Radioactivity

At the centre of every atom is a tiny, dense nucleus, which contains positively charged protons and neutral neutrons. These particles are collectively called nucleons. Orbiting the nucleus are negatively charged electrons. An atom is electrically neutral because the number of protons equals the number of electrons. The identity of an element is determined solely by its proton number (Z), also called the atomic number. The mass number (A), or nucleon number, is the total count of protons and neutrons in the nucleus. Different versions of the same element, called isotopes, exist. Isotopes have the same number of protons but different numbers of neutrons. For example, Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons, but Carbon-12 has 6 neutrons (A=12) while Carbon-14 has 8 neutrons (A=14). We represent a specific nucleus, or nuclide, with the notation A_Z_X, where X is the element symbol.

Some isotopes have unstable nuclei. To become more stable, they spontaneously break down, or 'decay', emitting energy and/or particles. This process is called radioactive decay, and the emissions are called nuclear radiation. There are three main types: Alpha (α), Beta (β), and Gamma (γ). An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons. A beta particle is a high-energy electron that is created and ejected from the nucleus when a neutron changes into a proton. Gamma radiation is not a particle; it is a high-energy electromagnetic wave, often emitted alongside alpha or beta particles as the nucleus settles into a more stable energy state.

The three types of radiation interact with matter differently, giving them distinct properties. Penetrating power describes how far radiation can travel through a material. Alpha is the least penetrating (stopped by paper or a few cm of air). Beta is more penetrating (stopped by a few mm of aluminium). Gamma is the most penetrating (requiring thick lead or concrete to significantly reduce it). Ionising power is the ability to knock electrons out of atoms, creating ions. This is how radiation damages living cells. Alpha is the most ionising, and gamma is the least. These two properties are inversely related: highly ionising alpha particles interact strongly with matter, lose their energy quickly, and therefore do not penetrate far. Because of their charge, alpha and beta particles are deflected in electric and magnetic fields (in opposite directions), while neutral gamma rays are not. Radiation is detected using devices like a Geiger-Müller (GM) tube, which works by detecting the ionisation the radiation causes in a gas.

The 'activity' of a radioactive sample is the rate at which its nuclei decay. It is measured in becquerels (Bq), where 1 Bq equals one decay per second. While the decay of any single nucleus is random, the overall rate of decay for a large sample is predictable. The activity of a sample decreases over time as the number of undecayed nuclei decreases. The time it takes for the activity of a sample to fall to half its initial value is called the half-life (t½). This is a constant for a given radioisotope. For example, if a sample has an activity of 800 Bq and a half-life of 10 days, its activity will be 400 Bq after 10 days, 200 Bq after 20 days, and 100 Bq after 30 days.

Nuclear equations are used to represent the changes that occur in a nucleus during radioactive decay. Like chemical equations, they must be balanced. This means the total mass number (A) and the total atomic number (Z) must be the same on both sides of the equation. In alpha decay, an alpha particle (a helium nucleus, ⁴₂He) is emitted. This causes the mass number (A) of the parent nucleus to decrease by 4 and the atomic number (Z) to decrease by 2. In beta decay, a beta particle (an electron, ⁰₋₁e) is emitted. This happens when a neutron turns into a proton. Therefore, the mass number (A) remains unchanged, but the atomic number (Z) increases by 1, forming a new element.

We are constantly exposed to low levels of ionising radiation from our surroundings. This is called background radiation. Natural sources are the biggest contributors and include: cosmic rays from space, radioactive rocks and soil (like granite), radon gas released from the ground, and radioactive elements in food and drink (like potassium-40 in bananas). Artificial sources include medical X-rays and treatments, and fallout from nuclear weapons testing or accidents. High doses of radiation are dangerous because ionisation can damage or kill living cells, or cause mutations in DNA that can lead to cancer. To work safely with radioactive sources, three principles are used: limit exposure time, increase distance from the source, and use shielding (e.g., lead aprons). Despite the dangers, radioisotopes have many beneficial uses. These include medical tracers (e.g., Technetium-99m to study blood flow), cancer therapy (radiotherapy), smoke detectors (Americium-241), and industrial thickness gauging.