radiation.

Apparatus and Materials

• Power supply, EHT, 0–5 kV (with internal safety resistor)
• Metal plates with insulating handles, 2

  Gold leaf electroscope

• Hook for electroscope
• Retort stands and bosses, 2
• Connecting leads
• Holder for radioactive source (e.g. forceps)

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 circuit so reducing 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

A web cam or ‘flexicam’ could be used to project an image of the electroscope's gold leaf onto a screen. Alternatively, use a bright lamp to cast a shadow of the leaf on a screen or wall.

The advantage of using an EHT is that it looks like an electric circuit – albeit a strange one. The electroscope plays the part of very sensitive meter and the air between the plates is a component whose resistance changes.

Take care when using the electroscope not to make it all seem like a sleight of hand – especially when moving leads around.

Radium is a source of alpha, beta and gamma radiation however the beta and gamma radiation do not cause enough ionisation of the air to start a spark.

Procedure

Setting up

1. Fix the two metal plates so that they are parallel to one another and about 1 cm apart.
2. Connect one of the plates to the positive terminal of the E.H.T. supply through the safety resistor.
3. Connect the other plate to the leaf of the electroscope through the hook.
4. Connect the case of the electroscope to the negative terminal of the E.H.T. supply.
5. Connect the negative terminal of the EHT supply to the earth terminal.
Getting a current to flow and charge the electroscope

6. Set the EHT supply to about 3 kV and switch it on. The leaf of the electroscope will rise due to induced charges; reset it by momentarily connecting the leaf to earth.
7. Hold the sealed radium source beside the gap and point it between the plates; watch what happens. The air in the gap is ionised and allows a charge to flow across the gap; this charges the leaf of the electroscope.
8. Discharge the electroscope by momentarily connecting the leaf to earth. Try recharging it by holding the source at different angles to and at different distances from the plates.
Discharging the electroscope...
9. You can also discharge the electroscope by ionising the air around it.

10. Disconnect the electroscope from the supply but keep the base earthed. Replace the disc in the top of the electroscope and charge it using a flying lead from the positive terminal of the power supply (via the safety resistor).
11. Point the sealed source over the top of the disc. This will ionise the air and allow the electroscope to discharge.
12. Charge the electroscope again and try holding the sealed source at different angles and at different distances from the disc.
13. The electroscope should discharge quickly to earth when the air above the its disc is ionised. If it is discharging too slowly, bring an earthed metal plate up close to the disc of the electroscope.

Teaching Notes

• In the demonstration

  Ions produced by a flame carrying a current

  students met the idea that ionising the air allows the EHT to charge the electroscope. This demonstration uses the same idea. But now the ionisation is being caused by some invisible radiation coming from the sealed source.
• As you bring the source towards the gap between the plates, point out that there is no flame and no obvious transfer of energy near the sealed source. Nevertheless, it must be producing ions in the gap. So it must be giving out some invisible radiation.
• Use the term “ionising radiation” to describe what the source is giving out.
• The circuit provides a visible means of detecting ionisation in the gap between the two plates. The electroscope provides the visibility and the high voltage across the air gap is the means of catching ions. This is the same principle as a more convenient detector (the Geiger-Müller tube) and it provides students with a conceptual step towards its construction. This is developed further in

  The spark counter

This experiment was safety-tested in May 2007

Demonstration

The spark counter is a highly visible (and audible) way of showing and counting ionisation of the air caused by alpha radiation (or a match). It is a useful step towards understanding the Geiger-Müller tube.

Apparatus and Materials

• Power supply, EHT, 0–5 kV (with option to bypass safety resistor)
• Spark counter
• Sealed source of radium, 5 μC (if available) or sealed source of americium-241, 5 μC
• Holder for radioactive source (e.g. forceps)
• Connecting leads

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 so reducing 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

The spark counter is a special piece of apparatus (see image above). It consists of a metal gauze with a wire running underneath. Philip Harris calls it a Spark Discharge Apparatus.

• Any kink or bend in the wire in the counter is liable to cause a spark discharge at that point. If that happens the wire should be replaced.
• A continuous spark (which will very soon damage the wire) shows the voltage is too high.
• The spark counter should be dust free. Dust around the stretched wire can usually be blown away.
• The gauze on top is connected to the earth on the EHT supply as a safety precaution.
• Radium is a source of alpha, beta and gamma radiation. Beta and gamma radiations do not cause enough ionisation of the air to start a spark.

Procedure

Setting up

1. 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.)
2. Connect the other terminal on the spark counter to the negative terminal of the power supply and connect this terminal to earth.
3. Turn the voltage up slowly until it is just below the point of spontaneous discharge. This is usually at about 4,500 V.

Carrying out

1. 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 each time an alpha source is brought near to the counter.
2. Move the source slowly away from the gauze and note the distance at which it stops causing sparks.

Teaching Notes

• Draw attention to the random nature of the sparks and hence of the radiation. By counting sparks you are counting the number of alpha particles emitted.
• You should find that the range of the alpha particles is about 5 cm.
• You could mention that this is alpha radiation, the most ionising of the three main types of radiation.
• The sparks are similar to those produced by a Van de Graaff generator. The alpha particles ionise the air forming positive and negative ions. When these ions recombine to form neutral atoms then blue light is emitted. The noise of the spark is due to warming the air in the narrow region of the avalanche current, which produces a sound wave just like in a lightning strike.
• A thin sheet of tissue paper or gold foil held between the spark counter and the source will show a reduced range for the alpha particles or even prevent them getting to the counter.
• A version of this apparatus can be seen in the CERN visitor centre (if you happen to be passing). It detects cosmic rays and makes them visible using a 3D array of wire meshes with high voltages between them. The paths of rays can be seen by the trail of sparks that they leave as they ionise the air between the wire meshes.
• This type of 3D array of high voltage meshes is the principle used to detect the paths of particles produced in the collision experiments at CERN.
• Before you use the spark counter to show ionisations from an alpha source, you could use the spark counter to count matches (as in

  Counting matches with an EHT supply

This experiment was safety-tested in June 2007

Demonstration

This is a general introduction to the Geiger-Müller (G-M) tube describing and explaining the basic principles of its operation.

Apparatus and Materials

• Scaler
• Holder for GM tube
• Thin window GM tube
• Gamma source, as pure as possible e.g. Co-60 with a filter to stop β, or Ra-226 with a thick filter
• Beta source, pure (Strontium 90)
• Holder for radioactive sources
• Gamma GM tube if available
• Box of matches

Health & Safety and Technical Notes

See guidance notes:

• Managing radioactive materials in schools

• Read our standard health and safety guidance

Geiger-Müller tubes are set up to operate at a voltage within their ‘plateau’. In self-contained systems, this is set automatically.

The voltage across a Geiger-Müller tube is generally kept low enough so as not to produce a roaring spark when an energetic particle enters it.

Geiger-Müller tubes are very delicate, especially if they are designed to measure alpha particles. The thin, mica window allows alpha particles to enter the chamber. It needs a protective cover to prevent it from being accidentally damaged by being touched. A good alpha detecting Geiger-Müller tube will also count photons. If you light a match in front of it, a few ultra violet photons will be detected.

Procedure

Setting up

This will depend on the type of Geiger-Müller tube you are using. If you have a self-contained system, then simply get it ready and switch it on. If you are using an older style Geiger-Müller tube that plugs into a separate ratemeter or scaler, you will need to set the voltage on the scaler. Do this by following these steps.

1. Put a radioactive source in a holder. Fix this in a clamp on a retort stand.
2. Put the Geiger-Müller tube in a stand. Adjust it so that it is pointing at the source, and is about 5 cm away from it.

Photograph courtesy of Mike Vetterlein
3. Plug the Geiger-Müller tube into the scaler (counter) and switch on.
4. Start the voltage at about 200 volts. Make a note of the number of counts in, say, a 15 second interval.
5. Increase the voltage in steps of 25 volts.
6. You will find that the counts vary with voltage and then reach a plateau. A graph would look like this (you do not need to plot the graph):

7. After the threshold voltage, the count will reach a plateau. It will stay constant over a range of voltages. Set the voltage at a value of between 50 to 100 V above the threshold.
8. If the clicking increases when you increase the voltage, then you have moved off the plateau. Turn the voltage back down.
9. Put the source back in a safe place until you carry out the demonstration.

Carrying out the demonstration

1. Switch on the Geiger-Müller tube counting system.
2. Highlight the fact that there is a background count.
3. Bring a radioactive source up to the Geiger-Müller tube and draw attention to the increase in counts.
4. You could measure the background count and the count with the source nearby. Do this over a period of 30 seconds. Draw attention to the difference.

Teaching Notes

• Discuss what is happening in the Geiger-Müller tube. Point out that it is more sensitive and more stable than the spark counter.
• Draw attention to the differences in count between the Geiger-Müller tube and the spark counter. The Geiger-Müller tube detects all of the ionisation events that take place inside it. Every one is registered.
• Discuss the Geiger-Müller tube as a development of the spark counter. You could say: "The wire of the spark counter is placed inside a metal shield, which acts as the other electrode, and the high voltage supply is incorporated in the scaler The scaler does the counting of the pulses of charge delivered by the electron avalanches to the central wire. The tube is filled with a suitable gas mixture to make sure that each spark does not last too long. When the spark is quenched, the tube and the scaler are ready to count a ‘bullet’ from another radioactive atom’s ‘explosion’. As you can see and hear, the tube can react very quickly to each avalanche."
• The Geiger-Müller tube works on the same principle as the spark counter: an ionisation between two high voltage electrodes produces a pulse of current (an avalanche of charge) between the electrodes. The differences are that the Geiger-Müller tube is sealed, it contains a low pressure gas (usually argon with a little bromine), and it is usually part of a circuit with a scaler counter.
• The scaler counter records and counts each pulse of charge.
• The actual phenomena inside a tube are much more complicated than the simple story of ionisation producing an avalanche of electrons. Inside the tube ultra violet photons probably play an important part, as well as colliding electrons and ions, and the detailed picture is extremely complex.
• An ionising particle will produce a pulse of charge of almost constant size. The size of the pulse does not vary with the energy or amount of ionisation produced by the ionising particle.
• The number of pulses represents the number of ionising particles coming into the tube.
• Geiger-Müller tubes do not distinguish between one kind of particle and another, or between a more energetic particle and a less energetic one, provided the particle enters the tube and does not pass right through (as most gamma rays do).

This experiment was safety-tested in August 2007

Class practical

The Taylor diffusion cloud chamber is a simple piece of equipment which will clearly show alpha particle tracks. It is cheap enough to allow students, in groups, the opportunity to do their own experiment. Students are fascinated by the tracks and watch them for a long time. This is something to be enjoyed and not hurried.

You can do this as a demonstration. However, students will prefer waiting for their own apparatus to produce results rather than yours. Also, if you have eight to ten groups of students, each with their own cloud chambers, you are more likely to get some results sooner or later.

Apparatus and Materials

For each student or student group:

• Taylor diffusion cloud chamber
• Lamp, 12 V, 24 W and power supply (shining through 1 cm wide slit)

Available to the class/teacher:

• Flexicam or webcam linked to a projector (optional)

Dry ice

or

Carbon dioxide cylinder

...and dry ice attachment (see guidance note).

Health & Safety and Technical Notes

This demonstration uses a weak radioactive source. If any radioactive paint has flaked off the source inside the chamber, do NOT use it.

Since ethanol is in use, there must be no naked flames in the room.

Wear eye protection and gauntlet-style leather gloves when making or handling solid carbon dioxide.

Read our standard health & safety guidance

• The cloud chamber works by allowing a super-saturated vapour to build up close to the base of the chamber. The air at the top of the chamber should become saturated with ethanol vapour. Any air that sinks to the bottom of the chamber is cooled by the dry ice underneath. This makes the air super-saturated and the vapour will condense if given the opportunity – i.e. one or more condensation nuclei. These are provided by alpha particles from the thoron source.
• When putting the alchol into the chamber it is essential that none of it falls on the source, otherwise alpha particles may not penetrate it.
• Surprisingly little dry ice is needed in these chambers. Practice will show you how much is required, usually about 2 or 3 cm³ .
• The radioactive source is normally a spot of radioactive paint containing thorium or radium.
• Insert the wire source holder in the cork and place the cork in the hole in the side of the chamber, with the source near the floor. Position the source in the gap between the metal foils by rotating the wire.

• Place the chamber on the three levelling wedges; clean the underside of the Perspex lid before replacing.
• Direct a flat beam of light across the chamber towards the radioactive source. (The foils should be bent back slightly so that they do not reflect light onto the chamber floor.)
• NB Suppliers of diffusion-type cloud chambers:
• Ideas for Education in Co. Fermanagh, N. Ireland, telephone number 028 6863 1209\. (Also supplied by:

• Timstar

  ...and...

  Scientific & Chemical Supplies

  ...Uses dry ice.
• PASCO SE-7943\. Uses ice water.
• An alternative radioactive source is fully described here...

  Alternative radioactive source for the diffusion cloud chamber

Procedure

1. It is very important that the class should have plenty of time for this experiment. Allocate the cloud chambers so that there is one for every three or four students.
2. The laboratory will need to blacked out, but the light from the 12 V lamps is enough for everyone to see what they are doing (see guidance note

   Classroom management in semi-darkness

3. To set up the chambers, put alchol on the padding inside the top of the chamber using a dropper. A drop or two may also be put on the black base of the chamber and allowed to spread over it. Make sure none gets onto the thoron source.
4. Unscrew the base of the whole apparatus and put a little

   dry ice

   in contact with the base plate. Put the foam back to keep the dry ice in contact with the plate. Screw the base cap on again, and turn the chamber the right way up.
5. It is important that the cloud chamber is level. Place it on the three wedges provided. These can be adjusted to get it level. If it is not level, you will see convection currents moving in the chamber and these can be used as guides in levelling.
6. The top must be put back on the chamber. Rubbing it with a clean duster will charge it sufficiently to provide an adequate electric field inside the chamber to sweep away old ions.
7. Illumination is important. Adjust the 12 V lamps so that there is a layer of illumination a few millimetres above the base plate.
8. Usually within 30 seconds of setting it up, you should see alpha tracks coming from the weak radioactive source which is inserted in the side of the chamber.
9. If the tracks are not sharp, try rubbing the top again to improve the electric field. This cleans out any stray ions in the air.

Teaching Notes

• Tell the class that what they can see is the effect of alpha radiation. They are not seeing the radiation itself, but the condensation which has formed on ions left behind by the radiation. By the time the condensation forms, the alpha particle has long gone. There is a nice analogy in the guidance note on

  Alpha particle tracks

• Draw attention to the amount of ionisation that each alpha particle produces and to the length of its track.
• You could also draw attention to the fact that the tracks are straight, showing that nearly all the collisions are with something much lighter (usually removing an electron from an atom). Forked tracks may be seen when the alpha particle strikes a more massive particle such as one of the constituents of air.
• If students watch the cloud chamber for long enough, and the chambers are well balanced, they may well see the tracks of high energy electrons from cosmic rays.
• Short, thin spiralling tracks may be seen which are electrons or β particles in the Earth's magnetic field.
• A fast group could swing the source behind the thin foil. This will absorb the α particles but let the β particles through. The wavering tracks of the β particles may be seen if conditions are optimum.
• If you start to get some good results, you could use a flexicam to project the live tracks onto a screen or whiteboard. You could even record a short movie for posterity and to refer back to in later lessons. Similarly, if you have access to a digital camera, you could take some still photographs and use them in a wall display or PowerPoint presentation in a follow-up lesson. You could offer a prize for a forked track!

This experiment was safety-tested in August 2007

Demonstration

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)
• Connecting leads
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)

• Alpha particle tracks showing their short range

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...

Read our standard health & safety guidance

Note that 5μCi is equivalent to 185 kBq.

Sealed sources for radium and plutonium are no longer available. 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 ( 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:

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:

The spark counter

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}.

Teaching Notes

• 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 three types of ionizing radiation}{/identifying-three-types-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.
• 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.

  Diffusion cloud chamber

• 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.
• 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.
• Refer to cloud chamber photographs of alpha particle tracks, showing them being deflected in a magnetic field

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"

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.
• 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 10
⁶ m/s.
• 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.
• Relate the hazard to the safety precautions that you are taking during the demonstration.

This experiment was safety-tested in August 2006

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.

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

Demonstration

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
• Geiger-Müller tube
• 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
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)

Health & Safety and Technical Notes

See guidance notes on...

Managing radioactive materials in schools

Read our standard health & safety guidance

Note that 5 μ Ci 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}

Procedure

1. Set up the Geiger-Muller Tube and attach it to the scaler if needed.
2. Put the source in its holder and clamp it a few centimetres from the Geiger-Muller tube.
3. 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.
4. Show that the gamma radiation will penetrate paper, cardboard, aluminium and thin lead, but is greatly reduced by thick lead.

Teaching Notes

• 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

Demonstration

Gamma radiation is part of the electromagnetic spectrum. It is not absorbed by the air, but its intensity decreases because it spreads out. Therefore, the intensity varies with the inverse square of distance: it follows an inverse square law. You can show this in the laboratory and use it as evidence to support the fact that gamma radiation is a part of the electromagnetic spectrum.

Apparatus and Materials

• Holder for radioactive sources
• Geiger-Müller tube
• Holder for Geiger-Müller tube
• Scaler
• Metre rule
• Sealed "pure" gamma source, cobalt-60 (⁶⁰Co), 5 μCi or sealed radium source
• Set of absorbers (e.g. paper, aluminium and lead of varying thickness)

Health & Safety and Technical Notes

See guidance notes on...

Managing radioactive materials in schools

Read our standard health & safety guidance

Note that 5 μCi is equivalent to 185 kBq.

Cobalt-60 is the best pure 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 will not. You can do a quick check by doubling and tripling the distance between the source and the axis of Geiger-Muller tube and seeing if the count follows an inverse square law (by dropping to a quarter and a ninth).

Using the Geiger-Muller tube sideways has an added advantage that you have an accurate measure of where the distance is zero. It is along the axis of the tube.

Education suppliers stock a set of absorbers ranging 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}

Procedure

Setting up...

1. Set up the Geiger-Muller tube and attach it to the scaler.
2. Clamp a metre rule to the bench and line it up with your zero point (in the Geiger-Muller tube).
3. With some Geiger-Muller tubes, the gamma radiation will pass through the side. So set the Geiger-Muller tube up at right angles to the metre rule. The zero point is then the axis of the tube.
4. You can check your zero point by doing some quick readings before the lesson. When you double the distance, the count should be a quarter. If it is more than a quarter, then move the tube towards the source to re-zero it. If it is less than a quarter, then your zero point is closer than you reckoned: move the tube away from the source to re-zero it.
Carrying out...
5. Measure the background count with the source far away.
6. Start with the gamma source 10 cm from the zero point.
7. Increase the distance and take measurements of count rate at 20 cm, 30 cm, 40 cm, 60 cm and 80 cm.
8. Correct the count rates for the background count.
9. Plot a graph of corrected count-rate against distance. You could use a spreadsheet program to do this.

Teaching Notes

• The shape of the graph shows that count rate decreases with distance. You can show that it is an inverse square by checking that the count rate quarters when the distance doubles (10 cm to 20 cm; 20 cm to 40 cm; 30 cm to 60 cm), falls to a ninth when it trebles (10 cm to 30 cm; 20 cm to 60 cm) and drops to a sixteenth when the distance is quadrupled (10 cm to 40 cm; 20 cm to 80 cm). (This is only true assuming the source is a small area compared with the cross-section of the detector. Keep minimum distance large!)
• A graph of count rate against 1distance² is a straight line.
• This is the same law that governs all electromagnetic radiation (see, for example

  The Sun's luminosity

  This is some evidence that gamma radiation is part of the electromagnetic spectrum.
• The moral of this story is that in order to protect yourself from gamma radiation the best thing to do is to move farther away. At 10 times the distance you will be 100 times as safe.

This experiment was safety-tested in May 2006

Demonstration

Measuring the half-life of a radioactive isotope brings some of the wonder of radioactive decay into the school laboratory. Students can witness one element turning into another and hear (or see) the decrease in the radiation it gives out as it transmutes.

This demonstration uses a protactinium generator to show the exponential decay of protactinium-234, a grand-daughter of uranium. It has a half-life of just over a minute, which gives students the chance to measure and analyze the decay in a single lesson.

Apparatus and Materials

• tray
• Holder for Geiger-Müller tube
• Geiger-Müller tube, thin window
• Scaler
• Stopclock
• Retort stand, boss, and clamp
• Ratemeter (OPTIONAL)

• Protactinium generator

Health & Safety and Technical Notes

See the following guidance note:

Managing radioactive materials in schools

To limit the risk of radioactive liquids being spilt, there should be special instructions in the local rules for handling (and preparing) this source.

Read our standard health & safety guidance

Preparation of the protactinium generator

It is now possible to purchase the chemicals already made up in a sealed bottle. One supplier is TAAB Laboratories Equipment Ltd, 3 Minerva House, Calleva Park, Aldermaston, RG7 8NA. Tel: 0118 9817775. However, you can make your own if you prefer.

These quantities make a total volume of 20 cm3. You can scale them up if you have a larger bottle. (A '30 ml' bottle has a capacity of about 35 ml, so there is still room to shake the solution when the total volume is 30 ml.)

1. Dissolve 1 g of uranyl nitrate in 3 cm3 of water. Wash it into a small separating funnel or beaker with 7 cm3 of concentrated hydrochloric acid.
2. To this solution, add 10 cm3 of iso-butyl methyl ketone or amyl acetate.
3. Shake the mixture together for about five minutes. Then run the liquid into the polypropylene bottle and firmly screw down the cap. It can help to shield the lower half of the bottle with some lead.
4. Place the bottle in a tray lined with absorbent paper.

Once you have made the protactinium generator, you can store it with other radioactive materials, taking care to follow your school code of practice and local rules: see the Managing radioactive materials in schools guidance note:

Managing radioactive materials in schools

A polypropylene bottle is preferable to polythene because it is somewhat more resistant to attack by the acid and ketone. Nevertheless, polythene bottles can be used, provided no attempt is made to store the liquid in them for more than a few weeks.

The organic layer which separates out contains the protactinium-234. This decays with a half-life of about 70 seconds.

An alternative to protactinium: A new, effective and extremely low hazard system for measuring half-life is available from Cooknell Electronics Ltd, Weymouth, DT4 9TJ. This uses fabric gas mantles designed for camping lights. Each mantle contains a small quantity of radioactive thorium. More details are available on the Cooknell Electronics website:

Cooknell Electronics

Procedure

1. Support the Geiger-Muller tube holder in a clamp, so that the tube is facing downwards towards the neck of the bottle.
2. Allow the bottle to stand for at least ten minutes. Take the background count by running the counter for at least 30 seconds. This is done with the bottle in position, because some of the count will come from the lower layer. You can do this before the experiment or some time after it has finished.

3. Alternatively, the GM tube can be clamped horizontally with the window close to the upper layer.

4. Shake the bottle vigorously for about 15 seconds to thoroughly mix the layers.
5. Place the bottle in the tray.
6. As soon as the two layers have separated, start the count and start the stop-clock.
7. Record the time from the beginning of the experiment - i.e. the time of day for the sample.
8. Record the count every 10 seconds. Or record it for 10 seconds every 30 seconds.
9. Run the experiment for about five minutes, ample time to reveal the meaning of the term half-life and to illustrate the decay process.
10. Provided you leave a few minutes between each attempt, you can repeat the experiment. In 5 minutes the activity of the protactinium in the aqueous layer grows to 15/16 of its equilibrium value.
11. It is possible to record the growth to equilibrium. Do this by moving the GM tube so that the aqueous layer at the bottom of the bottle is immediately above the end window of the GM tube.

Teaching Notes

The chemistry of the experiment:

• The first stages of the uranium-238 series are involved in this experiment.
• 
• The aqueous solution (at the bottom of the bottle) contains the uranium-238, its daughter thorium-234 and the short-lived granddaughter protactinium-234.
• Uranium and protactinium both form anionic chloride complexes but thorium does not. At high hydrogen ion concentrations, these complexes will dissolve in the organic layer (which is floating on top of the aqueous solution).
• When you shake the bottle, about 95% of the short-lived granddaughter (protactinium) and some of the uranium will be dissolved in the organic layer. The thorium stays in the aqueous layer.
• Since radioactivity is a property of the innermost nucleus of the atom it is not affected by chemical combination.
• The granddaughter (in the organic layer) decays without any more being produced by its parent (thorium) all of which is still in the aqueous layer. It emits beta particles which travel through the plastic wall of the bottle. Isolating the protactinium in the top (organic) layer allows it to decay without any top-up from its parent (thorium).
• The radiation from the thorium and uranium should not interfere with the results, for two reasons:
  1. The counter does not detect the alpha particles from the uranium or the low energy beta particles from the thorium. It only records the high energy (2 MeV) beta particles from the granddaughter (protactinium).
  2. The uranium-238 decays with an extremely long half-life. It yields a meagre, almost constant, stream of low energy alpha particles. Its daughter, thorium-234, decays with a half-life of 24 days. During the length of this experiment the decay rate can be assumed to be constant. If these two isotopes contribute to the count at all, it will be accommodated in the background count. The stockpile of thorium is also constantly topped up in the aqueous layer as long as the protactinium is present with the thorium.

Table of count rate: Get the students to make a table of count rate against time, and correct it for background count. The first 10-second reading should be allocated to a time of zero.

Plot a graph: Get the students to plot a graph of count rate against time. They should draw a smooth curve through the points.

• First point out the general pattern - that the count rate decreases with time. Then look for an exponential trend - that the best fit curve always takes the same amount of time to halve.
• Get students to measure the half-life from the curve.
• Point out the random nature of the points: although the decay follows a pattern, there is an element of randomness and it is not perfectly predictable.

How Science Works extension This experiment provides an opportunity to assess the accuracy of the measured half-life value and how the random nature of decay affects the answer.

The accepted value for the half-life of protactinium is about 70 seconds.

Explore different ways in which a half-life value can be obtained from this apparatus:

• Amend the procedure described above so that, instead of a scaler (counter), a ratemeter is used. One student just records the time it takes for the count-rate to halve. This will provide a very approximate value.
• Repeat the experiment with several members of the class timing how long it takes for the count-rate to halve. There is likely to be considerable spread in results across the group and the mean result may differ from the accepted value for half-life. In each case, ask students to identify errors and uncertainties in their measurement(s) and to suggest ways in which these could be reduced.

For example, ask: "how does the random nature of the decay affect the measured count-rate when the count is low, or high, compared the background count?"

• Either you or your students may suggest a graphical method as an improvement. The procedure described in the main experiment above could then be carried out, and then the accuracy of the half life value assessed and evaluated.

Radioactive materials raise significant safety issues, providing an opportunity to discuss the value and use of secondary data sources.

This experiment was safety-tested in February 2007

Class practical

In this activity, students model radioactive decay using coins and dice. By relating the results from the model to the experimental results in...

Measuring the half-life of protactinium

...students can see that the model helps to explain the way in which a radioactive substance decays. The model provides an insight into what might be happening within radioactive atoms.

This activity is a good analogy of radioactive decay as it is based on probability. The decaying trend will be noticeable and so too will the random nature.

Apparatus and Materials

• Pennies or other coins, plentiful supply
• Dice, plentiful supply (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance

The more coins each student has, the better the analogy of radioactive decay. You could use as few as one per student to keep it simple. Any more than four is quite difficult to manage.

Small coins will turn around more in their cupped hands.

A canvas bag containing 500 plastic cubes (each side 10 mm), each with one face identified, is available in the UK from Lascells, order code 60-010.

Lascells

Procedure

1. Explain the procedure (as follows) to the class.
2. Each student has a number of coins. This could be between one and four. They hold them in their cupped hands.
3. On your instruction "shake", the students shake their coins for at least 5 seconds (they should ensure that the coins are moving around inside their cupped hands). On the instruction "stop", they stop shaking and open their hands with one hand flat and facing upwards so that they can see their coins.
4. If any coins come down heads, they take them out of their palm and place them on the desk.
5. On your instruction "show", they put up a number of fingers corresponding to the number of coins they took out of their palm.
6. Record this number on the board.
7. They keep the remaining coins in their hands and repeat from step 3. If you can arrange it that you take a reading once every minute, then you can record the readings against time. It will then give results very similar to protactinium.
8. Analyze the result by plotting a graph.

Teaching Notes

• You might want to appoint a counter and a scribe to count the coins and record the results.
• Take care with how you ask students to signal the numbers. They may be tempted to add their own (rude) gestures.
• Draw out the similarities with the protactinium experiment. The trend is the same and there is also some randomness. The close match between the results from this model and the results from

  Measuring the half-life of protactinium

  show that radioactive atoms have a chance of decaying in any fixed time.
• Use the activity to explain the downward trend of the decay curve. Only coins that are left can decay. As there are fewer of them each time, fewer will decay.
• The activity raises the interesting question about how long a radioactive source will last and what happens to the last atom.
• An alternative to shaking the coins in students' palms is to flick them. But this takes longer.
• You could repeat the experiment with small dice to give a longer half-life. Combining results (as outlined here) makes for a smoother curve.

This experiment was safety-tested in May 2007