air shower detection methods

Shower detection

Extensive air showers with many particles arriving on the ground can be detected with different kinds of particle detectors. Most common are scintillation counters, allowing to measure the time of arrival with high accuracy. Other common devices include water Cherenkov counters, resistive plate chambers and a variety of position-sensitive devices which allow to measure the particle direction. These tracking detectors include different types of drift chambers, streamer tube detectors, and Geiger tube detectors.

Hundreds of single particles arrive per square meter per second but extensive air showers are less common. Therefore, coincidences of several particle detectors are required. When looking for small showers with perhaps a few thousand particle, tens or hundreds of detectors separated by 10-30 meters are usually used. When looking for the much less frequent very large showers with billions of particles, the detectors can be placed at separations of about a kilometer to the next neighbour.

Showers can be detected at even larger distances by the fluorescence light which is emitted by nitrogen molecules when charged particles are passing near-by. Imaging devices ('telescopes' with photomultipliers as cameras) can see the track of air showers through the atmosphere. This method was first used by the Fly's Eye experiment. Because the fluorescence light is very weak, only the most energetic showers can be detected this way.

With similar light-sensitive devices, the more intense Cherenkov light of showers can be detected. Since the Cherenkov light is only emitted at a narrow angle to the shower axis, it can be seen only when being at most a few hundred meters from the shower axis.

Further reading found on the net (Postscript or PDF format):
Particle Data Group: review of cosmic rays
Low threshold particle arrays
Additional reading in HTML format:
Detection of UHE Cosmic Rays from the Utah HiRes group.

This page was written by Konrad Bernlöhr.

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