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In experimentalparticle physics, acalorimeter is a type of detector that measures theenergy ofparticles. Particles enter thecalorimeter and initiate aparticle shower in which their energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its entirety, requiring total containment of the particle shower, or it may be sampled. Typically, calorimeters are segmented transversely to provide information about the direction of the particle or particles, as well as the energy deposited, and longitudinal segmentation can provide information about the identity of the particle based on the shape of the shower as it develops.Calorimetry design is an active area of research in particle physics.
Anelectromagnetic calorimeter (ECAL) is one specifically designed to measure the energy of particles that interact primarily via theelectromagnetic interaction such as electrons, positrons and photons. Ahadronic calorimeter (HCAL) is one designed to measure particles that interact via thestrong nuclear force. (Seetypes of particle showers for the differences between the two.) Calorimeters are characterized by theradiation length (for ECALs) andnuclear interaction length (for HCALs) of their active material. ECALs tend to be 15–30 radiation lengths deep while HCALs are 5–8nuclear interaction lengths deep.
An ECAL or an HCAL can be either asampling calorimeter or ahomogeneous calorimeter.
In asampling calorimeter, the material that produces the particle shower is distinct from the material that measures the deposited energy. Typically the two materials alternate. One advantage of this is that each material can be well-suited to its task; for example, a very dense material can be used to produce a shower that evolves quickly in a limited space, even if the material is unsuitable for measuring the energy deposited by the shower. A disadvantage is that some of the energy is deposited in the wrong material and is not measured; thus the total shower energy must be estimated instead of being measured directly.
Ahomogeneous calorimeter is one in which the entire volume is sensitive and contributes a signal.[1]
Mostparticle physics experiments use some form of calorimetry. Often it is the most practical way to detect and measure neutral particles from an interaction. In addition, calorimeters are necessary for calculating "missing energy" which can be attributed to particles that rarely interact with matter and escape the detector, such as neutrinos. In most experiments the calorimeter works in conjunction with other components like a central tracker and amuon detector. All the detector components work together to achieve the objective of reconstructing a physics event.