Note on the nature of cosmic rays

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Just how do cosmic rays reach such high energies?

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Where are the natural accelerators? The lowest energy cosmic rays arrive from the Sun in a stream of charged particles known as the solar wind, but pinning down the origin of the higher-energy particles is made difficult as they twist and turn in the magnetic fields of interstellar space. Clues have come through studying high-energy gamma rays from outer space. These are far fewer than the charged cosmic rays, but being electrically neutral they are not influenced by magnetic fields. They generate showers of secondary particles that can be detected on Earth and that point back towards the point of origin of the gamma rays.

Sources of the highest energy gamma rays in our own galaxy, the Milky Way, include the remnants of supernovae, such as the famous Crab Nebula ; the shock waves from these stellar explosions have long been proposed as possible natural accelerators.

Cosmic ray - Wikipedia

Other sources of ultra-high-energy gamma rays lie in other galaxies, where exotic objects such as supermassive black holes may drive the acceleration. There is also evidence that the highest energy charged cosmic rays also have similar origins in other galaxies. Could there be a link between galactic cosmic rays and cloud formation? The atoms involved in our everyday life are not too energetic. Take the air we breathe: its molecules have energies around 0. Such molecules bounce off each other like billiard balls, with not enough force to affect each other's structure by, say, tearing off electrons.

The Sun's plasma is much hotter , and that of the magnetosphere is hotter still. Auroral electrons typically have to 10, ev, as do protons in the magnetotail. Ring current protons have more, around 20, to , ev, while inner belt protons go higher still, typically 10,, to ,, ev.

How unusual is such an environment? How does the rest of the universe compare? Are the high-energy ions and electrons of the magnetosphere an exceptional and rare population? The unexpected answer is that even higher energies seem quite commonplace in the universe. One piece of evidence is a rain of fast ions constantly bombarding Earth, coming from distant space and much more energetic than any found in the magnetosphere.

They are known as cosmic rays or cosmic radiation.

The radiation is therefore not intense, giving us about as much energy as starlight. That does not sound like much, until one remembers what the stars are--distant suns, about a hundred billion of them traveling together in our galaxy, and untold billions in more distant galaxies. Actually, the source of cosmic rays is probably not quite as intense, because cosmic ray particles can stay around the galaxy much longer than starlight.

Starlight moves in straight lines, one pass through our galaxy and it is gone, into the great emptiness between galaxies. When cosmic rays enter the Earth's atmosphere they collide with atoms and molecules , mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including x-rays , muons , protons, alpha particles , pions , electrons , and neutrons. Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons.

Some of these subsequently decay into muons and neutrinos , which are able to reach the surface of the Earth. Some high-energy muons even penetrate for some distance into shallow mines, and most neutrinos traverse the Earth without further interaction. Others decay into photon , subsequently producing electromagnetic cascades. Hence, next to photons electrons and positrons usually dominate in air showers. These particles as well as muons can be easily detected by many types of particle detectors, such as cloud chambers , bubble chambers , water-Cherenkov or scintillation detectors.

The observation of a secondary shower of particles in multiple detectors at the same time is an indication that all of the particles came from that event. Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high-energy gamma ray emissions by gamma-ray telescope.

The flux of incoming cosmic rays at the upper atmosphere is dependent on the solar wind , the Earth's magnetic field , and the energy of the cosmic rays. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.

Solving a century-long mystery: the origin of galactic cosmic rays

In addition, the Earth's magnetic field acts to deflect cosmic rays from its surface, giving rise to the observation that the flux is apparently dependent on latitude , longitude , and azimuth angle. The combined effects of all of the factors mentioned contribute to the flux of cosmic rays at Earth's surface. The following table of participial frequencies reach the planet [66] and are inferred from lower energy radiation reaching the ground. In the past, it was believed that the cosmic ray flux remained fairly constant over time. However, recent research suggests one-and-a-half- to two-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.

There are two main classes of detection methods. First, the direct detection of the primary cosmic rays in space or at high altitude by balloon-borne instruments. Second, the indirect detection of secondary particle, i. While there have been proposals and prototypes for space and ballon-borne detection of air showers, currently operating experiments for high-energy cosmic rays are ground based.

Generally direct detection is more accurate than indirect detection. However the flux of cosmic rays decreases with energy, which hampers direct detection for the energy range above 1 PeV. Both, direct and indirect detection, is realized by several techniques. Direct detection is possible by all kind of particle detectors at the ISS , on satellites, or high-altitude balloons.

However, there are constraints in weight and size limiting the choices of detectors. An example for the direct detection technique is a method developed by Robert Fleischer, P.

Walker for use in high-altitude balloons. The nuclear charge causes chemical bond breaking or ionization in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path.

The resulting plastic sheets are "etched" or slowly dissolved in warm caustic sodium hydroxide solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves the plastic at a faster rate along the path of the ionized plastic. The net result is a conical etch pit in the plastic.


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The more extensive the ionization along the path, the higher the charge. In addition to its uses for cosmic-ray detection, the technique is also used to detect nuclei created as products of nuclear fission. There are several ground-based methods of detecting cosmic rays currently in use, which can be divided in two main categories: the detection of secondary particles forming extensive air showers EAS by various types of particle detectors, and the detection of electromagnetic radiation emitted by EAS in the atmosphere. Extensive air shower arrays made of particle detectors measure the charged particles which pass through them.

However, they are less able to segregate background effects from cosmic rays than can air Cherenkov telescopes.


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Most state-of-the-art EAS arrays employ plastic scintillators. Also water liquid or frozen is used as a detection medium through which particles pass and produce Cherenkov radiation to make them detectable. By the combination of several detectors, some EAS arrays have the capability to distinguish muons from lighter secondary particles photons, electrons, positrons. The fraction of muons among the secondary particles in one traditional way to estimate the mass composition of the primary cosmic rays.

A historic method of secondary particle detection still used for demonstration purposes involves the use of cloud chambers [72] to detect the secondary muons created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory.

A fifth method, involving bubble chambers , can be used to detect cosmic ray particles.