How to measure radioactivity ?

Radiation coming from a radioactive source is not directly perceptible. Radioactivity is invisible, inaudible, odorless, etc. It was only studied once instruments had been developed to detect the phenomenon, determine its characteristics and measure its intensity with precision.

All detection methods are based on the fact that radiation causes ionization (the stripping of electrons from atoms) and excitation (the transmission of a quantity of energy to impacted atoms that change from a fundamental state to an excited state) in exposed matter, leaving a footprint. By calculating the number of ionizations or excitations per unit time caused by the particles (in the case of alpha or beta radiation) or photons (in the case of gamma radiation), it is possible to quantify the energy transmitted by radiation to the material. This energy is characteristic of the emitting radionuclide species.

The three types of detectors most commonly used are gas-filled (e.g. the Geiger-Muller counter), scintillation and semiconductor radiation counters. They are all based on the same principle, i.e. the conversion of the photons or electrons created by the radiation into an electrical signal.

Most radiation detectors impose an electric field to separate and count the ions (or electrons) produced by radiation passing through the detector. Generally, the radiation penetrates into the sensitive volume of the detector comrising either a gas, solid or liquid, depending on the type of radiaton to be measured. The interaction produces a series of discrete events (the production of ionization electrons, photons, heat, etc.). However, no matter how a detector operates, and therefore whatever the underlying radiation detection principles, the following components are present:

- a probe in which the radiation interacts with the matter in the detector

- a system to shape and amplify the signal produced by the probe

- a signal processing system, if necessary