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Particle detectors

Today, highly sophisticated particle detectors are used to study traces and events caused by collision of electrically charged particles that have been brought close to the speed of light in huge particle accelerators . The following instruments greatly contributed to uncover the hidden particle world:

• Photomultipliers and Geiger counters. Photomultipliers measure low intensity electromagnetic radiation, down to a single photon, while Geiger counters measure ionizing radiation, including gamma, beta, and alpha radiation (see Radioactivity). Photomultipliers use the photoelectric effect of photons and the cascading effect of freed electrons. In a vacuum tube with an electric field for electron acceleration, each freed electron causes the release of 3-10 new electrons when impacting an anode of suitable material; the process is repeated several times to obtain a measurable current proportional to the number of photons that entered the instrument. In Geiger counters, the entering ionizing particles (photons, electrons, and sometimes nuclides) make an inert gas in the instrument conductive. The cascade effect then yields a measurable current or voltage.
• Cloud and bubble chambers. Historically, these chambers traced the path of particles exiting accelerators. Mass and charge of the particles could be determined from the particles’ trajectories caused by application of magnetic and electric fields. Cloud chambers were filled with a saturated air-alcohol mixture, and bubble chambers with liquid hydrogen at boiling point. On a sudden drop of pressure (adiabatic expansion) in sync with entering particles the gas would condense (cloud chamber) or the liquid evaporate (bubble chamber) along the path of particles inducing the phase change by ionization. Detailed evaluation of the tracks were made on photographs. Today, particle paths are captured electronically in multi-wired gaseous ionization chambers.
• Solid state detectors. In principle, these detectors are based on the photoelectric effect of semiconductors in similar ways as the CCD and CMOS sensors of digital cameras. Modern detectors of accelerators are very complex machines that incorporate a huge number of individual solid state detecting elements, together with gaseous ionization chambers, all designed to trace the paths of different particles electronically and provide data for determining charge, mass, spin, and momentum of the particles. Huge streams of data are generated, of which only a small portion can be recorded and evaluated ^[1] . In a modern particle collider the detecting elements are arranged around the location of collision, where the two crossing beams are squeezed by strong magnets into the smallest possible cross section for increased probability of collisions. The total assembly, called experiment, is a very large and heavy machine ^[2] .

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