particle detector, in physics, device for detecting, measuring, and analyzing particles and other forms of radiation entering it. Such devices play an important role not only in basic research, as in the study of elementary particles, but also in numerous applications of physics, from uses of radioactive tracers in medicine and biology to prospecting for natural ores that exhibit radioactivity. Almost all instruments used for detecting are based either on the ionization of matter caused by radiation or on the luminescence it can cause in certain materials. In devices based on ionization (the separation of neutral atoms or molecules into oppositely charged fragments), various methods may be used to convert the ionization to a useful measure of the radiation. In the ionization chamber and the Geiger counter, the radiation is measured by changes in an external electrical circuit; these changes are due to a current resulting from the increase of charges. In the bubble chamber, cloud chamber, and spark chamber, ionization is used to make visible the track of the charged particle causing the ionization. By adding a magnetic field across the path of incoming particles, a great deal can be learned about the nature and properties of the particles detected by the chambers; the presence of uncharged particles can be indicated indirectly as well. Another device used in studies of particles records a visible track made in a photographic emulsion. The most important device based on the luminescent effect of radiation is the scintillation counter. In addition to these larger instruments, there are also small detectors designed to be worn or carried by persons working near sources of potentially dangerous radiation. These are scaled down and sometimes simplified versions of the devices already described. Typical of these small detectors are pocket-size ionization chambers that resemble fountain pens and film detectors, embedded in badges, that register the amount of radiation by the degree of exposure of the film.