Radioactivity Study Guide

Radioactivity, the emission of energetic particles or rays from unstable atomic nuclei, can be measured and detected using various methods. Two common methods are photographic film and Geiger-Müller (GM) tubes. Photographic film darkens when exposed to radiation, with the degree of darkening indicating the amount of radiation absorbed. This happens because radiation ionises silver halide crystals in the emulsion, causing them to develop into visible silver grains. This method does not provide a real-time count but accumulates the effect of radiation over time, making it useful for personal dosimeters worn by individuals working with radioactive materials. A Geiger-Müller tube, on the other hand, provides real-time detection and measurement. It consists of a metal outer casing (cathode) and a central wire (anode) within a gas-filled tube, typically at low pressure. When ionising radiation enters the tube, it ionises the gas atoms, creating electrons and positive ions. These charged particles are accelerated by a high voltage applied between the anode and cathode, leading to a cascade of further ionisations. This results in a momentary electric current or pulse, which is detected by an electronic circuit and registered as a 'click' or a count. The rate of these pulses corresponds to the intensity of the radiation.

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.