lecture 10

lecture 10 - 1/23/2012 Health Physics Principles EEE 498,...

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1/23/2012 Lecture 1 1 Health Physics Principles EEE 498, EEE 591 Lecture 10 Interaction of Radiation with Matter: Neutrons Spring 2012, ASU R. Metzger, rlmetzger@asu.edu, BYEN335 Major Neutron Sources Reactor Californium-252 (Cf-252) Emits 1 neutron for every 313 alpha transformations Its alpha-decay half life is 2.73 y. Its effective half-life, including spontaneous nuclear fission, is 2.65 y. The most probable neutron energy is 1 MeV; the average neutron energy is about 2.3 MeV. Several fission fragments Very short half-lives Usually not very important in health physics Nuclear reactions Lecture 10 2
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1/23/2012 Lecture 1 2 Reactors The major source of neutrons is the nuclear reactor. Fissile isotopes are U-233, U235, Pu-239 and Pu-241 which emit neutrons during the fission process. The released neutrons have a wide energy range with a peak at 0.7 MeV and a mean of 2 MeV. Lecture 10 3 Nuclear Reactions Some reactions may be used to generate neutrons: The term in parentheses is called a “compound nucleus” the asterisk shows that that isotope is in an exited state. Po-Be is used in some reactors to initiate the fission reactions. Lecture 10 4 n C C He Be C n Be n B B D Be B n D Be 1 0 12 6 * 13 6 4 2 9 4 12 9 1 0 10 5 * 11 5 2 1 9 4 10 9 ) ( : ) , ( ) ( : ) , (
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1/23/2012 Lecture 1 3 Nuclear Reactions (Cont.) Unlike neutrons generated from a photodisintegration source, neutrons generated from α,n source have a wide range of energies. Lecture 10 5 Neutron Energy Classification Thermal neutrons have an energy of about 0.025 eV, which is the energy corresponding to most probable velocity at a temperature of 290 K. Fast neutrons have energy more than 0.1 MeV. Between these two, various names are used including intermediate neutrons , resonance neutrons , slow neutrons and epithermal neutrons . Lecture 10 6
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1/23/2012 Lecture 1 4 Thermal Neutrons Follow Maxwell-Boltzmann distribution of energy for gas molecules. where f(E) is the fraction of the neutrons of energy E per unit energy interval; k is the Boltzmann constant = 1.38x10 -23 J/K = 8.6x10 -5 eV/K; and T is the absolute temperature. The table below lists the average and most probable neutron velocities and energies: Lecture 10 7 Example: What are the most probable and average energies of air molecules at 38 o C (New York City subway in summertime)? T= 38
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This note was uploaded on 02/11/2012 for the course EEE 498 taught by Professor Barnaby during the Spring '08 term at ASU.

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lecture 10 - 1/23/2012 Health Physics Principles EEE 498,...

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