Collection Atoms and nuclei guidance notes
- Managing radioactive materials in schools
- Radioactive sources: isotopes and availability
- Nature of ionising radiations
- First models of the atom
- Developing a model of the atom: radioactive atoms
- Evidence for the hollow atom
- The great scattering experiments
- Alpha particles as tools
- Exponential decay of a radioactive substance
- Light behaving like a particle
- Electrons behaving as waves
- The electron
Atoms and nuclei guidance notes
The following guidance notes cover these practical collections:
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.
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)|
|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|
|caesium-137||beta, then gamma (from its decay product, metastable Ba-137)|
For safety information:
Nature of ionising radiations
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.
|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.
First models of the atom
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.
Developing a model of the atom: radioactive atoms
Teaching Guidance for 14-16
Initially, students may regard atoms as the fundamental chemical particles. True, electrons can be chipped off an atom, and possibly all an atom’s electrons stripped off to leave a bare nucleus; yet according to the simple story, the nucleus is still fixed and determines the element by its charge, Ze.
Therefore, to change one element into another, the alchemist’s dream of lead into gold, would require a change of nuclear charge. At first sight this seems impossible because the nucleus is buried deep in the atom bound together by tremendous forces. But it does happen in radioactive elements.
Soon after the discovery of radioactivity in 1896 by Becquerel, Marie Curie and her husband Pierre discovered a new element which they named radium. They extracted dangerously large samples of radium from vast quantities of rock and experimented on its radioactive behaviour.
You could say: Radioactive atoms do not just stay there as atoms of ordinary copper do; they are completely different: they are unstable, they suddenly break up, flinging out a particle such as an alpha particle, becoming an atom of a different element.
A radium atom remains a radium atom, with the chemical behaviour of a heavy metal, until it suddenly hurls out this alpha particle. (The alpha particle has such a huge energy that it must come from the nucleus.) The remainder of the radium atom is no longer a heavy metal, but a quite different element. This ‘daughter’ of radium is an atom of a heavy inert gas, the end of the helium, neon, argon, krypton, xenon series. It is called radon. The atomic masses have been measured directly, radium-226, radon-222 (a difference of 4 suggesting that the lost alpha particle is a helium nucleus). Separate measurements confirm this.
When you have a mixture of a parent element and a daughter element which have different chemical properties, then they can be separated by ordinary chemical methods.
Radon gas is itself unstable and radioactive. Each of its atoms suddenly, at an unpredictable moment, hurls out an alpha particle. The remainder is a new atom, very unstable, which is called polonium, the ‘daughter’ of radon and the ‘granddaughter’ of radium. The series continues through several more radioactive elements and stops at a stable form of lead. The series does not begin with radium: it begins with uranium several stages earlier. Radioactive uranium (Z=92) has turned into lead (Z=82).
Making unstable atoms
A century ago, radioactivity was a peculiarity of a few mostly heavy, elements: the last few at the end of the Periodic Table. Nowadays scientists can bombard samples of lighter elements with high speed, high energy protons or neutrons, provided directly or indirectly by an accelerator. They can make unstable isotopes of every element in the periodic table. This has opened up the field of nuclear chemistry. Radioactive isotopes behave chemically like their stable isotopes and can be mixed with them. Their progress as radioactive tags can be traced, like luggage labels, following the progress of a ‘labelled’ isotope through the human body or an industrial process.
All the unstable members of these strange families have a constant, reliable characteristic: the atoms show no signs of ageing, or growing weaker, however long they last. Each radioactive element has a constant chance of breaking up in each succeeding second. This is described by a useful length of time, the ‘half-life’ of the radioactive element. For each individual atom the betting is 50:50 for and against its breaking up at any time during one half-life from now. The break-up seems to be controlled by pure chance. That chance does not change and make the break-up more likely for atoms that need to survive longer.
For radium the half-life is 1650 years. Start with 1000 mg of radium now and 1650 years later you will have only 500 mg left. After a further 1650 years only 250 mg will be left and so on. For radium’s daughter, radon -222, the half-life is 3.8 days. In less than four days half the radon gas will have disappeared. You will find helium gas there instead, with the solid products.
The instability appears to be something inherent in the nuclear structure. Nowadays, taking a wave view of the behaviour of nuclear particles, you can picture a stationary wave pattern defining the life of an alpha particle inside the nucleus. But the wave is not completely confined, it leaks through the potential barrier round the nucleus and runs on as a faint wave outside. The wave is interpreted as describing probabilities of locations. It is not a mechanical wave carrying energy and momentum.
While the alpha particle is expected to be found inside the nucleus, there is a chance of finding it one day outside, despite what would seem an insurmountable potential wall. That chance of the alpha particle being outside, being emitted, is definite and constant, a part of the defining wave property, as long as the nucleus lasts. It suggests that high energy alpha particles go with a short half-life of the parent nucleus.
Evidence for the hollow atom
Teaching Guidance for 14-16
The main and first evidence for the hollow atom came from...
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.
The great scattering experiments
Hans Geiger was one of Rutherford’s students at Manchester University. He had been trying to make a workable detector to count alpha particles. During his investigations, he found that the alpha particles were deflected when they passed through a mica film. He told Rutherford of this effect.
Rutherford encouraged Geiger and Ernest Marsden, an undergraduate student, to investigate the deflections. They counted scattered alpha particles by the faint scintillations they make on a screen.
Only a few bounced back. Rutherford is widely quoted presenting this as an amazing result, saying 'it was as if you fired a 15" shell at a piece of tissue paper and it came back to hit you'. From Geiger and Marsden's results, Rutherford devised his new model of the atom: a very small massive nucleus with electrons so far out, and so light, that the alpha particles which were seriously deflected met the full force of a bare nucleus. He assumed that the nucleus carried a charge of +Ze, where Z is the serial number in the periodic table. Using this theory, the force between the alpha particle, itself a helium nucleus, and a gold nucleus is the inverse-square law Coulomb force of electrostatic repulsion.
Geiger and Marsden went on to make a great series of measurements of the deflections of a narrow beam of alpha particles that hits gold leaf in a vacuum. Their measurements served to test his new model.
Alpha particles making a very close approach to another nucleus are deflected through a large angle. Those missing the target widely are deflected through a small angle. Measurements over a big range of angles serve to investigate the field of force inside the scattering atoms over a large range of distances from the centre.
Rutherford assumed an inverse-square law of repulsion between the big electric charge on the massive nucleus of the gold atom and the smaller charge on the alpha particle flying past it. That is equivalent to Newton’s assumption of an inverse-square law attraction between the massive Sun and a planet. Instead of the simple circular orbits which serve approximately for planets, the change to a repulsive force predicts a different shape: hyperbolas. The alpha particle sails in, bends around, and sails out again on another almost straight track in a new direction. The simple calculation with circular orbits that predicts Kepler III becomes more complicated.
Instead of measuring the orbits of a few planets, Rutherford had to use hordes of little alpha particles to give him a statistical test. He made his theory predict the number of particles that an observer would count on a receiving screen in various directions, in some standard time. In calculating that prediction he simply used an inverse-square law of repulsive force and Newton’s laws of motion.
The table of actual measurements of scattered alpha particles for various angles (taken from Geiger and Marsden’s original paper) shows how the numbers counted fit the predictions for an inverse-square law of force.
Geiger and Marsden's table of results compared with predictions using Rutherford's model.
* Of path of alpha-particles. † Number of scintillations seen, for deflection A°, in a standard time.
In the actual experiments Geiger and Marsden made one set of measurements for the larger angles of deflection, and another set, with a much smaller radioactive source, for the smaller angles. To make one complete set in the table above, the numbers for smaller angles have been multiplied up to fit the set for larger angles. The multiplying factor was provided by experiment because counting was done for 30° in both data sets.
Finding the charge on the nucleus
Rutherford’s theory also predicted the way the count on a fixed screen would depend on the speed of the alpha particles:
N ∝ 1/ v 4 and on the electric charge of the scattering nucleus:
N ∝ (Ze)2 Chadwick used this to measure the charge on the nuclei of a number of elements. He used thin sheets of copper, silver and platinum instead of gold and measured the scattering of alpha particles from each. From his counts, with Rutherford’s theory, he calculated the charge on the nucleus of each of those nuclei.
His results were: copper 29.3 electron charges, silver 46.3 electron charges, platinum 77.4 electron charges, with expected errors of about 1%. The serial numbers of those elements, arranged in order of atomic weights and placed in the period table are: 29, 47, 78. Chadwick’s measurements showed that the nuclear charge is the atomic number.
Nowadays the charge of a nucleus is understood in terms of the proton number, and its value is measured in electron charges. Originally, from Geiger and Marsden's scattering experiments, it was deduced that the nucleus had a charge of about half the atomic weight multiplied by the electron charge.
Back-scattering to measure the size of the nucleus
From the known mass and speed of the alpha particles, Rutherford could calculate the distance of closest approach to a nucleus. This is the distance from a gold atom’s centre at which an alpha particle making a rare head-on collision would come to rest momentarily and bounce straight back.
Rutherford tried different energy alpha particles, and found some for which the measured number deviated from the predicted number. He suggested that, at this energy, the alpha particles were reaching the nucleus and being assimilated into it. This, he said, gave an indication of the radius of the nucleus. That radius turned out to be 10,000 times smaller than the radius of the atom.
Thanks to David Baum for pointing out an error on this page, now corrected. Editor
Alpha particles as tools
When Geiger and Marsden carried out the gold foil experiment in 1909, a lot was already known about alpha particles. Although the rare back scattering was a surprise to Rutherford, he knew that the bullets he was firing were helium nuclei.
Rutherford had been working on alpha particles for a number of years. This was before the development of the cloud chamber and mainly relied on recording alpha particles with photographic plates. Between 1903 and 1908, he had:
- deflected alpha particles in electric and magnetic fields and determined their charge to mass ratio (half the value for a hydrogen ion). This meant that they were probably either He2+or singly ionised hydrogen molecules; Rutherford favoured the former.
- with Frederick Soddy, published a paper in which they estimated the mass, energy and speed of alpha particles.
- noticed small deflections of alpha particles by air and mica (by firing them through the target at a photographic plate).
- tried to count and collect alpha particles to measure their charge; he later measured their charge (as +2e).
It was whilst trying to get a reliable counter that he and Geiger noticed the amount of scattering. At first, the scattering was a frustration. But once Geiger had noticed some large angles, they turned it into the famous investigation that began in 1908.
In 1909, Rutherford and Royds collected the gas that was formed when alpha particles were trapped in a tube and showed that it was helium. So they were now sure that alpha particles were doubly-ionised helium atoms, He2+.
However, because Rutherford still thought of atoms as plum puddings, he was still astonished when some of them were back scattered in the Geiger and Marsden experiment. It was sometime in 1910 that Rutherford put forward his idea of a nuclear atom and so the alpha particle itself could be referred to publicly as a helium nucleus.
Exponential decay of a radioactive substance
Teaching Guidance for 14-16
One of the most important characteristics of radioactivity is that it decays exponentially. This has two basic mathematical implications at this level.
- The rate falls by a constant ratio in a given time interval. The time it takes to fall by a half is always the same. It also falls to a tenth in equally regular, but longer, time intervals.
- The rate of decay is proportional to the amount that is left. This can be seen in the experiment to model radioactive decay. The number of coins that decay in any ‘shake’ is proportional to the number that is left.
From these features, you can argue, respectively, the following points.
- The chance of an atom disintegrating is constant in time. Radioactive decay is a series of many chance events, all with an unalterable chance.
- The rate of disintegrations is proportional to the total number of unchanged radioactive atoms at that moment. Both the rate and the stockpile itself die away exponentially with the same characteristic half-life.
Light behaving like a particle
No sooner do students see that light has a wave property (and have measured its wavelength) then this story is upset with further demonstrations that light has a particle property; it packages its energy in small quanta. The idea that radiation packages its energy in quanta proportional to frequency first arose in Planck’s mind when trying to fit the theoretical prediction for the energy distribution in the spectrum of a perfect radiator with the experimental results. The variation of the specific heat of materials with temperature also appeared to require a quantum rule. The photoelectric effect appeared to be pointing in the same direction when Einstein applied his clear vision to it in 1905 and was awarded the Nobel Prize for his efforts.
It is assumed that pupils have seen photocells at work in electric or electronic circuits where light releases a horde of electrons from a sensitive surface in a vacuum and the horde acts as a current to do jobs for us. That might be called the ‘wholesale photoelectric effect’. In this, light ‘flicks’ electrons out of a metal, ultra-violet light tearing them out with the crack of a whip and X-rays hurling them out. This strange interchange between radiation and electrons throws much ‘light’ on the micro-physical world.
A Geiger-Müller tube responding to gamma rays is demonstrating the photoelectric effect of those very energetic photons. However, the random counting is due to the random instability of the parent radioactive nuclei, not the effect of photons arriving at random from a steady stream of radiation. But if you shine a steady stream of ultra-violet light or light from a match onto a Geiger-Müller tube, with a thin mica window, then the Geiger-Müller tube will show random counts. A sheet of glass placed between the light source and the Geiger-Müller tube will show that it is not the visible light which is the active agent.
This experiment suggests some of the photo-electric effect story, but it does not show that the negative electricity is coming out in particles: electrons. It also does not show that light is arriving in bundles of energy: quanta. It only suggests that there is some connection between the wavelength of the light and its efficacy in ejecting negative charge.
More complex experiments, or perhaps a film, are needed to show:
- photons arriving one by one
- that the particles ejected are electrons with the usual value of e / m
- that the electrons emerge with a given illumination, with a variety of speeds, the slower ones having probably lost energy by travelling through the outer layers of the metal
- that with light of a given frequency, all the electrons ejected have the same maximum energy. This is the basis of Einstein’s equation, Eelectrons = hflight − Φ, where Φ is the ‘work function’ and h is the Planck constant.
- that the maximum energy of the ejected electrons is determined by the frequency of the light used and not by its intensity. Brighter light only produces more electrons and not faster ones
- that when the light is first turned on there is no delay in the production of electrons as one would expect if a continuous stream of light had to build up enough energy in the metal to eject each electron in turn. This is especially an impressive story with weak light. Sometimes an electron is ejected early, sometimes it may be later and so we are forced to conclude that the arrival of quanta is random in time.
Electrons behaving as waves
Students will know that electrons carry energy and momentum when they are moving. Yet these moving electrons seem to be guided to an interference pattern just like waves of light; or just like photons of light in the micro-physical world.
In the macro-physical world, large particles such as tennis balls and people do not display wave behaviour; wavelengths associated with such particles, at usual speeds, are so extremely minute that it won’t be possible to observe the diffraction or interference pattern associated with them. Nevertheless the behaviours of waves and particles in the micro-physical world are not entirely separate. Moving particles do follow wave directions, and it is the wave which predicts a probability of where to find the particle. The particles are guided by ‘matter waves’. Wave-particle duality was first suggested by Louis de Broglie about a century ago.
This raises the question of whether electrons (and other tiny particles) are particles or waves. Many observations in atomic physics can be treated using the particle model on its own. Others require the wave model. Both models prove to be useful and, despite their contradictory nature, must both be used for a full description. The two aspects of the electron both contradict and complement one another: both aspects are needed for a complete description. Niel’s Bohr’s solution was the principle of
The Complementarity Principle says that sometimes electrons have the properties of particles and sometimes the properties of waves, but never both together. Their two types of behaviour complement each other but never coexist. The type of behaviour that is shown usually depends on the measurement technique being used. To put it another way, ask a wave-type question and you will get a wave’s answer. Ask a particle-type question and you will get a particle’s reply.
Bohr’s interpretation was that the two irreconcilable descriptions should be applied in turn but cannot be applied simultaneously. They are never in direct conflict, because it is impossible to determine at the same time all the information required to make the two images precise.
This relates to Heisenberg’s Uncertainty Principle . The more precise the observations of one picture, the less precise the other becomes. Define the wavelength of an electron sharply enough and the attempt to apply the particle model will surely fail. Localize the electron definitely enough and the wave model fails.
Electrons were discovered by J J Thomson in 1897 – although he called them ‘corpuscles’. His discovery was based on experiments he and others had performed on cathode rays.
Many of these experiments can be reproduced in the school physics laboratory. Not only are students seeing historic demonstrations, they are seeing the behaviour of an extraordinary and influential particle, a particle which:
- shows that atoms are not indivisible;
- is fundamental – a member of the lepton family – and is therefore thought to be indivisible itself;
- carries the basic unit of charge;
- is responsible for electrostatics (and takes its name from the Greek word for amber);
- is the carrier of electric currents in conductors;
- through its behaviour in vacuum tubes, led to the birth of electronic devices, the computer and cathode ray screens;
- through its behaviour in semiconductors, led to the birth of solid state electronics;
- was the first particle to be observed showing wave properties, leading to wave mechanics and quantum theory.