Cosmic Ray - Introduction

Introduction

The variety of particle energies reflects the wide variety of sources. The origins range from explosions on our own Sun to different kinds of stellar explosions (novas, supernovas), taking place anywhere in the entire Universe. This doesn't mean that the supernova explosion itself gets the particles up to these speeds. The remnants of the explosions, expanding clouds of gas and magnetic field, can last for thousands of years, and this is where cosmic rays are accelerated. Bouncing back and forth in the magnetic field of the remnant randomly lets some of the particles gain energy, and become cosmic rays. Eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy. Because the cosmic rays eventually escape the supernova remnant, they can only be accelerated up to a certain maximum energy, which depends upon the size of the acceleration region and the magnetic field strength. However, cosmic rays have been observed at much higher energies than supernova remnants can generate, and where these ultra-high energies come from is a big question. Perhaps they come from outside the Galaxy, from active galactic nuclei, quasars or gamma ray bursts. Or perhaps they're the signature of some exotic new physics: superstrings, exotic dark matter, strongly-interacting neutrinos, or topological defects (see also topological string theory) in the very structure of the universe.

There is evidence that very high energy cosmic rays are produced over far longer periods than the explosion of a single star or sudden galactic event, suggesting multiple accelerating processes occurring over very long distances as compared to the size of stars. The obscure mechanisms of cosmic ray production at galactic distances is partly a result of the fact that the magnetic fields of our Solar system, our Galaxy and all other galaxies bend the path of cosmic rays, so that after a long journey through space, they arrive nearly randomly, i. e. from all directions, lacking information about the direction of their initial sources. Cosmic rays can have energies of more than 1020 eV (far higher than the 1012 to 1013 eV that Terrestrial particle accelerators can produce). There has been interest in investigating cosmic rays of even greater energies.

Certain primordial nuclides, i. e. some isotopes of lithium, beryllium and boron, are thought to have arisen not only during the Big Bang, but also (and perhaps primarily) to have been made after the Big Bang, but before the condensation of the solar system, by the process of cosmic ray spallation on interstellar gas and dust. This explains their higher abundance in cosmic rays as compared with their ratios and abundances of certain other nuclides on Earth.

They also produce both stable cosmogenic nuclides and unstable radioisotopes on Earth, such as carbon-14 or phosphorus-32. In the history of particle physics, cosmic rays were the source of the discovery of the positron, muon, and pi meson.

Persons living at relatively high altitudes (above 1000 m.a.s.) receives several times more cosmic radiation than people living at sea level. Airline crews increase their yearly ionizing radiation dose by 100-folds or more due to secondary cosmic rays.

Since both intensity (number of particles per unit area) and energy of cosmic rays is much larger outside Earth's magnetic field and Earth's atmosphere, this is expected to have an impact on the design of spacecraft that can safely transport humans in interplanetary space.