Atoms are almost entirely empty space with a tiny, dense nucleus at the centre. Unstable nuclei break down at random and emit radiation. This topic covers what that radiation is, what it does, and how it is used and managed.
An atom has a radius of about \(1\times10^{-10}\) m. Nearly all its mass is in a nucleus of protons and neutrons whose radius is less than \(\frac{1}{10000}\) of the atom's, so the atom is almost entirely empty space.
Electrons orbit the nucleus at distinct energy levels. Protons carry +1 charge, electrons −1 and neutrons none, and an atom has equal numbers of protons and electrons, so it is neutral overall.
The atomic number is the number of protons, and it alone decides which element an atom is. The mass number is the number of protons plus neutrons, so subtracting one from the other gives the number of neutrons.
Isotopes are atoms of the same element with different numbers of neutrons: the same atomic number but a different mass number. Because they have the same electron arrangement, isotopes have almost identical chemical properties, but can differ completely in nuclear stability.
Losing or gaining an electron makes an atom an ion, which is a change of charge, not of element.
Before the electron was discovered, atoms were thought to be tiny indivisible spheres. The discovery of the electron gave the plum pudding model: a ball of positive charge with electrons dotted through it.
The alpha scattering experiment overturned it. Most alpha particles passed straight through a thin gold foil, showing the atom is mostly empty space; a few were deflected strongly, showing a small concentrated positive mass. That gave the nuclear model. Bohr then proposed that electrons occupy particular energy levels at fixed distances, which explained observations the nuclear model alone could not. Later work identified the proton and, twenty years on, the neutron.
This sequence illustrates how scientific models change when new experimental evidence becomes available.
Some nuclei are unstable and break down at random, emitting radiation. This is radioactive decay, and the activity, the rate of decay, is measured in becquerels.
An alpha particle is two protons and two neutrons, identical to a helium nucleus. A beta particle is a high-speed electron emitted from the nucleus when a neutron turns into a proton. A gamma ray is a high-energy electromagnetic wave carrying no mass or charge. Neutrons may also be emitted.
The three types differ in how far they travel and how strongly they ionise, and the two go in opposite directions.
Alpha is stopped by a sheet of paper or a few centimetres of air, and is the most strongly ionising. Beta passes through paper but is stopped by a few millimetres of aluminium, and ionises moderately. Gamma is very penetrating and ionises weakly; its intensity is greatly reduced only by thick lead or metres of concrete.
So an alpha emitter is especially dangerous if it gets inside the body, where its ionisation is concentrated in a small volume of tissue, while outside the body it is easily shielded.
Alpha decay reduces the mass number by 4 and the atomic number by 2. Beta decay leaves the mass number unchanged and increases the atomic number by 1, because a neutron has become a proton.
Gamma emission changes neither: it carries away energy only. In any nuclear equation the mass numbers must balance and the atomic numbers must balance.
Radioactive decay is random: it is impossible to say which nucleus will decay next or when. What can be predicted is the behaviour of a large number of nuclei.
The half-life is the time for the number of undecayed nuclei to halve, or equivalently for the count rate to halve. After two half-lives a quarter remains, after three an eighth. Half-life does not depend on how much of the sample there is, or on temperature or chemical state.
Irradiation is being exposed to radiation from a source outside you. It does not make you radioactive, and it stops when you move away or the source is shielded.
Contamination is having radioactive atoms on or inside you. The source stays with you, so exposure continues until the material is removed or has decayed. Either can be serious; the dose received is what decides the harm. Alpha emitters are especially hazardous as contamination because their strong ionisation is deposited entirely within a small volume of tissue.
Findings on the effects of radiation are published and checked by other scientists, peer review, before being accepted.
Background radiation is around us all the time. Natural sources include radon gas from rocks, cosmic rays from space, and radioactive isotopes in food and in our own bodies. Man-made sources include medical procedures and fallout from nuclear weapons testing and accidents.
Any measurement of a source must have the background count subtracted from it. The dose a person receives depends on their location and occupation, and is measured in sieverts.
Isotopes have half-lives from fractions of a second to billions of years, and the right choice depends entirely on the use.
A medical tracer needs a short half-life, so its activity falls quickly after the investigation and the later dose is small, but long enough to be given and imaged. Waste management is the opposite problem: an isotope with a very long half-life stays hazardous for many thousands of years.
Gamma sources sterilise surgical instruments and food by killing microbes without heat. Gamma-emitting tracers are injected and followed with a detector outside the body, because gamma penetrates tissue.
Beta sources control the thickness of paper and foil in a mill, because the amount getting through depends on the thickness. High doses of gamma are used to destroy cancer cells in radiotherapy, so the dose is carefully directed and limited.
Fission is the splitting of a large unstable nucleus, usually uranium or plutonium. It rarely happens spontaneously: the nucleus usually has to absorb a neutron first.
It produces two smaller nuclei of roughly equal size, two or three neutrons, and a great deal of energy. Those released neutrons can be absorbed by other nuclei and cause further fission, which is a chain reaction. In a reactor, control rods absorb surplus neutrons so that on average one neutron from each fission goes on to cause one more. A nuclear weapon is built for the opposite: a chain reaction that grows extremely rapidly.
Fusion is the joining of two light nuclei to form a heavier one. Some of the mass is converted into energy, which is what powers the Sun and every other star.
Fusion needs enormous temperatures and pressures to force the positively charged nuclei close enough together, so it is much harder to achieve on Earth than fission.
Electrons occupy fixed energy levels. Absorbing electromagnetic radiation can lift an electron to a higher level, which is called excitation; it falls back and emits radiation carrying exactly the energy difference.
Absorbing enough energy can remove the electron altogether, leaving a positive ion. This is ionisation. Ionising radiation can damage living cells, and the risk depends on the type of radiation and the size of the dose.
Nuclear power produces waste of different grades. Low-level waste includes materials such as contaminated protective clothing. It is packaged and disposed of at authorised facilities, and some very low-level waste can go to permitted landfill sites. High-level waste from the reactor core stays intensely radioactive for thousands of years.
High-level waste is stored under water while it cools, then sealed in glass, encased in metal and stored deep underground in geologically stable rock. The difficulty is not chemistry but time: the storage has to remain secure for far longer than any human institution has ever lasted.
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