Radioactivity and Nuclear Decay

Radioactivity is the spontaneous decay of an unstable atomic nucleus. The nucleus releases energy or particles to become more stable. Radioactive decay is a nuclear process and is not normally affected by temperature, pressure or chemical reactions.

A stable nucleus has a combination of protons and neutrons that allows it to remain unchanged. An unstable nucleus has an unfavourable nuclear arrangement or excess energy and can undergo radioactive decay to reach a more stable state.

A radionuclide is an atom with an unstable nucleus that undergoes radioactive decay. Radionuclides can emit alpha particles, beta particles or gamma radiation. Different radionuclides have different decay modes and half-lives.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. A radioisotope is an unstable isotope that undergoes radioactive decay. Not all isotopes are radioactive.

Alpha radiation consists of particles containing two protons and two neutrons, identical to helium-4 nuclei. Alpha particles have a +2 charge, relatively large mass and strong ionising ability. They have a short range and low penetration.

During alpha decay, an unstable nucleus emits an alpha particle. Its mass number decreases by 4 and atomic number decreases by 2. Example: uranium-238 decays into thorium-234 and an alpha particle.

Beta minus radiation consists of high-energy electrons emitted from an unstable nucleus. It occurs when a neutron changes into a proton, releasing an electron and an electron antineutrino. The atomic number increases by 1 while the mass number stays unchanged.

In beta minus decay: neutron → proton + electron + electron antineutrino. Example: carbon-14 decays into nitrogen-14. The mass number remains 14, but the atomic number increases from 6 to 7.

Beta plus radiation consists of positrons, which have the same mass as electrons but a positive charge. It occurs when a proton changes into a neutron, emitting a positron and an electron neutrino. The atomic number decreases by 1.

In beta plus decay: proton → neutron + positron + electron neutrino. Example: fluorine-18 decays into oxygen-18. The mass number remains unchanged while the atomic number decreases from 9 to 8.

A positron eventually interacts with an electron, causing annihilation. Their mass is converted into energy, usually producing two 511 keV photons travelling in approximately opposite directions. This process is used in PET imaging.

Gamma radiation consists of high-energy electromagnetic photons emitted from an excited nucleus. Gamma photons have no mass or electric charge. They are penetrating and indirectly ionising, and their emission does not change the atomic or mass number.

Gamma decay occurs when an excited nucleus releases excess energy as a gamma photon and moves to a lower energy state. The number of protons and neutrons remains unchanged. Gamma emission can follow alpha or beta decay.

Alpha particles are heavy, positively charged and strongly ionising but weakly penetrating. Beta particles are lighter, charged and moderately penetrating. Gamma rays are uncharged photons, less densely ionising and generally more penetrating.

Ionising ability describes how effectively radiation removes electrons from atoms. Alpha particles generally produce dense ionisation along short tracks. Beta particles produce less dense ionisation, while gamma photons cause ionisation indirectly through secondary charged particles.

Penetrating ability describes how far radiation travels through matter. Alpha particles are stopped by paper or the outer dead layer of skin. Beta particles can be reduced using suitable plastic shielding. Gamma radiation requires substantial shielding such as lead or concrete.

Radioactive decay is a random process for individual nuclei. It is impossible to predict exactly when one nucleus will decay, but the behaviour of a large number of nuclei can be described statistically using decay laws and half-life.

Activity is the rate at which radioactive nuclei decay. It is measured in becquerels (Bq), where 1 Bq equals one nuclear decay per second. Higher activity means more decays occur each second, but activity alone does not determine absorbed dose.

Half-life is the time required for the number of undecayed radioactive nuclei, or their activity, to decrease to half its original value. Each radionuclide has a characteristic physical half-life. After one half-life 50% remains, after two 25%, and after three 12.5%.

The remaining activity can be calculated using A = A₀ × (1/2)ⁿ, where A₀ is the initial activity and n is the number of half-lives. Example: 800 MBq after three half-lives becomes 800 × (1/2)³ = 100 MBq.

The decay constant, λ, describes the probability of a nucleus decaying per unit time. It is related to half-life by λ = 0.693 / T½. A larger decay constant means faster decay and therefore a shorter half-life.

Radioactive decay follows an exponential relationship: A = A₀e^(−λt), where A is remaining activity, A₀ is initial activity, λ is the decay constant and t is elapsed time. The activity decreases continuously rather than by equal amounts.

Physical half-life describes radioactive decay. Biological half-life describes how quickly the body removes a substance through biological processes. Effective half-life combines both processes and is calculated using 1/T_effective = 1/T_physical + 1/T_biological.

Background radiation is ionising radiation naturally or artificially present in the environment. Sources include cosmic rays, radioactive materials in rocks and soil, radon gas and naturally occurring radionuclides within the body.

Contamination occurs when radioactive material is deposited on or inside a person or object. Irradiation occurs when radiation reaches a person or object from a source. Exposure to X-rays does not normally make a person radioactive.

Radioactive substances are used in nuclear medicine for diagnosis and treatment. Examples include technetium-99m in diagnostic imaging, fluorine-18 in PET and iodine-131 in thyroid treatment. Their usefulness depends on radiation type, energy, half-life and biological behaviour.