Theoretical descriptions of compound-nuclear reactions: open problems and challenges

Abstract

Abstract. Compound-nuclear processes play an important role for nuclear physics applications and are crucial for our understanding of the nuclear many-body problem. Despite intensive interest in this area, some of the available theoretical developments have not yet been fully tested and implemented. We revisit the general theory of compound-nuclear reactions, discuss descriptions of pre-equilibrium reactions, and consider extensions that are needed in order to get cross section information from indirect measurements. IntroductionCompound-nuclear reactions play an important role in basic and applied nuclear physics. They provide a prime example of chaotic behavior of a quantum-mechanical manybody system [64,77] and their cross sections are required for nuclear astrophysics, national security, and nuclear-energy applications. The theoretical formalisms used to describe compound reactions are typically considered well-established. R-matrix treatments [62,29] are employed for reactions proceeding through isolated resonances and a combination of Hauser-Feshbach theory [50] and pre-equilibrium descriptions is used for reactions involving strongly-overlapping resonances. Computational tools are readily available and extensively used in the nuclear science community for calculating a wide range of cross sections. Recommendations for input models and parameters have been formulated [25] and evaluations using these tools have been published.Hauser-Feshbach calculations require inputs that are typically obtained from complementary measurements (low-lying nuclear levels, separation energies, etc.) and nuclear-structure models. In the last decade or so, there have been increased efforts to move from phenomenological models (for level densities, γ-ray strength functions, etc.) to more microscopic structure descriptions. While the microscopic descriptions may not be as successful in reproducing measured cross sections as their phenomenological Theoretical descriptions of compound-nuclear reactions: open problems & challenges 2counterparts at this time, they are crucial for developing a more predictive treatment of compound reactions.Equally important in this context is a reconsideration of the reaction models that are implemented in the codes and of the underlying assumptions and approximations. Much less attention has been paid to this aspect, but modern computational capabilities and also the improved structure models that are now available should make it possible to develop better treatments of the reaction mechanisms involved in compound reactions. This is particularly relevant if one wants to calculate cross sections involving isotopes more than a few units away from the valley of stability. Here, some of the assumptions underlying current descriptions may no longer be valid and extensions of the reaction models may become necessary.For instance, as one moves away from the valley of stability, the nuclei under consideration (which may serve as targets in neutron-rich astrophysical environments) become weakly-bound. The ...

Statistical Features of the Thermal Neutron Capture Cross Sections

“…The door way states are also considered at higher energies in the so-called statistical multistep compound pre-equilibrium emission [7,8]. In these reactions the relative importance of the escape to the damping widths Γ [23]. The important feature that distinguishes the doorway resonance from the CN resonance is that the total width of the doorway is Γ D = Γ …”

“…In these reactions the relative importance of the escape to the damping widths Γ ↑ D i /Γ ↓ D i , of the different classes of the ever more complicated doorways populated in the reaction is very important. For a very recent review on compound nucleus reactions see [23]. The important feature that distinguishes the doorway resonance from the CN resonance is that the total width of the doorway is Γ D = Γ ↓ D + Γ ↑ D , while that of the CN is just an "escape" width to the open channels.…”