Cosmic ray

high-speed, i.e. high-energy particle, mainly originating in outer space, outside the Solar system

Cosmic rays are accelerated particles (mostly high-energy protons and heavier nuclei) that are accelerated by the shock waves of supernova remnant (SNR) expanding shells in our galaxy or by extra-galactic sources. Cosmic rays are believed to account for about 1% of the mass-energy of our universe.


  • The Universe is in fact observed not only through the different windows of the electromagnetic spectrum, but also through other cosmic messengers, i.e. through cosmic rays (CRs), neutrinos and gravitational waves (GWs). In general, gamma rays are the perfect companions for multi-messenger astronomy ... gamma-ray production is intimately related to the production of CRs. The latter are charged particles, mainly protons, whose energy spectrum covers a very wide range in energy and flux. Many questions regarding CRs are still open, especially looking at the most energetic ones above 1015 eV (1 PeV). The CR spectrum is approximately described by a power law: dN/dE ∼ E−Γ , where Γ is the spectral index. Γ is not constant, indicating a change in the properties of CRs, like their acceleration sites and chemical composition. For energies around ∼ 4 × 1015 eV, the flux starts to decrease more steeply: Γ changes from about 2.7 to about 3. This feature, marked with the term knee, is thought to indicate the maximum acceleration energy of Galactic sources ...
  • When I began life as a particle physicist fifty years ago, most of the major discoveries were made in Europe by people studying the cosmic rays that bombard the earth from outer space. Particle physics was done by observing the debris produced by cosmic rays as they pass through the atmosphere and the experimental apparatus. The debris consists of particles with short lifetimes and unfamiliar names. ... Three young Italians, Conversi, Pancini, and Piccioni, working with home-made particle counters in the chaos of postwar Italy, discovered that the common cosmic ray particle, later called the muon, had only weak interactions with matter. Cecil Powell, working with microscopes and photographic plates at Bristol in England, discovered the strongly interacting cosmic ray particle, which he called the pion. Other strange new particles were discovered by Rochester and Butler using old-fashioned cosmic ray cloud-chambers in Manchester.
  • Why does the atmosphere have conductivity? Here and there among the air molecules there is an ion—a molecule of oxygen, say, which has acquired an extra electron, or perhaps lost one. These ions do not stay as single molecules; because of their electric field they usually accumulate a few other molecules around them. Each ion then becomes a little lump which, along with other lumps, drifts in the field—moving slowly upward or downward—making the observed current. Where do the ions come from? It was first guessed that the ions were produced by the radioactivity of the earth. (It was known that the radiation from radioactive materials would make air conducting by ionizing the air molecules.) Particles like β-rays coming out of the atomic nuclei are moving so fast that they tear electrons from the atoms, leaving ions behind. This would imply, of course, that if we were to go to higher altitudes, we should find less ionization, because the radioactivity is all in the dirt on the ground—in the traces of radium, uranium, potassium, etc. ... To test this theory, some physicists carried an experiment up in balloons to measure the ionization of the air (Hess, in 1912) and discovered that the opposite was true—the ionization per unit volume increased with altitude! ... This was a most mysterious result—the most dramatic finding in the entire history of atmospheric electricity. It was so dramatic, in fact, that it required a branching off of an entirely new subject—cosmic rays.

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