### Speaker

### Description

Nucleon-induced pre-equilibrium reactions are predominantly direct reactions. At low incident energies, excitation of all but the lowest energy collective states can be well described in terms of one-step reactions that produce particle-hole pairs. As the incident energy is increased, more complex excitations involving two or more particle-hole pairs become accessible through multi-step reactions. Quantum mechanical models of such multi-step direct reactions were developed many

years ago [1,2,3] and have been studied and improved many times over since then [4,5,6,7]. In these models, a leading continuum particle initiates the reaction and remains in the continuum as it scatters repeatedly from the nucleus to produce successive particle-hole pairs. However, as the incident energy increases, the probability of exciting a nucleon to the continuum rather than to a bound particle state also increases. [8] These knockout nucleons can escape the nucleus or induce

secondary collisions that create still other continuum or bound particle-hole pairs. Calculations using Blann and Chadwick’s DDHMS pre-equilibrium simulation model [9,10] reveal that a 20 MeV neutron incident on 56Fe produces an additional continuum particle in 10% of its scatterings with nucleons in the nucleus and yields a knockout cross section of approximately 4% of the reaction cross section. At 200 Mev, knockout of at least one nucleon, that is, emission of two or more pre-equilibrium nucleons, corresponds to almost 80% of the pre-equilibrium reaction cross section. Here we discuss these calculations in more detail. We also analyze and compare the typical energy and angular distributions obtained from one-step quantum mechanical calculations of inelastic excitation and knockout reactions.

References

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