Radioactivity Study Guide

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. This process almost always results in a change to the nucleus's composition, specifically its atomic number (number of protons) and its mass number (total number of protons and neutrons, also known as nucleons). Alpha decay involves the emission of an alpha particle, which is identical to a helium nucleus (two protons, two neutrons). This causes the atomic number to decrease by 2 and the mass number to decrease by 4. Beta-minus decay occurs when a neutron in the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino. This increases the atomic number by 1, while the mass number remains unchanged. Gamma decay is the emission of electromagnetic radiation (gamma rays) from an excited nucleus. Since gamma rays are photons and have no mass or charge, both the atomic number and mass number remain unchanged during gamma decay; it only releases excess energy. Neutron emission, which is less common in typical decay chains, involves the direct ejection of a neutron from the nucleus. This reduces the mass number by 1, but the atomic number remains unaffected.

Radioactive substances are used in medical diagnosis to help visualise internal body structures and functions. They are primarily used as 'tracers', which means they are introduced into the body and their movement or concentration can be tracked by external detectors. The most common types of radiation emitted by these tracers are gamma rays, because they have a high penetrating power and can easily pass through body tissues to be detected outside without causing excessive harm. Alpha and beta radiation are generally not suitable for typical imaging inside the body due to their lower penetrating power and higher ionising effect within tissues where they decay. An important diagnostic tool is a PET (Positron Emission Tomography) scanner. This technique uses a radioactive tracer that emits positrons. When a positron meets an electron in the body, they annihilate, producing two gamma rays that travel in opposite directions. These gamma rays are detected by a ring of detectors around the patient, and a computer uses this information to build up a 3D image of the internal organ. Tracers are chosen based on their half-life (short enough to decay quickly, but long enough for the procedure) and their chemical properties, which allow them to accumulate in specific tissues or follow certain metabolic pathways.