Ionising Radiation
Quantum and Nuclear

Ionising the air

for 14-16

The air can be ionised by flames, radioactive sources and strong electric fields. Understanding this behaviour is a useful foundation for learning about spark counters, Geiger-Müller tubes and cloud chambers. It is also a step in building a model of atoms.

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Ions in a flame

Ionising Radiation
Electricity and Magnetism

Ions in a flame

Practical Activity for 14-16

Demonstration

A candle flame is electrically conductive. An EHT will visibly separate its free charges, causing a current to flow in an external circuit.

Apparatus and Materials

  • Light source, compact
  • Metal plates with insulating handles, 2
  • Power supply, EHT, 0–5 kV (with internal safety resistor)
  • Candle
  • Screen
  • Variable voltage supply, 0-12 V, 8 A

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 Van de Graaff generator or a Wimshurst machine can be used instead of the EHT power supply.

A flexcam and display screen could be used instead of the light source and screen.

Procedure

  1. Fix the plates in vertical planes parallel to each other and five to ten centimetres apart by means of their handles held in retort stands.
  2. Light the candle and place it so that its flame is lit a little below the plates.
  3. Set up the small bright light source so that a shadow of the plates falls on the screen beyond. Both the light source and screen should be at least a metre from the candle, and the source preferably more.
  4. Apply a high potential from the EHT power supply to the plates. The flame divides into two parts, one towards the positive plate and one to the negative. This can be seen clearly on the screen.

Teaching Notes

  • The flame divides into two. This shows that there are two streams of particles moving in opposite directions. One stream is luminous, probably with carbon particles, and the other is not so luminous.
  • The two streams will not look equal. One is a stream of negatively-charged particles, moving to the positive plate. The other is of positively-charged particles, moving to the negative plate.

This experiment was safety-tested in August 2007

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Ions produced by a flame carry a current

Ionisation
Quantum and Nuclear

Ions produced by a flame carry a current

Practical Activity for 14-16

Demonstration

A match flame will ionise the air. Use this to show ionisation and how it can be detected by charging an electroscope.

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
  • Matches
  • Retort stands and bosses, 2
  • Flexicam or webcam linked to a projector (optional)

Health & Safety and Technical Notes

See guidance note on Radioactive sources (UK guidelines) in

Managing radioative 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 gold leaf onto a screen. Alternatively, use a bright lamp to cast a shadow of the leaf on a screen or wall. The lamp should be on the same side as the observers and below the line of sight. Remove the glass plate from the back of the electroscope.

The advantage of using an EHT supply 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.

Procedure

Setting up

  1. Fix the two metal plates so that they are parallel to one another and about 5 cm apart.
  2. Connect one of the plates to the positive terminal of the EHT supply through the safety resistor.
  3. Place the hook in the electroscope and 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 EHT supply.
  5. Connect the negative terminal of the EHT supply to the earth terminal.

Getting a current to flow and charge the electroscope

  1. 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.
  2. Hold a match flame beneath the gap between the plates and watch what happens. The air in the gap is ionised and allows a charge to flow across the gap and charge the leaf of the electroscope.

Discharging the electroscope

  1. You can also discharge the electroscope by ionising the air around it.
  2. Disconnect the electroscope from the supply but keep the base earthed. Place 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.
  3. Light a match over the top of the disc. This will ionise the air and allow the electroscope to discharge.
  4. Charge the electroscope again and light a match to the side of the disc. To discharge the electroscope, you will need to blow gently over the flame so that the ions pass over the top of the disc.
  5. The electroscope should discharge quickly to earth when the air above 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

  • You could start the demonstration by asking students what they know about atoms and ions. The demonstration reveals something about the structure of atoms: that electrons are removed with relative ease. The temperature of the flame makes the air particles move so fast that, in collisions, they knock electrons off each other.
  • Discuss what happens when the air is ionised. Air atoms are losing electrons leaving behind positive ions. The electrons then move to the positive plate and ions move to the negative plate. You can demonstrate this with the experiment

    Ions in a flame


  • The demonstration is also a step towards measuring ionising radiations. The apparatus is, in effect, an ionisation measurer. It is a circuit that allows a small current to flow when there are ions between the plates. The current is too small to measure with a normal ammeter (there are extremely expensive ammeters that will do it). The best way to measure it is using an electroscope. This idea is developed in

    Ions produced by radiation carry a current


  • With more advanced students, you can discuss the capacitance of the circuit (a combination of the parallel plates and the electroscope). When you first switch on the EHT, there is a separation of charge induced on the plates (the positive plate induces a negative charge on the facing plate). In turn this draws some negative charge away from the electroscope, which is left charged. In effect, the electroscope measures the charge that has been displaced by the very small and very short-lived charging current of the capacitive circuit.

This experiment was safety-tested in May 2006.

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Ions produced by radiation carry a current

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

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.

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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.

Up next

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


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