Radioactivity Study Guide

An atom is the fundamental building block of matter, consisting of a central, positively charged nucleus surrounded by negatively charged electrons. The nucleus is composed of two types of subatomic particles: protons, which carry a positive charge, and neutrons, which have no charge (are neutral). The electrons orbit the nucleus in specific energy levels or shells. The vast majority of an atom's mass is concentrated within its nucleus, as protons and neutrons are significantly more massive than electrons. The nuclear radius is incredibly small compared to the overall radius of the atom. If an atom were the size of a football stadium, its nucleus would be no larger than a pea at the centre, demonstrating the vast empty space within an atom.

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.