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.

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.

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).

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.

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.

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.

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.

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.

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

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

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:

For safety information:

Managing radioactive materials in schools

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.