rpp2010-rev-cosmic-rays - 24. Cosmic rays 1 24. COSMIC RAYS...

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24. Cosmic rays 1 24. COSMIC RAYS Revised August 2009 by T.K. Gaisser and T. Stanev (Bartol Research Inst., Univ. of Delaware) 24.1. Primary spectra The cosmic radiation incident at the top of the terrestrial atmosphere includes all stable charged particles and nuclei with lifetimes of order 10 6 years or longer. Technically, “primary” cosmic rays are those particles accelerated at astrophysical sources and “secondaries” are those particles produced in interaction of the primaries with interstellar gas. Thus electrons, protons and helium, as well as carbon, oxygen, iron, and other nuclei synthesized in stars, are primaries. Nuclei such as lithium, beryllium, and boron (which are not abundant end-products of stellar nucleosynthesis) are secondaries. Antiprotons and positrons are also in large part secondary. Whether a small fraction of these particles may be primary is a question of current interest. Apart from particles associated with solar flares, the cosmic radiation comes from outside the solar system. The incoming charged particles are “modulated” by the solar wind, the expanding magnetized plasma generated by the Sun, which decelerates and partially excludes the lower energy galactic cosmic rays from the inner solar system. There is a signiFcant anticorrelation between solar activity (which has an alternating eleven-year cycle) and the intensity of the cosmic rays with energies below about 10 GeV. In addition, the lower-energy cosmic rays are a±ected by the geomagnetic Feld, which they must penetrate to reach the top of the atmosphere. Thus the intensity of any component of the cosmic radiation in the GeV range depends both on the location and time. There are four di±erent ways to describe the spectra of the components of the cosmic radiation: (1) By particles per unit rigidity. Propagation (and probably also acceleration) through cosmic magnetic Felds depends on gyroradius or magnetic rigidity, R ,wh ichis gyroradius multiplied by the magnetic Feld strength: R = pc Ze = r L B. (24 . 1) (2) By particles per energy-per-nucleon. ²ragmentation of nuclei propagating through the interstellar gas depends on energy per nucleon, since that quantity is approximately conserved when a nucleus breaks up on interaction with the gas. (3) By nucleons per energy-per-nucleon. Production of secondary cosmic rays in the atmosphere depends on the intensity of nucleons per energy-per-nucleon, approximately independently of whether the incident nucleons are free protons or bound in nuclei. (4) By particles per energy-per-nucleus. Air shower experiments that use the atmosphere as a calorimeter generally measure a quantity that is related to total energy per particle. The units of di±erential intensity I are [m 2 s 1 sr 1 E 1 ], where E represents the units of one of the four variables listed above.
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This note was uploaded on 06/07/2011 for the course PHYS 4132 taught by Professor Kutter during the Spring '11 term at University of Florida.

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rpp2010-rev-cosmic-rays - 24. Cosmic rays 1 24. COSMIC RAYS...

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