Class Notes on Neutron Activation Reactor Physics and criticality

Class Notes on Neutron Activation Reactor Physics and criticality

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Class Notes on Neutron Activation, Reactor Physics, and Criticality References: 1) http://en.wikipedia.org/wiki/Nuclear_chain_reaction 2) http://en.wikipedia.org/wiki/Criticality_accident 3) See also the references cited in 1 and 2 above. Neutron Activation Analysis In neutron activation, neutrons induce different kinds of nuclear reactions, depending on their energy. For most applications, it is the delayed gamma radiation from the radioactive product that is detected after activation. This type of analysis is called Neutron Activation Analysis (NAA). A different form of NAA called Prompt Gamma NAA (PGNAA) is also used. In this case, it is the prompt gamma rays emitted from the compound nucleus that are monitored. If a sample is placed in a neutron field, radiative capture reactions will occur: λ γ + + + C B n A p The target atoms (A) capture neutrons to form the radioactive species B, which then decay to product C. The rate of transformation, R, is given by: σ Φ = N R Where N is the number of the target atoms, φ is the neutron flux (neutrons/cm 2 -sec), σ is the cross section for the reaction (cm 2 ). The equation above assumes: N is remains constant (i.e. the number of the target atoms transformed is very small compared to the total); φ remains constant throughout the target. The net change in the number of transformed atoms, B, is: = dt dN B Creation rate – Loss rate= B N N λ Φ Therefore:

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( ) t B B e N A N λ σ Φ = = 1 Where A B is the activity of the sample at the end of the irradiation. The quantity N φσ is called the saturation activity, because it represents the maximum activity obtainable when the sample is irradiated for a long time compared to the half-life of the product, i.e. t>>(ln 2/ λ ). ------------------------------------------------------------------------------------------------- Example: An unknown sample containing Na-23 is irradiated for 24 hours in a neutron flux of 10 5 n/cm 2 -sec. The cross section for 23 Na(n, γ ) 24 Na is 0.167 barns. The activity of the sample is measured at the end of the irradiation and the
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Class Notes on Neutron Activation Reactor Physics and criticality

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