Radioactivity

Background radiation and how we detect it

Background radiation is the low-level ionising radiation that is present in the environment all the time, wherever you are. It comes from natural sources and from a small amount of human activity, and it is the reason a detector standing in an empty room still records a count rate instead of zero. Radon gas is the largest single contributor in the United Kingdom: uranium in rocks such as granite decays slowly and produces radon, a radioactive gas that seeps out of the rock and collects inside buildings, especially in cellars and on ground floors. Rocks and buildings are a source in their own right, because most rock and many building materials contain small amounts of radioactive isotopes such as uranium, thorium and potassium-40.

Food and drink contribute as well: every living thing contains carbon-14, and foods rich in potassium, such as bananas, potatoes and Brazil nuts, contain potassium-40. Cosmic rays are high-energy radiation arriving from the Sun and from outer space, and the atmosphere absorbs most of them, so the dose is greater at high altitude, which is why aircrew receive a larger dose than people who work at ground level. Artificial sources add a smaller amount, of which medical and dental X-rays are the largest.

Ionising radiation cannot be seen, felt or heard, so it is measured with a detector connected to a counter. The detector used in school is the Geiger-Muller (GM) tube: a metal tube containing a gas at low pressure. When an alpha particle, a beta particle or a gamma ray enters the tube it ionises the gas, a pulse of current flows, and the counter records one count. The counter gives a count rate, measured in counts per second (counts/s) or counts per minute (counts/min).

Because the background is always there, a reading is the source plus the background. Measure the background count rate with the source removed, or moved far away, and work out the corrected count rate: corrected count rate = count rate with the source present - background count rate. A reading of 620 counts/min with the source present and 20 counts/min with the source removed gives 620 - 20 = 600 counts/min. Keep the source at the same distance throughout, because the count rate falls rapidly as the distance increases, and count for a long time, because the emission is random and a short count gives an unreliable average.

• Radon gas in the air: produced by the decay of uranium in rocks and soils, and it collects inside buildings.
• Rocks and buildings: granite and many building materials contain small amounts of radioactive isotopes.
• Food and drink: potassium-40 in foods such as bananas and nuts, and carbon-14 in all living things.
• Cosmic rays: high-energy radiation from space, most of it absorbed by the atmosphere.
• Corrected count rate = count rate with the source present - background count rate.

Alpha, beta and gamma: the three types of nuclear emission

When a nucleus emits radiation it is described as radioactive. There are three types of emission, and for each one you must know its nature, how strongly it ionises and how far it penetrates. The emission from a nucleus is spontaneous, so nothing has to be done to the nucleus to start it, and it is random in direction, so there is no way of telling which way a particular particle or ray will travel.

An alpha particle is a helium nucleus: 2 protons and 2 neutrons, with a charge of +2e and a mass number of 4. It is heavy and slow compared with the other emissions, so it interacts strongly with the atoms it passes and ionises them. It gives up its energy within a few centimetres of air, and a single sheet of paper or the dead outer layer of skin stops it.

A beta particle is a fast-moving electron created inside the nucleus and thrown out of it. It has a charge of -1e and a mass about 1840 times smaller than a proton. Because it is so much lighter than an alpha particle it ionises less strongly, travels further, and is stopped by a few millimetres of aluminium. Gamma radiation is high-energy electromagnetic radiation: a photon with no charge and no mass. It ionises only weakly but penetrates the furthest, and several centimetres of lead are needed to absorb most of it.

Learn the order, because the same order describes both ionising power and penetrating power, and the two run in opposite directions: alpha is the most ionising and the least penetrating, gamma is the least ionising and the most penetrating, and beta lies between them.

Deflection in electric and magnetic fields, and why the ionising effects differ

A charged particle moving through an electric field or a magnetic field is deflected, because the field exerts a force on its charge. Alpha particles carry a positive charge of +2e, so they are pushed towards the negative plate. Beta particles carry a negative charge of -1e, so they are pushed towards the positive plate, in the opposite direction. Gamma radiation has no charge at all, so it is not deflected in either field.

Beta particles are deflected far more than alpha particles, and the reason is mass rather than charge. The size of the deflection depends on the ratio of the charge to the mass. An alpha particle has twice the charge of a beta particle, but its mass is about 7000 times greater, so its charge-to-mass ratio is much smaller and it is deflected much less. In a magnetic field the pattern is the same: alpha and beta curve in opposite directions, beta curves much more sharply, and gamma travels straight on.

This is how the three radiations are separated in an experiment, and it is how their charges were first established. If a beam from a source passes between charged plates and part of it is deflected towards the positive plate, that radiation must be negatively charged, so it is beta radiation.

The same two ideas, charge and kinetic energy, explain the ionising effects. Ionising an atom means removing an electron from it, so a radiation ionises strongly when it has a large charge and carries plenty of kinetic energy. An alpha particle has a charge of +2e and a relatively large mass, so it has a large kinetic energy and its strong positive charge pulls electrons off the atoms it passes. It transfers that energy over a very short distance, which is why it ionises very strongly but has a short range. Gamma radiation has no charge, so it slips through the electron clouds of atoms with almost no interaction, transfers its energy slowly and ionises only a few atoms along a very long path.

• Alpha particles: charge +2e, deflected towards the negative plate, smallest deflection because their mass is large.
• Beta particles: charge -1e, deflected towards the positive plate, largest deflection because their mass is very small.
• Gamma radiation: no charge, so it is not deflected at all.
• Ionising power, strongest first: alpha, then beta, then gamma.

Radioactive decay: the nucleus changes to that of a different element

Radioactive decay is a change in an unstable nucleus that can result in the emission of an alpha particle or a beta particle and/or gamma radiation. The change is spontaneous, which means it happens without anything being done to the nucleus and is not affected by temperature, pressure or chemical state, and it is random, which means you cannot predict which nucleus will decay next, when it will decay, or in which direction the particle will be emitted.

In alpha decay the nucleus loses 2 protons and 2 neutrons, so its mass number decreases by 4 and its proton number decreases by 2. Because the proton number changes, the nucleus becomes that of a different element. Radium-226 has 88 protons and 226 - 88 = 138 neutrons; it decays to radon-222, which has 86 protons and 136 neutrons. In words: radium-226 decays to radon-222 plus an alpha particle. Losing this heavy piece makes the nucleus smaller and more stable.

In beta decay a neutron changes into a proton and an electron, and the electron is thrown out of the nucleus as the beta particle. The mass number does not change, because a neutron has been replaced by a proton, but the proton number increases by 1, so again the element changes. Carbon-14 has 6 protons and 8 neutrons and decays to nitrogen-14, which has 7 protons and 7 neutrons. In words: carbon-14 decays to nitrogen-14 plus a beta particle. Beta emission reduces the number of excess neutrons, which is what makes the nucleus more stable.

Gamma emission is different. Gamma radiation carries away energy but no particles, so the mass number and the proton number both stay the same and the element does not change: the nucleus simply drops to a lower energy state and becomes more stable. Gamma emission often follows an alpha or a beta decay, when the new nucleus is left with surplus energy. Isotopes of an element may be radioactive for two reasons: the nucleus may contain too many neutrons compared with protons, which beta emission corrects, or the nucleus may be too heavy, with too many protons and neutrons altogether, which alpha emission corrects.

Half-life: how long a source stays useful

The half-life of a particular isotope is the time taken for half the nuclei of that isotope in any sample to decay. Equivalently, it is the time taken for the count rate from the source to fall to half of its value, once the background count rate has been subtracted. Half-life is a fixed property of the isotope: it does not depend on how much of the isotope you have, nor on temperature, pressure or chemical state.

Half-life does not mean that half the sample decays and then the rest stays put. The same fraction decays in every half-life: after one half-life half the nuclei remain, after two half-lives a quarter remain, after three an eighth, and so on. The count rate falls towards the background count rate but never reaches zero.

A worked example. A source gives a count rate of 800 counts/min. After 6 hours the count rate is 100 counts/min. The halvings are 800 -> 400 -> 200 -> 100, which is three halvings, so three half-lives have passed and the half-life is 6 / 3 = 2 hours.

At Extended level the background may not have been subtracted for you, so subtract it yourself first. The readings in the table below were taken with a background count rate of 30 counts/min. Subtracting 30 from each reading gives corrected count rates of 800, 400, 200, 100 and 50 counts/min, and each of these is half of the one before, so one half-life is 2 minutes. Halving the raw readings instead would give the wrong answer, because the background does not decay along with the source.

Choosing a radioisotope for the job

The type of radiation and the half-life together decide which isotope is used for a job. The type of radiation fixes how far it travels and how dangerous it is outside the body; the half-life fixes how long the source stays useful and how long the hazard lasts. A very short half-life means the source has to be replaced often, while a very long half-life keeps the activity steady but leaves material that stays radioactive for a long time, which is a problem when it has to be disposed of.

Smoke alarms contain americium-241, an alpha emitter with a half-life of 432 years. The alpha particles ionise the air inside the chamber so that a small current flows. Smoke entering the chamber absorbs some of the alpha particles, the current falls, and the alarm sounds. Alpha is chosen because it ionises air strongly but has a very short range, so the casing of the alarm stops it and it cannot reach anyone outside; the long half-life means the alarm keeps working for years without the source being replaced.

Sterilising equipment and irradiating food both use gamma rays from a source such as cobalt-60. Gamma is used because it is the most penetrating, so it passes through sealed packaging and through the item itself and kills the bacteria and other microorganisms inside. The food or the equipment does not become radioactive, because gamma radiation carries no particles and leaves no radioactive material behind.

Measuring and controlling the thickness of materials uses a source on one side of the sheet and a detector on the other. Beta radiation is used for thin material such as paper or plastic film: some beta particles are absorbed by the sheet, so the count rate falls as the sheet becomes thicker, and a control circuit uses the count rate to adjust the rollers. Alpha would be stopped by the air and by the sheet, giving no reading at all, and gamma is so penetrating that the count rate would hardly change, so neither would work.

Cancer. Gamma rays are used both to find tumours and to treat them. A gamma-emitting tracer collects in the tumour, and a detector outside the body builds up an image of where it has gathered. For treatment a strong gamma beam is aimed at the tumour from a source such as cobalt-60, and the beam is rotated around the patient so that the tumour at the centre receives a high dose while healthy tissue receives much less.

• Smoke alarm: americium-241, an alpha emitter with a long half-life.
• Sterilising equipment and irradiating food: gamma rays, which penetrate the packaging and kill microorganisms.
• Thickness gauge: beta radiation for thin sheets such as paper, because beta is partly absorbed by the sheet.
• Cancer diagnosis and treatment: gamma rays, which penetrate the body to reach the tumour.

Safety: the effects of radiation and the precautions that reduce the dose

Effects on living things. Ionising radiation damages living cells. It can kill cells outright, causing burns and radiation sickness; it can damage DNA and produce mutations, which may be passed on to future generations; and it can trigger cancer by causing cells to divide out of control. Which radiation is the greater hazard depends on where it is: alpha radiation is stopped by the dead outer layer of skin, so it is dangerous mainly if a source is breathed in or swallowed, where it delivers all of its energy to a small volume of tissue, while gamma radiation penetrates the body and can damage cells deep inside from outside.

Moving, using and storing sources. Radioactive materials are stored in lead-lined containers in a locked store, moved in sealed and clearly labelled containers, and handled with long tongs or by remote handling so that the source is never touched. Sources are used behind shielding and are never pointed at anyone, the area is monitored with a Geiger-Muller tube, and radioactive waste is sealed and buried deep underground so that it cannot enter the food chain.

The three precautions. Reduce the time spent near the source, increase the distance between the source and living tissue, and use shielding to absorb the radiation. Distance matters a great deal, because the intensity of the radiation falls with the square of the distance, so doubling the distance cuts the dose to a quarter. That is why a source is carried at arm's length in tongs rather than picked up by hand.

Match the shield to the radiation. A few centimetres of air or a sheet of paper absorbs alpha; a few millimetres of aluminium absorbs beta; and several centimetres of lead are needed to absorb most gamma radiation. Choosing the wrong shield is a common error: aluminium does almost nothing to a gamma beam, and paper does nothing at all.