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Unformatted text preview: Origin of Neutron, Neutron Sources, and Neutron Interactions Lecture Notes The neutron was discovered by Chadwick in 1932. He found that bombardment of Be alphas will generate a radiation that is neutral. He also found that that this same radiation would also knock nitrogen nuclei out of matter. He found that if this radiation was gamma , it had to have an energy of 50 MeV! This did not agree with the available energy from alpha bombardment of Be: Exercise: Show that Chadwick was correct, using alpha, Be interaction. If the reaction is Be ( , ) C; we can calculate the total energy available from this reaction as follows: Chadwick had the following data: M Be = 9.012185 amu; M He = 4.002603 amu; M c13 = 13.003354 amu m = (9.012185 + 4.002603)  13.003354 = 0.011434 amu X 931.5 MeV/amu = 10.65 MeV Q = 10.65 MeV Total Energy Available = 10.65 MeV + E alpha = 10.65 + 5 ~ 16 MeV Chadwick was correct; if the Be (alpha, ?) C generated a gamma radiation, the gamma energy would be 16 MeV; not enough to knock N from matter. Therefore, he concluded that this radiation is a neutral particle which he named Neutron . The characteristics of Neutron are: Mass = 1.0087 amu = 939.57 MeV = 1.6749 X 1027 Kg. Categorization of Neutrons Neutron is categorized according to its energy. There a few different nomenclatures used; here is one: Categorization of Neutrons I Energy (MeV) Neutron Type < 1010 Cold 1010 to 2.5 X 108 Thermal 2.5 X 108 to 104 Slow 104 to 0.1 Intermediate 0.1 to 100 Fast > 100 Ultra fast Categorization of Neutrons II Energy Neutron Type < 0.1 eV Thermal 0.1 < E<1 eV Epithermal 1 eV < E < 1 KeV Resonance E > 1 KeV Fast Thermal Neutrons Due to their importance in a variety of applications, such as neutron activation and fission reactors, we spend some time on their energy characteristics. Thermal neutrons behave much like a molecular gas and the number of neutrons with a range of speeds dv can be described by a Maxwellian (MaxwellBoltzmann) distribution: = ) ( v N KT mv v KT m N o 2 exp 2 4 2 2 2 / 3 ; where v = neutron speed m = mass of neutron = 1.675 x 10 27 Kg = K Boltzmanns Constant = 1.381 x 1023 Joule/ o K neutron temperature in degrees Kelvin (absolute temperature) = 273 + = T o C The most probable speed of neutron with such distribution is: m KT v p 2 = , and the average neutron speed is: m KT v 8 = At room temperature, 25 o C, the is approximately 2200 m/sec; this is the socalled thermal neutron speed. p v The energy distribution of thermal neutrons can also be described with Maxweelian Boltzmann distribution: dE KT E E KT N dE E N o = exp 1 2 ) ( 2 / 1 2 / 3 The most probable and average neutron energies are: KT E p 2 1 = and KT E 2 3 = ....
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This note was uploaded on 04/11/2008 for the course CHNE 524 taught by Professor Mohagheghi during the Fall '08 term at New Mexico.
 Fall '08
 Mohagheghi

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