Radioactivity Study Guide

Beta-minus (β-) decay is a common type of radioactive decay in which an unstable atomic nucleus transforms into a different nucleus by emitting a beta particle (an electron) and an antineutrino. This process occurs when there is an excess of neutrons in the nucleus. During β- decay, one of the neutrons within the nucleus effectively converts into a proton. To conserve charge and lepton number, an electron (beta particle) is ejected from the nucleus, and an electron antineutrino is also emitted. The atomic number of the atom increases by one, while its mass number remains unchanged, resulting in the formation of an isotope of a new element. For example, carbon-14 decays into nitrogen-14 through β- decay.

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.