Extended direct semidirect mechanism and the role of multistep processes in fast nucleon radiative capture

摘要

We have recently developed an extension of the direct-semidirect (DSD) radiative capture model to unstable final states and have confirmed its utility in explaining the spectrum of rays from capture of polarized 19.6-MeV protons on (sup 89)Y. It was found that the extended DSD model, supplemented by a Hauser-Feshbach contribution, successfully explains the observed spectra, angular distributions, and analyzing powers, without requiring additional mechanisms, such as precompound or multistep emission, or nucleon-nucleon bremsstrahlung. In this contribution we show that the model also successfully explains data at higher energies (34 MeV incident protons), and that there is no need for additional contributions other than Hauser-Feshbach at this energy as well. The extended DSD model treats capture to unbound final states and also to bound single-particle states that damp into a compound system. An optical (complex) potential is used to describe the propagation of the captured particle. Application of this model to the (gamma) spectrum in the (sup 89)Y(p, (gamma) ) reaction at 19.6 MeV is shown. We have performed new calculations at higher energy (34 MeV protons), and have compared them with the spectra and angular distributions measured in (2) on targets of natural Cu, Ag, and Au. An example of the results, for the spectrum from Cu, is shown in the right-hand part of the figure. In both cases the DSD calculation is shown by a solid line, and a Hauser-Feshbach calculation by a dashed line. The 34-MeV calculations were very similar to those at 19.6 MeV as described in (1). In both cases, the sum of DSD and Hauser-Feshbach calculations adequately describes the measured spectra. Although not shown, the angular distributions are also well described. There are no significant deficiencies in the comparison with experiment that indicate a need for multistep processes or other additional reaction mechanisms. Such processes are therefore required, if at all, only at significantly higher energies than reported here. On the basis of these results, we believe that the extended DSD model is the most appropriate tool for modeling the high-energy portion of the gamma spectrum from nu- cleon capture. On the other hand, simple precompound treatments such as the exciton model should remain important for emission in reactions with complex projectiles (e.g. d, (alpha), heavy ions), as long as an easily-implemented quantum mechanical model is not available.