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Neutron_flux_lab

# Neutron_flux_lab - INDIANA UNIVERSITY DEPARTMENT OF PHYSICS...

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INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P451 LABORATORY Neutron activation and flux determination A. Activation: basics Thermal neutrons react with the nuclei (A,Z) in the sample predominantly by an (n, γ ) reaction. The result is a new isotope (A+1,Z) of the same element. Typically, the isotope is formed in an excited state, but very quickly decays by gamma emission to the ground state. Typically, the new isotope is radioactive and decays with a half life T ½ . The so- called decay constant is given by 2 1 2 ln T λ . (1) If N is the number of active nuclei, the decay rate is given by N dt dN λ = . (2) The neutron flux Φ is defined by the number of neutrons that traverse a unit area per unit time in any direction. The probability that a reaction takes place is measured by the activation cross section σ . The units for cross section are cm 2 . Often 1b (barn) =10 -24 cm 2 is used. Now, consider a foil of area a (cm 2 ) and thickness d (nuclei per cm 2 ) exposed to flux Φ (s-1cm-2) during a time t (s). The number of active nuclei that are generated is then given by t t d e N e e a N λ λ σ λ + Φ = 0 ) 1 ( ) 1 ( (3) Here, N 0 is the number of active nuclei that are already present at the beginning of the irradiation. If the foil is inactive at the start ( N 0 =0) and thin ( σ d«1) and the irradiation time is short, the exponentials can be expanded and eq.3 reduces to t d a N Φ = σ (4) Note, that a ּ d represents the total number n of (A,Z) nuclei in the foil, and can be obtained from the mass M (in g) of the foil by M A n d a =

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Neutron_flux_lab - INDIANA UNIVERSITY DEPARTMENT OF PHYSICS...

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