Ionising Radiation
Quantum and Nuclear

Cloud chambers

for 14-16

Cloud chambers provide a satisfying and captivating activity for students, allowing them to see the ionisation trails left by alpha particles and sometimes cosmic rays. This collection provides some preparatory demonstrations and some follow-up activities to support the main event: the class practical with diffusion cloud chambers. It is well worth setting aside plenty of time to allow a class to experiment with cloud chambers. 
 
There are some good opportunities here for recording and displaying results using digital stills and movie cameras. 

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Jet of steam from a boiling flask

Ionising Radiation
Quantum and Nuclear

Jet of steam from a boiling flask

Practical Activity for 14-16

Demonstration

This demonstration shows that ions can act as condensation nuclei in a supersaturated vapour. This idea is used to reveal the tracks of alpha particles in both the expansion-type cloud chamber and the diffusion cloud chamber. This is useful preparation for both of these experiments.

Apparatus and Materials

  • Van de Graaff generator


  • Flask with bung and glass tube, 500 ml
  • Glass tube drawn to a jet
  • Bunsen burner
  • Tripod
  • Connecting leads
  • Flexicam or webcam linked to a projector (optional)
  • Compact light source and power supply (optional)

Health & Safety and Technical Notes

There is a risk of scalding from steam during this demonstration. Ensure the end of the steam tube is clamped in a suitable position to minimize this risk.

Read our standard health & safety guidance


You could use an EHT supply at 5,000 V rather than a Van de Graaff generator. However, the spark is not as impressive (due to the higher capacitance of the Van de Graaff). Two strips of Perspex may be needed as extra insulation at the place where the wires are clamped.

Like a boiling kettle, the white cloud that you can see consists of droplets of condensed water vapour formed when the water vapour meets the cold air.

The transparent gap between the jet nozzle and the white cloud contains the super-saturated water vapour.

Procedure

Setting up

  1. Keep the nozzle about 1.5 m away from the Bunsen flame otherwise the flame will spoil the demonstration because it makes so many ions itself.
  2. Arrange two wire electrodes to form a spark gap about 0.5 mm wide and about 0.3 mm above the nozzle. This will ensure it is in the water vapour and not in the white cloud.
  3. To make the effect clearly visible to the class:
  • Either project a shadow onto the wall or a screen, using the compact light source at a distance of 1 m from the nozzle.
  • Or use a flexicam or webcam connected through a computer to a projector or Interactive whiteboard.

Carrying out the demonstration

  1. Boil the water in the flask and observe the cloud formation as the vapour emerges from the jet.
  2. Highlight the gap between the cloud and the nozzle.
  3. Switch the Van de Graaff generator on so that a stream of small sparks passes through the vapour jet. The cloud will be seen to intensify due to the production of ions which act as condensation nuclei.

Teaching Notes

  • Draw attention to the gap between the cloud and the nozzle. Right next to the nozzle, this is water vapour – i.e. the gaseous state of water. As it moves a few millimetres from the nozzle, it cools down and becomes a supersaturated vapour. It then starts to condense into water droplets forming the white cloud.
  • In the region where it is a supersaturated vapour, the water molecules are ready to condense. The ions act as condensation nuclei on which the water molecules can condense. This produces a condensation trail.

This experiment was safety-tested in August 2007

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Making a cloud by expansion

Ionising Radiation
Quantum and Nuclear

Making a cloud by expansion

Practical Activity for 14-16

Demonstration

This shows the principle of cooled, supersaturated air producing clouds. It is more directly relevant to the expansion (Wilson) cloud chamber than the diffusion cloud chamber, in which the method of cooling the air is different. However, the basic ideas are the same and this experiment is a good lead in to either.

Apparatus and Materials

  • Aspirator or large flask, 10 litres
  • Bung to close lower outlet of aspirator
  • Bung with glass tube
  • Short length of rubber tubing
  • Compact light source and power supply
  • Matches

Health & Safety and Technical Notes

Read our standard health & safety guidance


The air inside the aspirator contains water vapour. When the bung is removed, the expansion causes the air to cool and a cloud is formed. This is similar to cloud formation when air is forced upwards into the atmosphere.

Procedure

Setting up

  1. For the cloud to be clearly visible by the class, it needs bright lighting from the side and a very dark background, in a partially dark room (see teaching notes below). Set up the black card behind the aspirator and illuminate it from the side using the compact light source.
  2. Put a few cm3 of water in the aspirator. Close with the bung and tubing attached.

Carrying out the demonstration

  1. Blow down the rubber tubing to raise the pressure inside, and then pinch the end of the tubing. Wait about a minute to allow the air to cool down again.
  2. Pull the bung out sharply. This allows the air to expand rapidly. You should see a cloud in the aspirator.
  3. After the cloud has formed, replace the bung and tube. Blow into the bottle in order to show the cloud disappearing.
  4. Repeat the procedure to produce another cloud. This time allow the cloud to settle.
  5. After you have produced a few clouds and allowed them to settle, they will begin to look thinner and may stop appearing. This is because the clouds are cleaning the air inside the aspirator and any dust is settling to the bottom. The number of condensation nuclei in the air is reduced.
  6. Once the clouds stop appearing, throw a lighted match into the aspirator. You will then see good clouds again. The match provides ions and smoke particles to act as condensation nuclei.

Teaching Notes

  • Make the point that it is the cooling of the air that makes it supersaturated and ready to form a cloud.
  • The presence of condensation nuclei allows the cloud to form.
  • The smoke particles act as condensation nuclei. This is similar to the smogs that occurred in London up to the 1950s and 1960s, with the worst ‘pea souper’ in 1952 leading to the Clean Air Act (in 1956).
  • Read:

    Classroom management in semi-darkness


This experiment was safety-tested in April 2006

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Expansion cloud chamber

Ionising Radiation
Quantum and Nuclear | Forces and Motion

Expansion cloud chamber

Practical Activity for 14-16

Demonstration

This is a useful demonstration to introduce students to what they can expect to see in the diffusion cloud chamber, and possibly to see forked paths resulting from collisions.

Apparatus and Materials

  • Expansion cloud chamber
  • Source of alpha radiation (if it is not part of the cloud chamber)
  • EHT power supply (NOT an HT one)
  • Large forceps or pliers, if required
  • Bicycle pump or other device for producing expansion
  • Illuminant (lamp, lens and power supply)

Health & Safety and Technical Notes

See guidance note:

Radioactive sources (UK guidelines)


The radioactive sources supplied with some expansion cloud chambers screw into the base of the chamber and are moderately strong. Handle them with large forceps or pliers.

Read our standard health & safety guidance


Various types of expansion cloud-chamber are available commercially. They differ greatly in effectiveness and clarity for small groups of students so it's advisable to try before buying. Much depends on the illumination. Some require alcohol, but those which work with water are preferable.

The above diagrams show a Princeton University design used (i) for a few students viewing directly; or (ii) for projecting a large image by an overhead projector.

The above diagram shows another design.

All types need an electric field to sweep away ions left by earlier events. In each case the manufacturer's instructions should be followed carefully.

Expansion cloud chambers are relatively expensive so schools are unlikely to have more than one.

By contrast, the Taylor diffusion cloud chamber is inexpensive and can be used for class experiments. Also, it maintains a constant state of supersaturation:

Taylor diffusion cloud chamber


Procedure

  1. Expansion cloud chambers are all slightly different in their operation, and you will need to refer to the manufacturer's instructions. The position of the lamp is often critical.
  2. Some have an evacuation mechanism, such as a bicycle pump, which removes the air whilst others change the pressure of a water column.

Teaching Notes

  • The tracks produced in cloud chambers are always fascinating and a forked track even more exciting. If the cloud chamber is filled with helium gas then alpha particles will produce a 90° forked track. Alpha particles in a chamber filled with air will be deflected at more than 90° (oxygen and nitrogen have almost the same atomic mass, about four times that of alpha particles).
  • Display collections of cloud-chamber photographs, keeping them on view for some time. Include examples of fork-tracks resulting from collisions and in particular an example of a 90° fork of an alpha-particle collision with a helium nucleus.
  • The first cloud chamber was invented in 1895 at the Cavendish Laboratory by the Scottish physicist CTR Wilson. He was trying to imitate the way that clouds formed as wet air cooled on expansion. Initially, he used it to examine clouds and condensation rather than to track ionising radiation. He invented it to save him having to climb up to the observatory on the top of Ben Nevis. It wasn’t until 1911 that he developed the cloud chamber as a measuring device and photographed the paths of alpha and beta particles. For this he was awarded the Nobel Prize in 1927.
    • The Wilson cloud chamber works by expanding a fixed volume of wet air. The air cools as it expands, forming a supersaturated vapour. The vapour will condense into droplets if it is provided with condensation nuclei, such as ionised air molecules. In this way, the cloud chamber produces a visible trail of droplets left behind by, for example, an ionising alpha particle.
    • The air will soon warm up again. So the results are fleeting. For this reason, the Wilson cloud chamber is sometimes known as the pulsed cloud chamber.
    • Cloud chambers and even their successors, bubble chambers, are now no more than historic curiosities. Analysis of cloud chamber photographs assumes that momentum is conserved in nuclear events. A uniform magnetic field applied to a cloud chamber perpendicular to the direction of motion of the charged particles produces tracks which are bent into a circle. From this, the momentum of the particle can be calculated and hence its velocity if its mass is known.
    • The kinetic energy before and after collision can then be calculated and if it is not conserved either it was a rare inelastic collision or the target nucleus was wrongly identified (due to impurities or the liquid used to make the drops). If it was an inelastic collision then a nuclear transformation has been effected by the alpha particle.

This experiment was safety-tested in April 2006

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Diffusion cloud chamber

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

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Display of cloud chamber photographs

Ionising Radiation
Quantum and Nuclear

Display of cloud chamber photographs

Practical Activity for 14-16

Demonstration

Displays of cloud chamber photographs are very worthwhile as a focus for discussion. As well as the straight alpha particle tracks, you can show other types to illustrate the power of the cloud chamber in the early days of studying radioactivity. Photographs showing collisions with a fork in them should provoke questions from the students as to what is happening.

You can set this up as a wall display or use digital images and a projector.

Apparatus and Materials

  • Some photographs of cloud chamber tracks

Health & Safety and Technical Notes

Read our standard health & safety guidance


Even at the beginning of the twentieth century, a few stout-hearted critics maintained that the existence of atoms was not proven. It was cloud chamber pictures (in around 1911) that convinced them that there are atoms. The forks showed that there were collisions between individual particles.

Procedure

The aim is to get students to look at and discuss the cloud chamber photographs. There are a number of ways you could achieve this.

Here are some possibilities:

  1. Print out sets of the photographs to give to groups of students. Ask them to look at the pictures, and arrange them according to some criteria. This could be the type of radiation, or it could be something more abstract like how easily they could explain what is happening, or how interesting the photograph looks.
  2. Ask students to match captions to photographs. This could be done in groups, or you could give each photograph a number and pin them up around the room. Put the captions on a work sheet or a projector slide, and ask students to move around the display and match the photographs to the captions.
  3. Make a wall display or projector slide with four photographs on it, and ask students which one they think is the odd one out. There need not be a right answer – the idea is to elicit discussion in groups or in the class.
  4. Bubble chamber photographs or modern electronically produced pictures could be added to the display.
  5. You could use a PowerPoint file rather than a wall display.

Teaching Notes

  • Although students may see these cloud chamber photographs before they meet the Rutherford scattering experiment, Rutherford’s experiment came first. He did not have the evidence from these photographs when he developed his model of a hollow atom with a tiny, massive nucleus.
  • The paths of nearly all the alpha particles do not bend, despite their many collisions. Therefore, they must be much more massive than the electrons which they pull off. In these collisions, the alpha particle passes through the hollow part of the atom and is nowhere near what is now called the nucleus.
  • Very rarely, an alpha particle collides with something more massive (Rutherford’s nucleus) producing a forked track. The rarity of these events is an indication that the nucleus is much smaller than the atom.
  • In collisions with a hydrogen nucleus, the alpha particle and the hydrogen nucleus both continue forwards. This indicates that the alpha particle has more mass than the hydrogen nucleus. In collisions with nitrogen and other heavy atoms, the alpha particle sometimes bounces backwards, showing a collision with something more massive. In collisions with helium nuclei, the fork shows a 90° angle, which indicates an elastic collision with an object of equal mass.
  • Bubble chamber photographs or modern electronically produced photographs can be added to the display.

This experiment was safety-tested in May 2007

Up next

Elastic collisions with bodies of equal mass

Ionising Radiation
Quantum and Nuclear

Elastic collisions with bodies of equal mass

Practical Activity for 14-16

Demonstration

This is useful to help students understand the shape of the tracks when an alpha particle collides with the nucleus of an atom (particularly a helium atom) in a cloud chamber.

Apparatus and Materials

Steel balls demonstration

  • Steel balls with hooks, 5 cm diameter, 2

Floating pucks demonstration

Health & Safety and Technical Notes

If the pendulums are attached to the ceiling, ensure that an adult is available to hold the step-ladder while another one works at a height.

Remember to wear leather gloves and eye protection when handling dry ice.

Read our standard health & safety guidance


You can use either method.

Although the pucks require specialist apparatus and take a bit more setting up, they show the paths of the pucks very clearly in a single plane.

The dry ice provides a cushion of carbon dioxide gas to make the pucks float. You have some dry ice available from cloud chamber experiments.

You could set up a flexicam underneath the pendulums (facing upwards) and project it onto a screen. This will show their motion in the horizontal plane. Similarly, you could set up a flexicam above the pucks to watch their motion.

In either experiment, you could capture short movie clips of collisions. You could use interactive whiteboard software to draw lines over the motion and measure the angles between the paths.

The pucks can also be recorded using

Multiflash photography


You will need a digital camera with a shutter that can be held open and a bright strobe light.

Procedure

Steel balls demonstration

  1. Suspend the two massive steel balls from the ceiling to make two very long pendulums.
  2. Adjust them so that they are at the same height.
  3. Draw one of the pendulums aside and release it so that it makes an oblique collision with the other stationary ball.
  4. Draw attention to the directions of the balls after the collision. They should set off at right angles to each other.
  5. Try a number of collisions with different lines of impact. Show that, although the angles of the paths after the collision change, they are always at right angles to each other.

Floating pucks demonstration

  1. Make sure that the glass base is level.
  2. Put some dry ice in the cavity underneath the pucks. They should float on a cushion of CO2.
  3. Put one of the pucks in the centre of the base.
  4. Gently push the other one towards it so that they make an oblique collision. The metal of the pucks should not come in contact this will make the collision less elastic.
  5. Draw attention to the motion of the pucks after the collision. When they have equal mass, they should set off at right angles to each other.
  6. Try a number of collisions with different lines of impact. Show that, although the angles of the paths after the collision change, they are always at right angles to each other.
  7. Increase the mass of the target puck (by putting another puck on top or by attaching a 500g mass with some Blu-tak or Velcro). Try some more collisions and draw attention to the angle between the paths after the collision.
  8. Swap the pucks around and use the less massive one as the target. Try some more collisions and draw attention to the angle between the paths each time.

Teaching Notes

  • See photograph of
  • Alpha particle tracks including a collision with a helium nucleus


    • When the colliding masses are equal, the angle between the paths after the collision is 90°. You can show this in photographs of a cloud chamber filled with helium gas instead of air. This shows that the alpha particle has the same mass as a helium atom and is evidence that it is itself a helium nucleus.
    • When the alpha particle collides with a more massive nucleus, like nitrogen, then the angle between the forked tracks is more than a right angle. This is modelled in step 7 of the floating pucks demonstration.
    • When the alpha particle collides with a less massive nucleus like hydrogen, the angle between the tracks is less than a right angle. This is modelled in step 8 of the floating pucks demonstration.
  • Such forks occur so rarely that there is little chance of seeing one of them unless you have a lot of observing time. Human scanners were employed to find interesting photographs which could then be analyzed. Similar triage processes are now used at CERN. Results are analyzed by software and humans to find interesting events.
  • Advanced students might appreciate a mathematical proof of the right angle result for equal masses. Using the conservation of momentum, you can draw a closed triangle of vectors. Conservation of kinetic energy produces an equation linking the squares of the velocities (before and after). In the special case where the masses are equal, these two equations give a triangle whose longest side squared equals the sum of the squares of the other two sides. This shows that there is a right angle between the two shorter sides. Further analysis is described in
  • Two dimensional momentum interchanges with pucks


This experiment was safety-tested in April 2006

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Managing 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

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

Sparks in the air

Ionising Radiation
Quantum and Nuclear

Sparks in the air

Teaching Guidance for 14-16

A high voltage will produce a spark in an air gap. This is easily demonstrated with the spheres of a Van de Graaff generator, see the experiment:

Showing that a spark can pass through air


A spark will spontaneously jump across a gap of 1 cm if there is a potential difference, of about 10,000 V across it. A larger gap needs a bigger potential difference so a spark will jump across a 3 cm gap if there is a potential difference of 30,000 V and so on.

Advanced students will understand the idea of an electric field. A spark is produced by a field strength of 10,000 V cm-1. The force on particles will increase if the charge on the spheres increases OR if the spheres are closer together.

The spark discharge process

When a positive ion is produced between the spheres of the Van de Graaff generator, it is accelerated towards the negative sphere, gaining kinetic energy. The bigger the force on the ion (or field strength), the bigger the acceleration. The ion collides with a neutral atom (after, on average, one mean free path). If it is going fast enough, it will knock an electron off the neutral atom turning it into another ion (this is an inelastic collision – kinetic energy is not conserved). If it is not going fast enough, the two will bounce away from each other with some sharing of energy.

Although it may be slowed down, the first ion will be accelerated again and make another ion in its next inelastic collision. Each new ion will also accelerate towards the negative sphere, producing new ions when they collide with air atoms.

The spark is a cascade or avalanche of ions – like a chain reaction. The picture below shows a simple experiment (possibly a thought experiment) that illustrates this.

The gradient of the ramp represents the electric field strength. If the field strength is too small, then it will not accelerate the ions enough to produce an avalanche. Any ions that are produced in the field will be drawn to the side but won’t cause a spark. [The actual mechanism that produces a spark is more complicated: there are negative ions as well as electrons; there will be excited atoms producing light; and there will be ultraviolet radiation and even X-rays produced by decelerating ions and electrons.]

Initiating a spark

At 10,000 V cm-1, the air breaks down and a spark is produced spontaneously. The field strength is enough to pull some molecules apart and produce ions that start the avalanche. It is sometimes useful to reduce the field strength slightly so that the air isn’t breaking down. Then you can initiate the avalanche by producing ions in the electric field. The two most common ways of producing ions are with a flame or a radioactive source (usually an alpha source like radium).

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First models of the atom

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 stored kinetically 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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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 it no longer has energy stored kinetically. 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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Developing a model: the nuclear atom

Ionising Radiation
Quantum and Nuclear

Developing a model of the atom: the nuclear atom

Teaching Guidance for 14-16

Students will have seen signs of electrons and positive ions (perhaps carrying a current through ionized neon or helium). On this evidence, Thomson proposed his ‘plum pudding’ model, in which the negative electrons sit in the positive nucleus like currents in a bun.

However, students may well have been exposed to images, stories, films and textbooks which incorporate a version of a planetary model. They may find it strange that you even mention the plum pudding model. It is not difficult to persuade them that this model has been superseded. What is more challenging is to take them beyond the simple pictures that they may have seen and to give them some idea of the size of atoms and nuclei – i.e. that the atom is not just hollow but extremely hollow (too hollow to represent in simple images).

The Rutherford-Bohr model

Rutherford proposed the idea of a central nucleus in an atom in around 1908. The nucleus contains the atom’s positive charge whilst the electrons are outside. From back scattering experiments, he showed that the radius of the nucleus was 100,000 times smaller than the radius of an atom. This is equivalent to the head of a pin (the nucleus) in the middle of a large stadium (the atom). Consequently, the relative sizes of the atom and its nucleus cannot be shown in simple diagrams.

back scattering experiments


Given that the electrons were easily removed, Rutherford assumed that they were on the edge of the atom. There emerged a picture of orbiting electrons that mirrored the planets orbiting the Sun.

Problems with the planetary model

However, whilst the existence of the hollow atom is well accepted, there have always been serious objections to a classical planetary model. The main objection is that the orbiting electrons are moving charges and should radiate electromagnetic waves, losing energy. This loss of energy would cause them to spiral into the nucleus. In other words, there was no way of explaining why an atom with orbiting electrons is stable.

The Bohr model

This issue of instability was addressed by Niels Bohr in 1913. He combined Rutherford’s model with the quantum ideas put forward by Max Planck at the turn of the century. Bohr no longer referred to orbits but only to ‘stationary states’. Atoms could exist in a stationary state and be stable. Spectral lines were a result of transitions between these stationary states. Bohr’s model was the first step towards an atom that is described by quantum mechanics. It no longer followed classical laws – particularly those of electrodynamics. The reason for this was straightforward: classical electrodynamics could not explain how the electron could be bound to the nucleus and be stable.

In the late 1920s, the quantum mechanics of Schrödinger and Heisenberg offered two further developments to Bohr’s model.

The atom: a quantum mechanical model


These models become ever more difficult to represent in simple images. Perhaps because of this, the simple planetary picture has endured as one of the icons of twentieth century atomic physics, even though it was superseded within a few years of being proposed.

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Alpha particle tracks

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

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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Using 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

Radioactive sources: isotopes and availability

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

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.

IOP DOMAINS Physics CPD programme

The IOP is proud to present our new Forces CPD Domain

This is the first of a collection of videos for you to view ahead of our live support sessions for teachers and coaches of physics.

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