Radioactivity Study Guide

Radioactive decay results in the emission of alpha (α), beta (β), and gamma (γ) radiation, each possessing distinct characteristics regarding their ability to penetrate materials and cause ionisation. Penetration refers to how far radiation can travel through a substance, while ionisation is the process of removing electrons from atoms, creating ions. Alpha particles, being relatively large and carrying a +2 charge, interact strongly with matter, leading to high ionisation but very low penetration; they can be stopped by a sheet of paper. Beta particles, which are fast-moving electrons, have less mass and charge (-1) than alpha particles, resulting in lower ionisation but greater penetration, typically stopped by a few millimetres of aluminium. Gamma rays, high-energy electromagnetic waves, have no charge or mass. This allows them to penetrate deeply through materials, requiring thick lead or concrete to significantly reduce their intensity, but they cause the least ionisation per unit path length. Understanding these differences is crucial for radiation protection.

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.