A cosmic ray cascade in the Earth’s atmosphere

As the incident cosmic ray particle collides with an atom or a molecule of the air, it produces lots of secondary particles. If it is a heavy ion, it will be broken into lighter nuclei, protons, or neutrons. All these particles continue to move downward and can again react with other air molecules, if they have enough energy. This generates a cosmic ray cascade. The deeper the particles penetrate into the atmosphere, the more energy they lose. The primary cosmic ray must have a minimum energy of about 450 MeV to produce a significant number of secondaries that can reach sea level. One has to go to high mountains or use air planes, balloons or spacecraft to detect traces of primary cosmic rays of lower energies.

A typical cosmic ray cascade is shown in the Figure. The schematic drawing of the cascade is from Simpson et al. (1953, Phys. Review 90, 934). Let us have a look on the individual reactions from the first impact to the arrival of the disintegration products at the ground:

• Among the particles produced by the impact of the primary cosmic ray are charged (π^±) and neutral (π⁰) pions, i.e. particles with masses between the electron and the proton. These particles are unstable: neutral pions decay into gamma-ray photons (γ), which can produce electron-positron pairs (e^±); charged pions decay into muons (μ^±), which may also produce electrons and positrons. Some muons undergo nuclear interactions and produce neutrons (n).
• Another branch of reactions creates fast nucleons, neutrons (N) and protons (P), with a broad energy spectrum that extends to the energy of the incident nucleon. If they are energetic enough, they can interact with other air nuclei. At each reaction, evaporation nucleons are also created, both neutrons (n) and protons (p – lower case letters to show that these are particles of rather low energy), with energies of a few MeV.

The particles produced during this cascade all interact more or less strongly with the atmosphere. Muons are least likely to interact, and they are therefore the most abundant secondary cosmic rays at sea level. Evaporation protons and neutrons collide with air nuclei, and lose their energy. Protons also lose energy by ionisations, when they remove electrons from the atmospheric atoms, but neutrons are not able to ionise. Therefore protons suffer stronger energy losses in the air than neutrons, and are, at energies below 2 GeV, far less abundant at sea level than neutrons.

Electromagnetic waves and electrons are the third component of cosmic rays that can be observed from the ground.

Ground-based cosmic ray detectors can be divided into subgroups according to the components they measure: nucleonic (protons and neutrons), meson (muons) and electromagnetic (photons, electrons, etc) components. Neutron monitors detect mainly the nucleonic component, i.e. N and P, through their production of further neutrons in the lead producer. The evaporation nucleons n and p produced in the atmosphere around the neutron monitor are prevented from entering into the detector.