Topic E: Radioactivity and Nuclear Energy

Radioactivity and Nuclear Energy looks inside the nucleus to explain why some atoms are unstable and emit radiation. You will identify the three main types of radiation, use half-life to describe decay, compare nuclear fission and fusion as sources of energy, weigh the uses and dangers of radiation, and connect mass and energy through E = m c squared.

The atomic model and the nucleus

An atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by negative electrons. The number of protons is the atomic number Z, which identifies the element, and the total number of protons and neutrons is the mass number A. A nuclide is written with A as a superscript and Z as a subscript before the chemical symbol. Isotopes are atoms of the same element with the same Z but different numbers of neutrons, so they have the same chemistry but different nuclear stability. The strong nuclear force holds protons and neutrons together against the electrical repulsion of the protons, but in some nuclei this balance fails and the nucleus is unstable.

Radioactivity: alpha, beta and gamma

Radioactivity is the spontaneous and random emission of radiation from unstable nuclei as they decay to become more stable. There are three main types. An alpha particle is a helium nucleus (two protons and two neutrons), is highly ionising but has low penetration, stopped by paper or a few centimetres of air. A beta particle is a fast electron emitted when a neutron changes into a proton, is moderately ionising and penetrating, stopped by a few millimetres of aluminium. Gamma radiation is a high-energy electromagnetic wave, weakly ionising but very penetrating, reduced only by thick lead or concrete. Emission of alpha or beta changes the nucleus into a different element, which can be tracked with decay equations that balance mass number and atomic number.

Decay and half-life

Radioactive decay is a random process: it is impossible to predict when a particular nucleus will decay, but for a large number of nuclei the behaviour is statistically predictable. The half-life is the average time taken for half the radioactive nuclei in a sample to decay, or equivalently for the activity (the number of decays per second, measured in becquerel) to fall to half its value. Each half-life reduces the remaining undecayed nuclei by another half, giving an exponential decay curve. Half-lives range from fractions of a second to billions of years. Knowing the half-life lets us calculate how much of a source remains after a given time and is the basis of radioactive dating, such as carbon-14 dating of once-living material.

Nuclear fission

Nuclear fission is the splitting of a large unstable nucleus, such as uranium-235, into two smaller nuclei when it absorbs a neutron, releasing several more neutrons and a large amount of energy. The extra neutrons can trigger further fissions, producing a chain reaction. In a nuclear reactor this chain reaction is controlled: a moderator slows the neutrons so they are more easily absorbed, and control rods absorb surplus neutrons to keep the reaction steady. The energy released heats a coolant, which raises steam to drive turbines and generators. Fission produces highly radioactive waste with long half-lives, which must be stored safely, and carries the risk of meltdown if cooling fails.

Nuclear fusion

Nuclear fusion is the joining of two very light nuclei, such as isotopes of hydrogen, to form a heavier nucleus, releasing even more energy per kilogram than fission. Fusion is the process that powers the Sun and other stars, where hydrogen nuclei fuse into helium. To overcome the strong electrical repulsion between the positively charged nuclei, fusion requires extremely high temperatures and pressures, which makes it very hard to achieve and sustain on Earth. Its great attraction is that the fuel (hydrogen) is abundant and the products are far less radioactive than fission waste, so controlled fusion is seen as a potential clean energy source for the future.

Uses, hazards and safety of radiation

Radiation has many beneficial uses: gamma sources sterilise medical equipment and treat cancer, tracers diagnose problems in the body and detect leaks in pipes, and the penetrating power of beta or gamma radiation is used in thickness gauges. However, ionising radiation damages living cells by knocking out electrons and breaking molecules, which can cause radiation sickness, cancer or genetic mutations. Safety measures include keeping a large distance from sources, limiting exposure time, using shielding such as lead, handling sources with tongs, and wearing film badges to monitor dose. Background radiation, from rocks, cosmic rays and medical sources, is always present and must be subtracted when measuring a source.

Mass-energy equivalence

Einstein's equation E = m c squared states that mass and energy are equivalent, where c is the speed of light, about 3 x 10^8 metres per second. Because c squared is enormous, even a tiny loss of mass releases a huge amount of energy. In both fission and fusion the total mass of the products is slightly less than the mass of the original nuclei; this missing mass, the mass defect, is converted into the energy released. This is why nuclear reactions release millions of times more energy per kilogram of fuel than chemical reactions such as burning fossil fuels, and it explains the binding energy that holds a stable nucleus together.