Radioactivity Study Guide

Electrons within an atom can only exist in specific energy levels, often visualised as distinct orbits around the nucleus. When an electron absorbs electromagnetic radiation, it gains energy and can transition to a higher energy level or orbit, provided the absorbed energy matches the exact energy difference between the initial and final orbits. Conversely, when an electron falls from a higher energy level to a lower one, it emits the excess energy as electromagnetic radiation. The frequency and wavelength of the emitted radiation are directly proportional to the energy difference between these orbits. This phenomenon is fundamental to understanding how atoms interact with light.

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.