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- Cosmic rays : high energy particles from the Universe
- Solar Wind, Heliosphere, and Cosmic Ray Propagation
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- A few technical details
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- Muon detectors
- Underground muon experiments
- Extensive Air Shower arrays
- Cherenkov detectors
- Balloon detectors
- Further reading
The muon component of the atmospheric cascade is measured by muon detectors. It is important to note that only primary cosmic ray (CR) nucleons >4 GeV have sufficient energy to generate muons that can penetrate through the atmosphere. The detection of the muons is realized e.g. by utilizing Geiger-Müller counters or scintillation counters. The Geiger-Müller counters require high voltage, which creates a very high electric field near the anode of the detectors. When a CR particle enters a detector, it strips off some electrons from the counting gas and from the counter tube wall. These electrons are accelerated towards the positively charged wire and gain enough energy to strip more electrons from the counter gas molecules. In turn, these electrons are also accelerated and strip off more and more electrons. This electric avalanche consisting of more than a billion negative charges rains down on the positively charged wire, causing a current that flows into the simple detection circuit.
As a single Geiger counter is sensitive to particles coming from any direction, such a detector assembly does not permit the selection of specific orientations and of the particle family. The use of two or more Geiger detectors with the coincidence technique (simultaneous count signal in two or more counter tubes) offers the possibility of carrying out more sophisticated experiments, e.g. discriminate muons and determine the direction of incidence. It also allows to exclude the detection of terrestrial radiation.
The high-energy part of the muon component is studied by underground detectors. These detectors use the good penetration capability of muons in matter to easily distinguish muons from other CR components (except for neutrinos). The underground muon detector may be either a single detector or a small array. (Note that atmospheric, solar and cosmic neutrinos can also be studied deep underground. However, the size of the detector must be very large in order to compensate for the small cross section of neutrinos).
Extensive air showers are detected with different kinds of particle detectors. Most common are scintillation counters that allow to measure the time of arrival with high accuracy. Further used devices are water Cherenkov counters, drift chambers, streamer tube detectors, and Geiger-Müller tubes. Position-sensitive devices allow to measure the incidence direction of the particles.
To detect extensive air showers coincidences of several particle detectors of an array of tens or hundreds of detectors separated by 10-30 meters are required. For the very large showers with billions of particles, the detectors have to be placed in a network with mash size of typically one kilometer. Therefore, the size of an air shower array varies from hundreds of meters to tens of kilometers. Such arrays allow to study primary CRs with energies in the range 1012-1021 eV.
Relativistic electrons and positrons produced in the atmospheric cascade generate Cherenkov emission in visible light when propagated at a speed greater than the speed of light in that medium. The Cherenkov array collects these light pulses from a large volume (thousand cubic kilometers). A similar technique is also used to study neutrinos where Cherenkov light pulses are produced under water (e.g. Deep Underwater Muon And Neutrino Detector (DUMAND)) or in ice (e.g. IceCube Neutrino Observatory or Antarctic Muon And Neutrino Detector Array (AMANDA)).
Modern balloons bring detectors up to altitudes of 40-70 km. Earlier, rather small and simple detectors were flown on balloons. However, today rather large and complicated telescopes such as the BESS (Balloon Borne Experiment with Superconducting Solenoidal Spectrometer) detectors are flown on balloons. At these high altitudes, the atmosphere above the balloon is negligible for CR, and therefore the balloon borne detectors observe directly primary CR particles. In this sense they are like low-orbit satellites, only much cheaper and easier to operate.
The geomagnetic rigidity cutoff is still a significant effect for balloon observations. Moreover, the atmospheric albedo particles (particles reflected or scattered back into space from the atmosphere) are also measured by balloon detectors and therefore have to be taken into account. The main disadvantage of balloon-borne experiments is that they are campaign-like experiments, operating only for a short time interval.
M.L. Duldig, ``Muon observations'', Space Science Review, vol. 93, pp. 207-226, 2000