preshower

A preshower is an electromagnetic cascade that begins before the main detection medium or calorimeter, typically triggered when a high-energy photon or electron interacts with a material or a strong external field and produces secondary particles that start shower development outside the primary detector. The term is used in both particle physics detectors and astroparticle physics, and its presence can alter the longitudinal and lateral development of the subsequent shower.

In detector systems, preshower detectors are placed in front of electromagnetic calorimeters to initiate a shower early. They often employ a layer of high‑Z material, such as lead or tungsten, followed by an array of active sensors (for example silicon strips or scintillators). The early conversion of photons into electron–positron pairs and the subsequent energy deposition help with photon identification, electron–photon separation, and improved determination of the shower direction and energy. These layers aid in distinguishing electromagnetic showers from hadronic ones and can enhance angular and energy resolution.

In astroparticle physics, a preshower can form when ultra-high-energy photons traverse a strong external field, notably the Earth's geomagnetic field, before entering the atmosphere. The field can induce pair production and subsequent radiation, causing the initial shower to develop partly in space rather than entirely in the atmosphere. This geomagnetic preshower changes the observed air shower profile, affecting interpretation by ground-based detectors such as extensive air shower arrays and Cherenkov telescopes. The effect depends on photon energy, arrival direction, and the local magnetic field.