Radioactivity Study Guide

Background radiation refers to the ionising radiation that is naturally present in our environment originating from various sources, making it detectable almost everywhere at all times. This radiation is typically low-level but can vary depending on geographical location and other factors. It primarily comes from natural sources like radioactive rocks in the ground, cosmic rays from space, and radioactive gases such as radon that diffuse from the Earth's crust into the atmosphere. A smaller fraction of background radiation also originates from artificial or man-made sources, which include medical procedures like X-rays, nuclear power generation, and fallout from historical nuclear weapons testing. Constant monitoring of background radiation levels is important for safety and for understanding its potential effects on living organisms.

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.