Particle-matter interaction - Detectors

Detectors are assemblies capable of measuring the properties of radiation (mass, charge, trajectory...), one by one, and in a 4 π geometry most often: as an example, the central Delphi detector at CERN (microvertex detector) consists of 3 x 10 ⁷ detection cells corresponding to 126,000 signal paths to be processed, to identify all the trajectories of particles created in an e ⁺ + e reaction ^- at 100 GeV in the center of mass. In this case, the particle deposits a tiny fraction of its energy in each detector plane crossed, enabling its trajectory to be reconstructed. The particle's complementary characteristics are measured by other detectors, also numbering in the tens of thousands, so the particle's energy is measured in massive detectors (calorimeters) that stop it dead in its tracks.

Twentieth-century physics is characterized by the indirect observation of the phenomena under study, as the eye is no longer suited to the direct perception of these phenomena, contrary to everything that had been practiced until then. The discovery of X-rays by Wilhelm Conrad Röntgen and the indirect detection of a physical phenomenon can be precisely dated back to 1895: it was the fluorescence of a barium platinocyanide screen that revealed (indirectly) to Röntgen's eye the emission of X-rays from an electric discharge tube.

Detecting ionizing radiation (directly ionizing, such as a charged particle, or indirectly ionizing, such as X and γ photons or neutrons) involves taking all or part of the radiation's energy and transforming it into a more manageable form: following the ionizations created by the particle as it passes through, the positive charges (ions) and negative charges (electrons) can be separated under the action of an electric field: this gives rise to an electric current. This is the principle behind gaseous detectors (ionization chamber, Geiger-Müller counter, proportional multi-thread chamber, etc.), liquid detectors using argon, for example, and solid-state semiconductor detectors such as Si and Ge junctions. When the charges created by primary ionization are not separated, recombination occurs, which may be accompanied by light emission in the case of scintillators, a "memory effect" of the medium passed through (emulsions, bubble chambers...), a rise in temperature (thermal detectors) and other less detectable phenomena.

In the case of scintillators, the energy lost through radiation in a given thickness of material will be recovered in the form of a certain number of luminescent photons (proportional to the energy transferred); an appropriate sensor (photomultiplier tube, photodiode) will transform...