Topic 5: Nuclear physics

Nuclear physics studies the tiny, dense core at the centre of every atom and the radiation some nuclei give out as they become more stable. In this topic you will learn how atoms are built, how to read nuclide symbols, and how unstable nuclei decay over time. You will also meet half-life calculations, radiation safety, and the energy-releasing processes of fission and fusion.

The nuclear model of the atom

An atom is mostly empty space. At its centre sits a very small, very dense nucleus that contains positively charged protons and uncharged neutrons. Negatively charged electrons orbit this nucleus at relatively large distances. Because the proton charge is positive and the electron charge is negative and equal in size, a neutral atom has equal numbers of protons and electrons, so its overall charge is zero. Almost all of the atom's mass is concentrated in the nucleus, since electrons are roughly two thousand times lighter than protons or neutrons. The scattering of alpha particles by thin metal foil gave the first evidence for this model: most particles passed straight through (empty space), while a few were deflected sharply back (the small, dense, positive nucleus).

Proton number, nucleon number and nuclide notation

The proton number (also called atomic number), given the symbol Z, is the number of protons in a nucleus. It decides which element the atom is. The nucleon number (also called mass number), given the symbol A, is the total number of protons and neutrons together, since protons and neutrons are jointly called nucleons. The number of neutrons therefore equals A minus Z. A particular nucleus is described using nuclide notation, written with the nucleon number as a superscript and the proton number as a subscript in front of the chemical symbol. For example, carbon with twelve nucleons is shown with A equal to 12 and Z equal to 6, telling you it has six protons and six neutrons.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same proton number, they have the same chemical properties and occupy the same place in the Periodic Table, but their nucleon numbers differ. For example, chlorine exists naturally as two isotopes, one with 18 neutrons and one with 20 neutrons, both having 17 protons. Some isotopes are stable, while others have an unstable balance of protons and neutrons and are radioactive, meaning their nuclei break down and give out radiation. The chemical behaviour of an element does not depend on which isotope is present, but the nuclear behaviour can be very different.

Background radiation and detecting radioactivity

Background radiation is the low-level ionising radiation that is present everywhere, all the time. It comes from natural sources such as rocks and soil (especially radon gas), cosmic rays from space, food, drink, and the human body, as well as man-made sources such as medical X-rays and the fuel industry. When measuring the radioactivity of a sample, you must first measure this background count rate and then subtract it from your readings to get the true count from the source. A common detector is the Geiger-Muller tube connected to a counter, which clicks or counts each time an ionising particle enters it. Radioactivity is a random process: you cannot predict when any single nucleus will decay, only the average rate of decay for a large number of nuclei.

The three types of nuclear emission and their properties

Unstable nuclei emit three main kinds of radiation. Alpha radiation is a helium nucleus made of two protons and two neutrons, so it has a charge of plus two and a relatively large mass. It is the most strongly ionising but the least penetrating, stopped by a sheet of paper or a few centimetres of air. Beta radiation is a fast-moving electron with a charge of minus one and very small mass. It is moderately ionising and is stopped by a few millimetres of aluminium. Gamma radiation is a high-energy electromagnetic wave with no charge and no mass. It is the least ionising but the most penetrating, needing thick lead or concrete to reduce it greatly. Because alpha and beta are charged, they are deflected by electric and magnetic fields in opposite directions, while uncharged gamma is not deflected at all.

Radioactive decay and nuclear equations

Radioactive decay is the spontaneous change of an unstable nucleus into a different, more stable one, with the release of radiation. The process is random and cannot be sped up, slowed down, or stopped by temperature, pressure, or chemical change. In a nuclear equation, the total nucleon number and the total proton number must each be the same on both sides. When a nucleus emits an alpha particle, its nucleon number falls by 4 and its proton number falls by 2, so it becomes a new element. When a nucleus emits a beta particle, a neutron turns into a proton and an electron is ejected, so the nucleon number stays the same while the proton number rises by 1. Gamma emission carries away energy only, so it does not change the nucleon or proton numbers; it usually follows alpha or beta decay as the nucleus settles.

Half-life with a worked example

The half-life of a radioactive isotope is the average time taken for half the unstable nuclei in a sample to decay, or equally the time for the count rate (above background) to fall to half its value. Because decay is random but predictable on average, the activity falls by the same fraction in each equal time interval. Worked example: a sample has an activity of 800 counts per second and a half-life of 5 hours. After 5 hours (one half-life) the activity falls to 400 counts per second. After 10 hours (two half-lives) it is 200, after 15 hours (three half-lives) it is 100, and after 20 hours (four half-lives) it is 50 counts per second. To find how many half-lives have passed, count how many times you must halve the starting value to reach the final value, then multiply by the half-life to get the total time.

Safety, handling, and uses of radioactivity

Ionising radiation is dangerous because it can damage or kill living cells and may cause cancer, so exposure must be kept as low as possible. Safe handling means keeping sources at a distance using tongs or tools, limiting the time of exposure, storing sources in lead-lined boxes, and never pointing a source at people. People who work with radiation wear badges to monitor their dose. Despite the dangers, radioactivity has many valuable uses. Gamma sources are used to sterilise medical equipment and food, and to treat cancer by killing tumour cells. Beta sources are used in thickness gauges for paper or metal sheets. Radioactive tracers help doctors follow processes inside the body and engineers find leaks in underground pipes. Carbon-14 dating uses a known half-life to estimate the age of once-living material.

An introduction to fission and fusion

Nuclear fission is the splitting of a large, unstable nucleus, such as uranium, into two smaller nuclei when it absorbs a neutron, releasing energy and more neutrons. Those extra neutrons can split further nuclei in a chain reaction, which is controlled in a nuclear power station to generate electricity. Nuclear fusion is the joining together of two very light nuclei, such as hydrogen isotopes, to form a heavier nucleus, also releasing energy. Fusion is the process that powers the Sun and other stars, but it requires extremely high temperatures and pressures to make the positively charged nuclei get close enough to join. Both processes convert a tiny amount of mass into a large amount of energy, but fusion releases more energy per unit mass than fission and produces less long-lived radioactive waste.