Ionising radiations and their properties

NB Return each source to its box before removing another one so that only one source is in use at any one time. This is necessary because the dose rate calculations have been done for single sources only.

Radioactive sources – isotopes, radiation and availability

Note that 5 μCi is equivalent to 185 kBq.

Sealed sources for radium and plutonium are no longer available (see the guidance note...

...However, if you have them in your school, you can use them as long as you follow your school safety policy and local rules.

Solid state (semiconductor) detectors are no longer available.

Nevertheless, you may have one in your school. They are effective at detecting alpha radiation. However, they are often very sensitive to light so you get a large background count. They use the energy of the incoming particle without a flash of light playing a part. The particle pushes some electrons into upper energy levels in the semiconductor, leaving holes which act as positive charges. A strong electric field makes a pulse of current which can be counted.

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

www.mindsetsonline.co.uk

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.

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 americium-241), this is the recommended method because the spark counter does not respond to beta or gamma. See...

Spark counter

...experiment for technical notes.

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 kit to have in your prep room.

Procedure

Point each of the available sources in turn to the Geiger-Müller tube, the spark counter, and the solid state detector (connected to the scaler). The scaler responds by counting the ionizing events which occur.
You can show that only one type of source produces sparks. The others register a count on the Geiger-Müller tube, showing they are producing some kind of radiation but they do not produce sparks. Tell students "the radiation that produces sparks is the most ionising and is known as alpha."
In each case, put absorbers between the source and the detecting device. You can quickly show that paper stops alpha radiation. Of the remaining two types of radiation, one is stopped by a sheet of Perspex, an exercise book, or thin aluminium: "we call this beta radiation". The remaining type of radiation is very penetrating and needs thick lead to reduce it to a low level: "we call this gamma radiation".

Having identified the three types of radiation, try moving each one away from the detector. You can quickly show that alpha is very short range, beta has a range of about 10 cm in air, and gamma gets weaker with distance but doesn’t come to a stop at any particular distance.

Teaching Notes

At this stage, you can use this practical in two ways.
As a quick introduction to the three types of radiation. You may not want to dwell too much on the different properties. You can investigate these in more detail in later experiments.
As a more detailed round-up of the range and penetrating properties of the three types of radiation. In effect, you can merge this experiment with...

Alpha radiation: range and stopping

Beta radiation: range, stopping and deflecting

Gamma radiation: range and stopping

Relate the range and poor penetration of alpha to its strong ionisation. You can also refer to cloud chamber tracks if students have seen the experiment or photographs of the tracks.
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 ionising the radiation, the more harmful it is.
Relate the hazard to the safety precautions that you are taking during the demonstration.
Unstable radioactive atoms send out particles. The remaining atom is different from the initial atom, and it becomes an atom of a different chemical element.

This experiment was safety-tested in April 2006

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...

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...

Radioactive sources: isotopes, radiation and availability

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:

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

"alpha particles are thought to be a doubly ionized helium atom"

Alpha particle tracks including a collision with 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

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

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...

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

Set up the Geiger-Müller tube in a clamp and connect it to a scaler if needed.
Fix the beta-source in its holder and clamp it near to the G-M tube.
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.
A graph of count rate against separation distance could be plotted.
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.

Identifying the three types of ionizing 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

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.

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

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°.
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°.

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.
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.
Remove the magnet and return the beta source to the safe.

Carrying out

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.
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.
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

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...

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

Set up the Geiger-Muller Tube and attach it to the scaler if needed.
Put the source in its holder and clamp it a few centimetres from the Geiger-Muller tube.
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.
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

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...

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...

Set up the Geiger-Muller tube and attach it to the scaler.
Clamp a metre rule to the bench and line it up with your zero point (in the Geiger-Muller tube).
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.
You can check your zero point by doing some quick readings before the lesson. When you double the distance, the count should be a quarter. If it is more than a quarter, then move the tube towards the source to re-zero it. If it is less than a quarter, then your zero point is closer than you reckoned: move the tube away from the source to re-zero it.

Carrying out...

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

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.

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-60	pure gamma (provided the low energy betas are filtered out)
strontium-90	pure beta
americium-241	alpha 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-226	alpha, beta and gamma
plutonium-239	pure alpha
caesium-137	beta, then gamma (from its decay product, metastable Ba-137)

For safety information:

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.

	alpha	beta	gamma
property	highly ionising	fairly ionising	weakly 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 paper	stopped by lead or thick aluminium	attenuated by thick lead
	deflected slightly in magnetic field	deflected in magnetic field	Undeflected in electric and magnetic fields
	deflected in electric field	deflected in electric field
nature	positive charge	negative charge	no charge
	large mass (same as helium nucleus)	small mass
identity	helium nucleus	fast moving electron	high 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.

Quantum and Nuclear

Experiments with alpha radiation

Teaching Guidance for 14-16

If you do not have a plutonium-239 source, you can use americium-241. However, you need to be aware that it gives off gamma radiation as well as alpha radiation. Therefore, if you use a Geiger-Muller tube to detect the radiation, you will find that the source still produces a count at long range or when you try to block it with paper. This is due to the gamma radiation that it is emitting.

With advanced students, you could try accounting for this radiation (i.e. treat it as a background count). However, the most effective method of demonstrating the properties of alpha radiation is to use a spark counter. The spark counter responds only to alpha radiation so you do not need a pure source. You can use radium-226, plutonium-239 or americium-241. There is the added benefit that the spark counter makes for an impressive and captivating demonstration.

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.