Cosmic Rays
Cosmic rays consist of subatomic particles which have high energy (those not from our own sun) due to their high velocities. Victor Franz Hess (1883-1964) showed in 1911 that the ionization of the Earth's atmosphere increases with altitude. Hess made ten ascents in balloons with detectors over 1911-12. He reached altitudes of 5000 meters. He came to the conclusion that a powerful radiation entered Earth's atmosphere and then diminished as it progressed toward the ground. This radiation he called "cosmic rays".
Bruno Rossi (1905-) demonstrated how penetrating cosmic rays were. At first thought to be gamma radiation it was proved in the 1920's that cosmic rays are actually high energy particles and are a constant "cosmic rain" falling onto the Earth. It was also discovered that the intensity of the radiation was dependent on latitude. This finding indicated that the particles which make up cosmic rays are electrically charged and are deflected by the Earth's magnetic field.
Each cosmic ray particle has an electric charge, a rest mass, and energy which is derived from its rest mass and velocity. As the particles hurl through the atmosphere they collide with the nuclei of atoms in the upper atmosphere producing showers of secondary particles such as protons, neutrons, light nuclei, and charged and neutral pions. The pions shortly decay into gamma ray photons which go on to produce electrons and positrons. Charged pions can transmutate into penetrating muons and neutrinos which can travel deeply underground.
About 87% of primary cosmic rays are protons and 12% are alpha particles (helium nuclei) but heavier elements are also present. They are classified as light (lithium, beryllium, and boron), medium (carbon, oxygen, nitrogen, and fluorine), and heavy (the remainder of the elements--even up to uranium.)
The origin of cosmic rays remains somewhat enigmatic. The sun emits low energy cosmic rays during solar flares. But the heavier and highly energetic particles for the most part are believed to come from supernovas, giant exploding stars. There is disagreement about whether the supernovas involved are only from the Milky Way or are also from distant galaxies. It is considered as of the year 2000 that the majority of cosmic rays arise within our own galaxy. This conclusion is based on fragmentation of particles which should be much different if greater distances were involved--there should be far fewer large particles. Also, with the great distances that would have to be traveled a large number of particle--photon collisions would occur by the cosmic ray electrons, greatly decreasing the energy from what is actually recorded. Finally, satellite mapping has shown a strong concentration of high-energy gamma rays toward our Milky Way galactic equator. A more widely spread distribution would be expected if the primary 700MeV (million electron volts) protons which collide with interstellar hydrogen to produce neutral pions which then decay into high-energy gamma ray photons were extragalactic.
There is also some disagreement as to whether Type I or Type II supernovas are the primary producers. Type I supernovas are thought to occur in the late stages of binary star system evolution. One of the stars is a white dwarf while its partner is a normal star with an extended atmosphere allowing material to collect at the surface of the high gravity white dwarf. Once the mass of the white dwarf reaches a certain limit a massive explosion occurs blasting particles into space.
The Type II supernovas are more abundant and probably are the final stages in the evolution of a star similar to our sun. These stars eventually become as small as 6.2 mi (10 km) across but with a magnetic field 10 the strength of the Earth's. Rapid rotation and strong magnetic field combine to hurl cosmic ray particles into space. Recent data using the VLA (very large array) radiotelescopes in New Mexico indicate that indeed the Type II supernovas are the source of the great majority of cosmic rays, if not the only source. And again most of those sources are considered to be intragalactic.
The paths of cosmic rays are incredibly tangled. The particles are subject to the interstellar magnetic field since the particles are charged. It can be deduced that a typical carbon or oxygen particle has traveled a distance of almost thirty times the Milky Way galactic diameter between its origin and its entry into Earth's atmosphere. Obviously, there is interstellar matter with which the particles may react thus transmutating, or undergoing multiple glancing collisions.
There are cosmic rays with little or no mass. Essentially this category includes electrons and neutrinos. All photons are not included but are used as diagnostics of distant cosmic rays.
Electrons are produced along with protons and heavier nuclei in supernova explosions. To these electrons are added others which result from interstellar collisions of nuclear particles. Most of these collisions are cosmic ray protons colliding with hydrogen nuclei (also a proton). Approximately equal numbers of positively and negatively charged pions are produced which decay almost instantaneously into muons, which, after about 2 microseconds, decay into electrons and positrons. About 20% of these primary cosmic rays are positrons. As these particles collide with Earth's atmosphere a cascade of photons occurs.
Right now you are being pierced again and again by neutrinos, you do not feel them and they have no effect on you. But they do represent our third and final type of cosmic ray. Along with the heavier nuclear particles, and the nearly massless electron (or positron), the neutrino is a true cosmic ray. But it has neither mass or electric charge. It does carry energy and momentum and therefore can interact with nuclei and electrons but rarely does so. In fact while passing through the entire Earth a neutrino has only a few chances in a trillion of having a collision.
Neutrinos can be emitted in the decay of radioactive nuclei or come from mesons. When a pion decays to a muon a neutrino is produced, and when the muon decays to an electron 2 neutrinos are produced. Neutrinos are generated from supernova explosions and in our sun's core. Our galaxy is in a hailstorm of neutrinos.
But neutrinos are difficult to study, Since they have no charge we must wait for them to be picked up by special detectors such as deep underground chambers filled with ultrapure water or extremely pure perchlorethylene (a cleaning fluid), and even then the results are confusing. Particle physicists know that there are 3 types of neutrinos: the electron-neutrino, the muon-neutrino, and the tau-neutrino. And as of the year 2000 they are manipulated at will by scientists at high-energy physics facilities. But they remain fundamentally elusive. Some theorists have arrived at a mass for the neutrino, usually no more than 1/50,000 of an electron mass. Some have said that neutrinos may change our ideas regarding our sun, and supernovas. But speculation begs for proof, and we can only await the next intragalactic supernova to gain more information. Such data may change many of our present cosmological ideas and theories.
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