Definition of cross sections 1 - Moodle

The SLBW formula 71. The SLBW formula can only be applied to the case of an isolated resonance. However,most isotopes feature many resonances at increasing energies. A crude approximationconsists to neglect resonance interactions, so that the cross section of a reactionx ≠ e is written by summing the contributions of I resonances as(36)σ x (e exc ) =I∑i=1σ 0γ x,iγ i11+u 2 .A similar expression can be written for σ e (e exc ).2. In the general case, many levels of the compound nucleus interact together and amultilevel formula is required. In this case, the cross section are obtained by summingthe contributions from each individual resonance, taking care to introduce interactionterms in the sum.3. At energies above ≃ 10 keV, resonances become unresolved and are represented bystatistical parameters. At higher energies, the resonance become tighter up to thepoint where they overlap, leading to the continuum energy domain.ENE6101: Week 4 Cross sections – 30/37