Radioactivity Study Guide

The nucleus of an atom contains protons and neutrons, collectively called nucleons. The atomic number (proton number), symbolised by Z, represents the count of protons in the nucleus. This number uniquely identifies an element and determines its chemical properties. The mass number (nucleon number), symbolised by A, is the total count of protons and neutrons in the nucleus. Isotopes are atoms of the same element, meaning they have the same atomic number (Z), but differ in their mass number (A) due to different numbers of neutrons. For example, Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons (Z=6), but Carbon-12 has 6 neutrons (A=12) while Carbon-14 has 8 neutrons (A=14). We represent an isotope using the notation ⁱ⁳C₆, where A is the mass number and Z is the atomic number.

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.