Lecture13-16

O there has always been radiaon on earth o and somemes

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Unformatted text preview: 014 s. D) There is not enough information to determine. E) None of the above. 29 Interac;ons of Par;cles with MaFer u༇  Excluding really high energy par;cles from cosmic rays or accelerators: o  charged par;cles lose energy by ionizing and exci;ng atoms. o  α par;cle’s range is very small due to the slow velocity. o  protons have a bit longer range (s;ll slow) o  electrons move much faster and have a much longer range (minimum ionizing) o  γ rays have a s;ll longer range and may lose energy by pair produc;on in the field of a nucleus or at lower energy by Compton scaFering. u༇  At high energies: o  maFer is almost transparent to neutrinos o  muons have a very long range o  strongly interac;ng par;cles shower (interact producing many secondaries) rather quickly o  electrons and photons shower most quickly, par;cularly in high Z 30 Solar “Wind” Solar ac;vity causes a large rate of protons to be emiFed at low energies. u༇  The solar wind dominates radia;on in the solar system. u༇  But we are protected from it by the earth’s small magne;c field. u༇  Solar wind spirals into the atmosphere at the magne;c poles. u༇  o  aurora borealis u༇  But many ;mes in the earth’s history the field has flipped, going to zero for long periods of ;me. o  lots of radia;on u༇  Solar flares can greatly increase the radia;on and impact air travelers. 31 Cosmic Rays u༇  protons and other nuclei accelerated by large magne;c fields from (rota;ng) neutron stars and perhaps other sources, are all over the universe. u༇  They strike the earth’s atmosphere and interact. o  some very high energy showers reach the earth’s surface o  but the main component down here is muons which we considered in the ;me dila;on studies. u༇ Cosmic rays produce a typical dose of 0.4 millisieverts per year u༇ but much higher doses in the mountains in airplanes. 32 Carbon 14 Da;ng u༇ The ra;o of 14C to 12C in the carbon dioxide molecules of our atmosphere has a constant value of approximately 1.3 X 10- 12. u༇ The 14C is created in the upper atmosphere by cosmic rays. u༇ When an organism dies, it stops exchanging Carbon with the air and the frac;on of 14C decreases exponen;ally with a half life of 5730 years. 33 Clicker: Natural Radioac;vity u༇  Natural radioac;ve elements are found on the earth because: A) Heavy isotopes produced in supernovae have very long lifetimes so they are still present on earth. B) Cosmic rays produce radioactive isotopes continuously. C) Decay products of long lived heavy isotopes are produced continuously. D) All of the above. E) None of the above. 34 Radia;on u༇  We live in a good bit of radia;on u༇  There is no evidence that the natural radia;on harms us. o  there has always been radia;on on earth o  and some;mes much more than today u༇  Our DNA gets repaired...
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This note was uploaded on 02/24/2014 for the course PHYS 2D taught by Professor Hirsch during the Winter '08 term at UCSD.

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