Speaker
Description
Multistep Direct (MSD) and Multistep Compound (MSC) mechanisms when combined account for emission of nucleons from a composite nucleus before it attains compound nucleus equilibrium. In spite being better founded than exciton model or other classical pre-equilibrium models, MSD and MSC were only occasionally employed in practical calculations. Initially, it was due to higher complexity of these quantum-mechanical theories and higher computational cost. Both these factors, however, are not major obstacles today since MSD (TUL) and MSC (NVWY) were implemented in the EMPIRE code by the end of the last century and modern computational capabilities make such calculations feasible on a single-processor laptop. The major cause of avoiding MSD/MSC in practical calculations (e.g., nuclear data evaluations) was the fact that these two models tend to underestimate the middle-energy range of neutron spectra.
In our recent work we were able to overcome this deficiency and obtain very good reproduction of experimentally measured neutron spectra coming from neutron interaction with Ta181 and Pu239 targets. The default MSD/MSC calculations on Ta181 are already acceptable. Similar result with exciton model requires DWBA calculations to a large number of fake levels embedded in the continuum to simulate MSD mechanism. This success comes, however, at the price of turning off gradual absorption to the MSC chain. This is at odds with the fundamental distinction between MSD and MSC mechanisms that should proceed through the chain of open (P-space) and closed (Q space) respectively. By blocking gradual absorption we allow the first stage of MSC to be fully populated from the incident channel. This ignores the fact that at high enough incident energies creation of a bound three-quasiparticle state is energetically impossible.
We will discuss various attempts of addressing the problem that, so far, remains open.
