Radioactivity Study Guide

Unstable atomic nuclei undergo radioactive decay, a process where they emit radiation to achieve a more stable state. This emission is fundamentally random, meaning it's impossible to predict precisely when a specific nucleus will decay, though the decay rate of a large sample is predictable. The types of radiation commonly emitted include alpha particles, beta-minus particles (electrons), beta-plus particles (positrons), gamma rays (electromagnetic radiation), and neutrons. Each type of radiation has distinct properties, affecting its penetrating ability and ionising power, and is a consequence of intricate changes within the nucleus.

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.