Quantum and Nuclear

Teaching Radioactivity

Practical Activity for 14-16

The purpose of this collection of resources is to support the teaching of ionizing radiation in school and, in particular, to encourage teachers to use practical activities. The guidance and advice on setting up and managing activities includes the use of the spark counter, the Geiger-Müller tube, and the diffusion cloud chamber. The videos were produced in collaboration with CLEAPSS and all of the activities can be carried out safely in schools.

How to use this collection of resources:

Useful links:

  • Visit CLEAPSS for further safety information and advice on all aspects of practical science in schools.
  • SSERC provide information and advice on practical science for schools in Scotland.
  • HPA - The Health Protection Agency - has a section on radiation with useful information about background radiation.

Up next

Radioactivity illustrative animations

This resource contains 4 different animations to aid in the teaching of radioactivity. 

Ionising Radiation
Quantum and Nuclear

Teaching Radioactivity Animations

Classroom Activity for 14-16

These 4 animations will aid in the teaching of radioactivity. 

Radiation Ionises the Air

The Cloud Chamber

The Spark Counter

The Properties of Alpha, Beta and Gamma

Up next

Ions produced by radiation carry a current

This demonstration uses a radioactive source to produce radiation that will ionise the air and complete a circuit to charge an electroscope. 

Ionising Radiation
Quantum and Nuclear

Ions produced by radiation carry a current

Practical Activity for 14-16

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.
  6. Getting a current to flow and charge the electroscope
  7. 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.
  8. 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.
  9. 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.
  10. Discharging the electroscope...
  11. You can also discharge the electroscope by ionising the air around it.
  12. 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).
  13. Point the sealed source over the top of the disc. This will ionise the air and allow the electroscope to discharge.
  14. Charge the electroscope again and try holding the sealed source at different angles and at different distances from the disc.
  15. 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

 

 

Up next

The spark counter

This demonstration is a highly visible way of showing and counting ionisation of the air caused by alpha radiation.

Ionising Radiation
Quantum and Nuclear

The spark counter

Practical Activity for 14-16

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

Up next

The Geiger-Muller tube

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

Ionising Radiation
Quantum and Nuclear

The Geiger-Müller tube

Practical Activity for 14-16

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:

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.
  3. Photograph courtesy of Mike Vetterlein
  4. Plug the Geiger-Müller tube into the scaler (counter) and switch on.
  5. Start the voltage at about 200 volts. Make a note of the number of counts in, say, a 15 second interval.
  6. Increase the voltage in steps of 25 volts.
  7. 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):
  8. 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.
  9. If the clicking increases when you increase the voltage, then you have moved off the plateau. Turn the voltage back down.
  10. 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

Up next

Diffusion cloud chamber

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.

Ionising Radiation
Quantum and Nuclear

Diffusion cloud chamber

Practical Activity for 14-16

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:

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

Up next

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.

Ionising Radiation
Quantum and Nuclear

Alpha radiation: range and stopping

Practical Activity for 14-16

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

Up next

Beta radiation: range and stopping

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

Ionising Radiation
Quantum and Nuclear

Beta radiation: range and stopping

Practical Activity for 14-16

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.

Up next

Beta radiation: deflection in a magnetic field

This demonstration focuses on the properties of beta particles.
You can show that beta radiation is deflected in a magnetic field; this is an impressive and striking demonstration.

Ionising Radiation
Quantum and Nuclear

Beta radiation: deflection in a magnetic field

Practical Activity for 14-16

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.
  6. Carrying out
  7. 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.
  8. 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.
  9. 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

Up next

Gamma radiation: range and stopping

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.

Ionising Radiation
Quantum and Nuclear

Gamma radiation: range and stopping

Practical Activity for 14-16

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

Up next

Gamma radiation: inverse square law

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.

Ionising Radiation
Quantum and Nuclear

Gamma radiation: inverse square law

Practical Activity for 14-16

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.
  5. Carrying out...
  6. Measure the background count with the source far away.
  7. Start with the gamma source 10 cm from the zero point.
  8. Increase the distance and take measurements of count rate at 20 cm, 30 cm, 40 cm, 60 cm and 80 cm.
  9. Correct the count rates for the background count.
  10. 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

Up next

Measuring the half-life of protactinium

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.

Exponential Decay of Activity
Quantum and Nuclear

Measuring the half-life of protactinium

Practical Activity for 14-16

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

Up next

Simple model of exponential decay

In this activity, students model radioactive decay using coins and dice.

Exponential Decay of Activity
Quantum and Nuclear

Simple model of exponential decay

Practical Activity for 14-16

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

Up next

Managing radioactive materials in schools

Teacher guidance on the legislating and regulations governing radioactive materials in schools

Ionising Radiation
Quantum and Nuclear

Managing radioactive materials in schools

Teaching Guidance for 14-16

Countries have national laws to control how radioactive materials are acquired, used and disposed of. These laws follow internationally agreed principles of radiological protection.

The following principles apply to schools:

  • There should be a person designated to be responsible for the security, safety and proper use of radioactive sources.
  • Sealed radioactive sources should be of a safe design and type suitable for school science.
  • Sealed sources should be used whenever possible in preference to unsealed sources. Unsealed sources can only be justified when the scientific demonstrations would not be practicable using sealed sources.
  • Records of all radioactive sources should be properly kept, showing what they are, when they were bought, when and by whom they have been used, and eventually, how they were disposed of.
  • Radioactive sources should be used only when there is an educational benefit.
  • Radioactive sources should be handled in ways that minimize both staff and student exposures.
  • Sealed sources should be carefully checked periodically to make sure they remain in a safe condition.
  • The school should have a suitable radioactivity detector in good working order.

UK regulation & guidance

Generally, school employers will insist you obtain their permission before acquiring new radioactive sources.

You must follow your employer’s safety guidance relating to the use the radioactive sources. Most school employers will require you to use either SSERC or CLEAPSS safety guidance, as follows:

In Scotland, safety guidance for use of radioactive sources in schools is issued by the Scottish Schools Equipment Research Centre (SSERC) and is available to members through their website.

In the rest of the UK and British Isles Crown Dependencies, guidance is available from CLEAPSS, the School Science Service. Their guidance document, L93, is freely available from their website, even to non-members.

In the UK...

  • In classes where children are under the age of 16, the use of radioactive material shall be restricted to demonstrations by qualified science teachers, (which includes newly qualified teachers). However, closer inspection of devices containing low-activity sources such as diffusion cloud chambers is permitted provided the sources are fully enclosed within the devices and not removed during the inspection.
  • Young persons aged 16 and over may use radioactive sources under supervision. Although the use of radioactive material is regulated, it should not be used as an excuse to avoid practical work. As the ASE points out, "Using the small sources designed for school science gives a good opportunity to show the properties of radioactive emissions directly, and to discuss the radiation risks. Just as importantly, it is an opportunity to review pupils' perception of risks, as they are likely to have constructed their own understanding from a variety of sources, including science fiction films and internet sites. If the work is restricted just to simulations, it may reinforce exaggerated perceptions of risk from low-level radiation.”

Summary of legislation (UK)

Updated October 2008

The following summarizes the somewhat complicated legislative framework in which schools are expected to work with radioactive sources in the UK. However, teachers do not need to obtain and study this legislation; this has been done by CLEAPSS and SSERC, and it is incorporated into their guidance in plain English.

In the European Union, member states have implemented the 1996 EU Basic Safety Standards Directive (as amended) that in turn reflects the 1990 International Commission on Radiological Protection recommendations. In the UK, this has been done through the Radioactive Substances Act 1993 (RSA93), which controls the security, acquisition and disposal of radioactive material, and the Ionising Radiations Regulations 1999 (IRR99) which controls the use of radioactive material by employers. Transport of radioactive material is controlled by The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2007.

There are exemptions from parts of the RSA93 and schools can make use of The Radioactive Substances (Schools etc.) Exemption Order 1963, The Radioactive Substances (Prepared Uranium and Thorium Compounds) Exemption Order 1962, and others. These exemption orders are conditional and to make use of them and avoid costly registration with the Environment Agency (or SEPA in Scotland, or the Environment and Heritage Service in Northern Ireland) you must adhere to the conditions. Note that currently, these exemption orders are being reviewed.

The way in which these laws are implemented in England, Wales, Northern Ireland and Scotland varies. The Department for Children, Schools and Families (DCSF) has withdrawn its guidance AM 1/92, and the associated regulations requiring this have been repealed. Consequently, purchase of radioactive sources by maintained schools in England is no longer regulated by the DCSF. The DCSF commissioned CLEAPSS to prepare and issue ‘Managing Ionising Radiations and Radioactive Substances in Schools, etc L93’ (September 2008) and has commended it to schools in England. Similar regulations relating to other educational institutions in the UK have not changed; English institutions for further education remain regulated through the Department for Innovation, Universities and Skills. Similarly, schools in Wales should follow the guidance from the Welsh Assembly Government Department for Children, Education, Lifelong Learning and Skills. Schools in Scotland should follow the guidance from the Scottish Government Education Directorate and associated guidance issued by SSERC. Schools in Northern Ireland should follow the guidance from the Department of Education Northern Ireland (DENI). The Crown Dependencies Jersey, Guernsey and Isle of Man are not part of the UK and schools and colleges should follow the guidance from their own internal government departments responsible for education.

In the UK, if an employer carries out a practice with sources of ionising radiations, including work with radionuclides that exceed specified activities (which is 100 kBq for Co-60, and 10 kBq for Sr-90, Ra-226, Th-232, Am-241 and Pu-239), the practice must be regulated according to the IRR99 and the employer must consult with a Radiation Protection Adviser (RPA). Since 2005, the RPA must hold a certificate of competence recognized by the Health and Safety Executive. Education employers are unlikely to have staff with this qualification, so the RPA will usually be an external consultant. Education employers need to notify the HSE 28 days before first starting work with radioactive sources. This is centralized at the HSE’s East Grinstead office.

Note: For higher risk work with radioactive material, the IRR99 requires designated areas, called controlled areas and supervised areas, to be set up if special procedures are needed to restrict significant exposure – special means more than normal laboratory good practice. It should never be necessary for a school to designate an area as controlled, and only in special circumstances would it be necessary to designate an area as supervised. The normal use of school science radioactive sources, including the use of school science half-life sources, does not need a supervised or controlled area.

Disposal of sources in the UK

Sources that become waste because they are no longer in a safe condition, or are no longer working satisfactorily, or are of a type unsuitable for school science, should be disposed of. In England and Wales, the Environment Agency has produced a guidance document through CLEAPSS that explains the available disposal routes. Similarly, SSERC has produced guidance for schools in Scotland. Schools in Northern Ireland should refer to DENI.

Health and safety statement

See the health and safety notes in each experiment. This is general guidance.

Health and safety in school and college science affects all concerned: teachers and technicians, their employers, students, their parents or guardians, and authors and publishers. These guidelines refer to procedures in the UK. If you are working in another country you may need to make alternative provision.

Up next

Making dry ice

Teaching guidance on different methods of making dry ice. 

Ionising Radiation
Quantum and Nuclear | Forces and Motion

Making dry ice

Teaching Guidance for 14-16

Solid carbon dioxide is known as dry ice. It sublimes at –78°C becoming an extremely cold gas. It is often used in theatres or nightclubs to produce clouds (looking a bit like smoke). Because it is denser than the air, it stays low. It cools the air and causes water vapour in the air to condense into tiny droplets – hence the clouds.

It is also useful in the physics (and chemistry) laboratory.

The Institute of Physics has kindly produced this video to explain how dry ice is formed.

Safety

Dry ice can be dangerous if it is not handled properly. Wear eye protection and gauntlet-style leather gloves when making or handling solid carbon dioxide.

Uses

Dry ice has many uses. As well as simply watching it sublime, you could also use it for cloud chambers, dry ice pucks, and cooling thermistors and metal wire resistors in resistance experiments. It can also be used in experiments related to the gas laws.

Obtaining dry ice

There are two main methods of getting dry ice.

1. Using a cylinder of CO2

It is possible to make the solid snow by expansion before the lesson begins and to store it in a wide-necked Thermos flask.

Remember that the first production of solid carbon dioxide from the cylinder may not produce very much, because the cylinder and its attachments have to cool down.

What type of cylinder, where do I get CO2 , and what will it cost?

A CO2 gas cylinder should be fitted with a dip tube (this is also called a ‘siphon type’ cylinder). This enables you to extract from the cylinder bottom so that you get CO2 in its liquid form, not the vapour.

NOTE: A plain black finish to the cylinder indicates that it will supply vapour from above the liquid. A cylinder with two white stripes, diametrically opposite, indicates it has a siphon tube and is suitable for making dry ice. A cylinder from British Oxygen will cost about £80 per year for cylinder hire and about £40 each time you need to get it filled up. (The refill charge can be reduced by having your chemistry department cylinders filled up at the same time.)

Don't be tempted to get a small cylinder, it will run out too quickly.

If the school has its own CO2 cylinder there will be no hire charge, but you will need to have it checked from time to time (along with fire extinguisher checks). Your local fire station or their suppliers may prove a good source for refills.

CLEAPSS leaflet PS45 Refilling CO2 cylinders provides a list of suppliers of CO2.

A dry ice attachment for the cylinder

Dry ice disks can be made using an attachment that fits directly on to a carbon dioxide cylinder with a siphon tube. Section 13.3.1 of the CLEAPSS Laboratory Handbook explains the use of this attachment (sometimes called Snowpacks or Jetfreezers). This form is most useful for continuous cloud chambers and low-friction pucks.

You can buy a Snowpack dry ice maker from Scientific and Chemical. The product number is GFT070010.

2. Buying blocks or pellets

Blocks of solid carbon dioxide or granulated versions of it can be obtained fairly easily with a search on the Internet. Local stage supply shops or Universities may be able to help. It usually comes in expanded foam packing; you can keep it in this packing in a deep freeze for a few days.

The dry ice pellets come in quite large batches. However, they have a number of uses in science lessons so it is worth trying to co-ordinate the activities of different teachers to make best use of your bulk purchase.

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Health and safety statement

Health and Safety statement

Teaching Guidance for 14-16 16-19

Health and safety in school and college science affects all concerned: teachers and technicians, their employers, students, their parents or guardians, as well as authors and publishers. These guidelines refer to procedures in the United Kingdom. If you are working in another country you may need to make alternative provision.

See the health and safety notes in each experiment. This is general guidance.

Health & safety checking

As part of the reviewing process, the experiments on this website have been checked for health and safety. In particular, we have attempted to ensure that:

  • all recognized hazards have been identified,
  • suitable precautions are suggested,
  • where possible, the procedures are in accordance with commonly adopted model (general) risk assessments,
  • where model (general) risk assessments are not available, we have done our best to judge the procedures to be satisfactory and of an equivalent standard.

Assumptions

It is assumed that:

  • the practical work is carried out or supervised by a qualified science teacher with adequate knowledge of physics and the equipment used,
  • practical work is conducted in a properly equipped and maintained laboratory,
  • rules for student behaviour are strictly enforced,
  • equipment is regularly inspected and properly maintained,
  • with appropriate records are kept,
  • care is taken with normal laboratory operations such as heating substances and handling heavy objects,
  • good laboratory practice is observed,
  • eye protection is worn whenever risk assessments require it,
  • hand-washing facilities are readily available in the laboratory.

Teachers' and their employers' responsibilities

Under the Health and Safety at Work Act and related Regulations, UK employers are responsible for making a risk assessment before hazardous procedures are undertaken or hazardous materials are used. Teachers are required to co-operate with their employers by complying with such risk assessments. However, teachers should be aware that mistakes can be made and, in any case, different employers adopt different standards.

Therefore, before carrying out any practical activity, teachers should always check that what they are proposing is compatible with their employer’s risk assessments and does not need modification for their particular circumstances. Any rules or restrictions issued by the employer must always be followed, whatever is recommended here. However, far less is banned by employers than is commonly supposed. Be aware that some activities, such as the use of radioactive material, have particular regulations that must be followed.

Reference material

Model (general) risk assessments have been taken from, or are compatible with:

  • CLEAPSS


  • ASE Safeguards in the school laboratory 11th edition 2006
  • ASE Topics in Safety 3rd edition, 2001
  • ASE Safety reprints, 2006 or later

Procedures

Clearly, you must follow whatever procedures for risk assessment your employers have laid down. As far as we know, almost all the practical work and demonstrations on this website are covered by the model (general) risk assessments detailed in the above publications, and so, in most schools and colleges, you will not need to take further action, other than to consider whether any customisation is necessary for the particular circumstances of your school or class.

Special risk assessments

Only you can know when your school or college needs a special risk assessment. But thereafter, the responsibility for taking all the steps demanded by the regulations lies with your employer.

External websites

The Institute of Physics are not responsible for the content of external websites which may be linked from this website's pages.

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Radioactive sources: isotopes and avaliability

Teaching guidance covering the different isotopes that may be available in your school.

Ionising Radiation
Quantum and Nuclear

Radioactive sources: isotopes and availability

Teaching Guidance for 14-16

In the UK, education suppliers stock only these three isotopes in sealed sources:

cobalt-60pure gamma (provided the low energy betas are filtered out)
strontium-90pure beta
americium-241alpha and some gamma

They are shown with the radiations that they emit.

However, you may have other sources in your school or Local Authority and, as long as you follow your school safety policy and local rules, you can use these in schools. The ones that are useful for practical work are:

radium-226alpha, beta and gamma
plutonium-239pure alpha
caesium-137beta, then gamma (from its decay product, metastable Ba-137)

For safety information:

Managing radioactive materials in schools


Up next

Using an electroscope

Teaching guidance on the use of an electroscope. 

Ionising Radiation
Quantum and Nuclear

Using an electroscope

Teaching Guidance for 14-16

A gold leaf electroscope measures potential difference between the leaf and the base (or earth).

The leaf rises because it is repelled by the stem (support). The leaf and its support have the same type of charge. A typical school electroscope will show a deflection for a charge as small as 0.01 pC (the unit pC is a pico coulomb, 1 × 10-12 coulombs, equivalent to the charge on over 6 million electrons).

Charging an electroscope

There are a number of ways of charging an electroscope. They include:

Charging by contact. Rub an insulator to charge it up. Then stroke it across the top plate of the electroscope. This will transfer charge from the insulator to the electroscope. This method is direct and clear to students. However, the charge left on the electroscope will not always leave it fully deflected.

Charging by induction. This is a quick way to get a larger charge onto the electroscope. However, it can look a bit magical to students. So it should be used with some care.

Rub an insulator to charge it up. Bring it close to the top plate of the electroscope – but don’t let it touch. This will induce the opposite charge on the plate of electroscope leaving a net charge on the gold leaf, which will rise. Now touch the plate with you finger momentarily to earth it (still holding the charged insulator near the top plate). The charge on the top plate will be neutralised but there will still be a charge on the gold leaf. Let go of the plate and then take the charged insulator away. The charge that had been pushed down to the gold leaf will now redistribute itself over the plate and the leaf, leaving the whole thing charged. The leaf will show a good deflection.

Charging with an EHT or Van de Graaff generator. You can use a flying lead connected to one of these high voltage sources to charge up the gold leaf electroscope. This is quick, effective and obvious to students. The other terminal of the supply should be earthed. Connect the flying lead to the supply through a safety resistor.

Detecting small currents

The electroscope can be used to demonstrate that a small current is flowing in a circuit – for example in experiments to show the ionisation of the air.

Using the hook rather than the plate makes the electroscope more sensitive to small amounts of charge. A charge of around 0.01 pC will cause a noticeable deflection of the gold leaf. So it is possible to watch it rise (or fall) slowly due to a current as small as 1 pA.

Put the electroscope in series (as though it were an ammeter). Any charge that flows in the circuit will move onto the electroscope making the gold leaf rise. You may need to discharge the electroscope when you first switch on the power supply because there will be an initial movement of charge due to the capacitance in the circuit.

Alternatively, you can use the electroscope as a source of charge and watch it discharge. It is like a capacitor with its own display. Charge it up and then connect it into a circuit. If the circuit conducts, the electroscope (capacitor) will discharge and, at the same time, the leaf will display how much charge is left.

Using the electroscope as a voltmeter or electrometer

The electroscope has a very high (as good as infinite) resistance. If you earth the electroscope case, the electroscope measures potential so it is well suited to detecting potentials in electrostatic experiments. Without earthing, the quantity it is measuring is charge. This is related to p.d. (by its capacitance C , i.e. V = Q/C ). But it isn’t the same as p.d. because the capacitance can vary a lot – even during an experiment. Capacitance depends on the position of the electroscope, people nearby and so on.

So although the electroscope is useful as an indication of a voltage, it isn’t a reliable means of measuring it.

Cosmic radiation

School electroscopes are open to the air (more refined ones are in a vacuum). Cosmic radiation will ionise this air and cause a small leakage current. So the electroscope will discharge over time. Historically, the discharging of electroscopes led to the suggestion of the existence of cosmic radiation. Victor Hess and Carl Anderson shared the Nobel Prize for Physics in 1936, for discoveries related to cosmic radiation. The Nobel Award ceremony speech describes their work:

Nobel Prize in Physics 1936 Award ceremony speech


Up next

First models of the atom

Teaching guidance on the models students may have of the atom and how this may develop. 

Plum Pudding Model
Quantum and Nuclear

First models of the atom

Teaching Guidance for 14-16

As students start experiments on ionisation, they will have a fairly basic model of atoms and molecules, as portrayed by the simple kinetic theory. They will know that solids, liquids and gases are made up of atoms and molecules. They may think of these as round blobs with no internal structure. These particles exert attractions on each other at short ranges of approach and, necessarily, repulsions at very short range. They bounce off each other in elastic collisions (energy in the kinetic store is conserved) – more advanced students may understand that this is because the forces are the same on the way in as they are on the way out.

They will have heard of ions – probably in the context of chemical reactions, solutions and electrical conductivity. However, using ions to explain sparks may be a new idea. Ionisation and sparks show that electrons are easily knocked off neutral atoms and molecules. In these collisions, energy is not conserved – some of it is lost to remove the electrons. So the collisions are inelastic. This shows that the energies needed to remove electrons are of the order of the energy of a very fast moving particle (a few 100 m/s).

Their picture of the atom will develop. They will learn that it contains electrons, which are fairly easily detached. There must also be some positive material, probably holding most of the mass of the atom. The atom is held together in some unknown way.

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How clouds form

Teaching guidance explaining how clouds form. 

Ionising Radiation
Quantum and Nuclear

How clouds form

Teaching Guidance for 14-16

Clouds form in the atmosphere when warm, wet air is pushed up. As the air goes higher its pressure decreases and it gets colder. The colder air cannot contain so much water vapour and the air becomes supersaturated with vapour. Given the right conditions, some of the vapour condenses to form water droplets in a cloud.

The droplets need something to form around. An extremely small droplet of just a few molecules cannot form a cloud all by itself. Its molecules would escape from each other again. If the air contains attractive particles (usually minute particles of salt) that the water can wet, a droplet will start to form. The salt particles serve as condensation nuclei on which larger drops can grow.

A similar process is put to use in a cloud chamber containing alcohol vapour. Ions in the air can serve as excellent condensation nuclei. The alcohol molecules are electrically ‘oblong’ with positive and negative charges at the ends, so they can cluster easily around a charged particle. The other small particles can be cleared out. This stops droplets forming into unwanted clouds (even when the air is supersaturated). Instead, the droplets form on the trail of ions left behind by ionising radiation, typically an alpha particle.

Up next

Alpha-particle tracks

Teaching guidance on Alpha-particle tracks in a cloud chamber.

Ionising Radiation
Quantum and Nuclear

Alpha particle tracks

Teaching Guidance for 14-16

Nuclear bullets from radioactive atoms make the tracks in a cloud chamber. They hurtle through the air, wet with alcohol vapour, detaching an electron from atom after atom, leaving a trail of ions in their path. Tiny drops of alcohol can easily form on these ions to mark the trail.

The trail of ions is made up of some ‘air molecules’ that have lost an electron (leaving them with a positive charge) and some that have picked up the freed electrons, giving them a negative charge.

There is no sighting of the particle which caused the ionisation, because it has left the ‘scene’ before the condensation happens. If you count the number of droplets an alpha particle might produce 100,000 pairs of ions by pulling an electron from 100,000 atoms.

When the alpha particle has lost all its energy in collisions with the ‘air molecules’ it stops moving and is absorbed.

Up next

Evidence for the hollow atom

Teaching guidance on evidence for the hollow atom.

Ionising Radiation
Quantum and Nuclear | Forces and Motion

Evidence for the hollow atom

Teaching Guidance for 14-16

The main and first evidence for the hollow atom came from...

Rutherford's alpha scattering experiment


However, the first evidence students see for a hollow-atom often comes from cloud-chamber photographs. Although this may be historically back to front, it is reasonable to use the cloud chamber photographs as the first indication that atoms are mainly empty.

Chronology of evidence

Rutherford had devised his model of a nuclear atom by 1910, before alpha particle tracks were photographed in cloud chambers (c1911). However, Rutherford and Wilson worked in the same laboratory so it is likely that Rutherford had seen tracks in cloud chambers.

The evidence provided by cloud chamber photographs and the inferences that can be made are extremely useful whether you present them as preparation for the Rutherford model or follow-up support for it.

Evidence from cloud chambers

Most of the time there is just a straight track produced when an alpha particle passes through the cloud-chamber, producing ions. Mostly, these ions are produced by inelastic collisions with electrons in neutral particles. An alpha particle will have around 100,000 inelastic collisions before all of the energy has been shifted from it's kinetic store. The number of collisions shows that electrons are easily removed.

The straightness of the tracks shows that:

  • an electron has a mass that is much smaller than the mass of an alpha particle (now known to be about 7000 times smaller).
  • the atom is hollow: each straight track represents about 100,000 collisions without any noticeable deviation. All of these collisions missed anything with significant mass. During a session, the class might observe 1000 tracks between them – all of which are straight.

Therefore, in all of these 100 million collisions with atoms, the alpha particles never hit anything with significant mass. So most of the atom is empty.

However, students will see photographs that show large deflections of alpha particles. These are rare events (requiring thousands of photographs to be taken). They show that:

  • there is something in an atom that has a mass that is similar to the mass of an alpha particle; only a target with a comparable mass could cause a large deviation.
  • this mass is very concentrated; the rareness of the forked tracks shows that most alpha particles miss this massive target.

Evidence from alpha particle scattering

The hollowness of the atom is treated more quantitatively in the Rutherford scattering experiment. In this, 99.99% of the alpha particles are undeflected. This gives an indication of how tightly the positive charge of the nucleus is packed together.

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Classroom management in semi-darkness

Interference
Light Sound and Waves

Classroom management in semi-darkness

Teaching Guidance for 14-16

There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.

Shadows on the ceiling will reveal movements that are not in your direct line of sight.

Up next

The nature of ionising radiation

Students' models of each of the radiations will develop through this topic. They will start with an idea of a generalized invisible radiation. As they see more evidence for the nature of the radiations, their model will become more sophisticated.

Ionising Radiation
Quantum and Nuclear

Nature of ionising radiations

Teaching Guidance for 14-16

Students' models of each of the radiations will develop through this topic. They will start with an idea of a generalized invisible radiation. As they see more evidence for the nature of the radiations, their model will become more sophisticated. This will be reflected in the developing language that you use to describe the radiations:

  • the radiations come from radioactive materials and cause ionisation: they are ionising radiations.
  • natural radioactive materials produce three types of ionising radiation: alpha radiation, beta radiation and gamma radiation.
  • alpha radiation and beta radiation are made up of streams of charged particles, alpha particles and beta particles; gamma radiation is an electromagnetic wave.
  • an alpha particle is a helium ion (an atom that has lost two electrons), He2+; a beta particle is a fast moving electron, e-.
  • an alpha particle is a helium nucleus (because it only has two electrons per atom); all three radiations originate in the nuclei of atoms.

Eventually, the properties and nature of alpha, beta and gamma radiations can be summarized as follows.

alphabetagamma
propertyhighly ionisingfairly ionisingweakly ionising (depends on intensity)
short range in air (3 to 5 cm)medium range in air (~15 cm)long range (inverse square law)
stopped by paperstopped by lead or thick aluminiumattenuated by thick lead
deflected slightly in magnetic fielddeflected in magnetic fieldUndeflected in electric and magnetic fields
deflected in electric fielddeflected in electric field
naturepositive chargenegative chargeno charge
large mass (same as helium nucleus)small mass
identityhelium nucleusfast moving electronhigh frequency electromagnetic wave

At each stage in this developing picture, you can link the properties of the type of radiation with its nature. Alpha radiation is highly ionising because of the large momentum, though relatively modest speed (~10 7m/s) of the alpha particles and their double positive charge. But, given its propensity to interact with atoms (in the air and solids), it has a shorter range and lower penetrating power than the other two types of radiation.

Beta radiation is made up of a stream of beta particles moving extremely fast (about 98% the speed of light). They have less momentum than alpha particles and are less ionising, tending to pass through the air and matter more easily than alpha particles.

Beta particles are noticeably deflected in a magnetic field, much more so than alpha particles, whose deflection cannot easily be measured in a school laboratory. This is because the beta particles have a smaller momentum and experience a bigger force because they are moving faster (although they also have a smaller charge, their speed is more than twice as much as that of an alpha particle).

The deflection of alpha particles can be more noticeable in an electric field. Here the force depends on the charge but not on the speed.

Gamma radiation is an electromagnetic wave. This means it has no charge and is not deflected by magnetic or electric fields. It is weakly ionising and its effects on matter depend among other factors on the intensity of the radiation.

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The chemistry of the protactinium generator

Teaching guidance on the protactinium generator.

Exponential Decay of Activity
Quantum and Nuclear

Protactinium generator

Teaching Guidance for 14-16

When using the protactinium generator you will also need a:

  • Small polypropylene bottle (30 ml capacity)
  • Separating funnel or beaker
  • Uranyl nitrate (or uranium oxide dissolved in nitric acid)
  • Concentrated hydrochloric acid, 7 ml
  • Iso-butyl methyl ketone, or amyl acetate
  • Tray lined with absorbent paper

The chemistry of the protactinium generator

The first steps 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.

Radioactivity is a property of the innermost nucleus of the atom, so 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:

• 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).

• 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 thoruim is also constantly topped up in the aqueous layer as long as the protactinium is present with the thoruim.

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Developing a model of the atom: radioactive atoms

Initially, students may regard atoms as the fundamental chemical particles. True, electrons can be chipped off an atom, and possibly all an atom’s electrons stripped off to leave a bare nucleus; yet according to the simple story, the nucleus is still fixed and determines the element by its charge, Ze.

Therefore, to change one element into another, the alchemist’s dream of lead into gold, would require a change of nuclear charge. At first sight this seems impossible because the nucleus is buried deep in the atom bound together by tremendous forces. But it does happen in radioactive elements.

Exponential Decay of Activity
Quantum and Nuclear

Developing a model of the atom: radioactive atoms

Teaching Guidance for 14-16

Initially, students may regard atoms as the fundamental chemical particles. True, electrons can be chipped off an atom, and possibly all an atom’s electrons stripped off to leave a bare nucleus; yet according to the simple story, the nucleus is still fixed and determines the element by its charge, Ze.

Therefore, to change one element into another, the alchemist’s dream of lead into gold, would require a change of nuclear charge. At first sight this seems impossible because the nucleus is buried deep in the atom bound together by tremendous forces. But it does happen in radioactive elements.

Soon after the discovery of radioactivity in 1896 by Becquerel, Marie Curie and her husband Pierre discovered a new element which they named radium. They extracted dangerously large samples of radium from vast quantities of rock and experimented on its radioactive behaviour.

You could say: Radioactive atoms do not just stay there as atoms of ordinary copper do; they are completely different: they are unstable, they suddenly break up, flinging out a particle such as an alpha particle, becoming an atom of a different element.

A radium atom remains a radium atom, with the chemical behaviour of a heavy metal, until it suddenly hurls out this alpha particle. (The alpha particle has such a huge energy that it must come from the nucleus.) The remainder of the radium atom is no longer a heavy metal, but a quite different element. This ‘daughter’ of radium is an atom of a heavy inert gas, the end of the helium, neon, argon, krypton, xenon series. It is called radon. The atomic masses have been measured directly, radium-226, radon-222 (a difference of 4 suggesting that the lost alpha particle is a helium nucleus). Separate measurements confirm this.

When you have a mixture of a parent element and a daughter element which have different chemical properties, then they can be separated by ordinary chemical methods.

Radon gas is itself unstable and radioactive. Each of its atoms suddenly, at an unpredictable moment, hurls out an alpha particle. The remainder is a new atom, very unstable, which is called polonium, the ‘daughter’ of radon and the ‘granddaughter’ of radium. The series continues through several more radioactive elements and stops at a stable form of lead. The series does not begin with radium: it begins with uranium several stages earlier. Radioactive uranium (Z=92) has turned into lead (Z=82).

Making unstable atoms

A century ago, radioactivity was a peculiarity of a few mostly heavy, elements: the last few at the end of the Periodic Table. Nowadays scientists can bombard samples of lighter elements with high speed, high energy protons or neutrons, provided directly or indirectly by an accelerator. They can make unstable isotopes of every element in the periodic table. This has opened up the field of nuclear chemistry. Radioactive isotopes behave chemically like their stable isotopes and can be mixed with them. Their progress as radioactive tags can be traced, like luggage labels, following the progress of a ‘labelled’ isotope through the human body or an industrial process.

Half-lives

All the unstable members of these strange families have a constant, reliable characteristic: the atoms show no signs of ageing, or growing weaker, however long they last. Each radioactive element has a constant chance of breaking up in each succeeding second. This is described by a useful length of time, the ‘half-life’ of the radioactive element. For each individual atom the betting is 50:50 for and against its breaking up at any time during one half-life from now. The break-up seems to be controlled by pure chance. That chance does not change and make the break-up more likely for atoms that need to survive longer.

For radium the half-life is 1650 years. Start with 1000 mg of radium now and 1650 years later you will have only 500 mg left. After a further 1650 years only 250 mg will be left and so on. For radium’s daughter, radon -222, the half-life is 3.8 days. In less than four days half the radon gas will have disappeared. You will find helium gas there instead, with the solid products.

The instability appears to be something inherent in the nuclear structure. Nowadays, taking a wave view of the behaviour of nuclear particles, you can picture a stationary wave pattern defining the life of an alpha particle inside the nucleus. But the wave is not completely confined, it leaks through the potential barrier round the nucleus and runs on as a faint wave outside. The wave is interpreted as describing probabilities of locations. It is not a mechanical wave carrying energy and momentum.

While the alpha particle is expected to be found inside the nucleus, there is a chance of finding it one day outside, despite what would seem an insurmountable potential wall. That chance of the alpha particle being outside, being emitted, is definite and constant, a part of the defining wave property, as long as the nucleus lasts. It suggests that high energy alpha particles go with a short half-life of the parent nucleus.

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Exponential decay of a radioactive substance

Teaching guidance discussing the exponential decay of radioactive substances and the mathematical implications.

Exponential Decay of Activity
Quantum and Nuclear

Exponential decay of a radioactive substance

Teaching Guidance for 14-16

One of the most important characteristics of radioactivity is that it decays exponentially. This has two basic mathematical implications at this level.

  1. The rate falls by a constant ratio in a given time interval. The time it takes to fall by a half is always the same. It also falls to a tenth in equally regular, but longer, time intervals.
  2. The rate of decay is proportional to the amount that is left. This can be seen in the experiment to model radioactive decay. The number of coins that decay in any ‘shake’ is proportional to the number that is left.
  3. Radioactive decay experiment


From these features, you can argue, respectively, the following points.

  • The chance of an atom disintegrating is constant in time. Radioactive decay is a series of many chance events, all with an unalterable chance.
  • The rate of disintegrations is proportional to the total number of unchanged radioactive atoms at that moment. Both the rate and the stockpile itself die away exponentially with the same characteristic half-life.

Up next

Some useful equations for half-lives

Teaching guidance covering some useful equations on half-lives and exponential decay. 

Exponential Decay of Activity
Quantum and Nuclear

Some useful equations for half-lives

Teaching Guidance for 14-16

The rate of decay of a radioactive source is proportional to the number of radioactive atoms (N) which are present.

dNdt = -λN

is the decay constant, which is the chance that an atom will decay in unit time. It is constant for a given isotope.

The solution of this equation is an exponential one where N0 is the initial number of atoms present.

N = N0e -λt (Equation 1)

Constant ratio

This equation shows one of the properties of an exponential curve: the constant ratio property.

The ratio of the value, N1, at a time t1 to the value, N2, at a time t2 is given by:

N1N2 = e -λt1e -λt2

N1N2 = e -λ(t1-t2)

In a fixed time interval, t2t1 is a constant. Therefore the ratio

N1N2 = a constant

So, in a fixed time interval, the value will drop by a constant ratio, wherever that time interval is measured.

Straight line log graph

Another test for exponential decay is to plot a log graph, which should be a straight line.

Since

N = N0e -λt

Taking natural logs of both sides:

lnN=lnN0-λt

Therefore a graph of N against t will be a straight line with a slope of QuantitySymbol{-λ}.

Half-life and decay constant

The half-life is related to the decay constant. A higher probability of decaying (bigger λ) will lead to a shorter half-life.

This can be shown mathematically.

After one half life, the number, N of particles drops to half of N0 (the starting value). So:

N = N02 when t=T½

By substituting this expression in equation (see above),

N02 = N0e -λT½

Taking natural logs of both sides gives:

ln½=-λT½

ln2 = +λT½

T½ = 0.693λ

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