Pawel Danielewicz and Filomena Nunes plan to conduct research on nuclear reactions, with particular emphasis on theory that applies to and can be tested by experiments at current accelerator facilities, such as of the Michigan State University and of the Brookhaven National Laboratory. In the latter context, the proposal aims at advancing the theory for reactions with rare isotope beams and for learning on bulk nuclear properties at different bombarding energies. Specific goals in those respective areas include the study of breakup effects in reactions with weakly bound nuclei and advancing of the quantum transport theory for central reactions.
Pawel Danielewicz and Filomena Nunes conducted research in nuclear theory, concentrating on problems of interest in connection with reaction experiments at existing accelerator facilities, such as NSCL, and those planned for the future, such as FRIB. Research tied to central reactions concentrated on the symmetry energy which is the contribution to the nuclear energy associated with neutron-proton imbalance. Knowledge of the symmetry energy is needed to tie laboratory measurements of nuclei to astronomical observations of neutron stars, the densest objects in the Universe that have not yet turned into black holes. Relations have been established between features of the symmetry energy and properties of nuclei that can be extracted from measurements. Data have been in turn used to constrain features of the symmetry energy of importance in astrophysics. In the context of central reactions, progress was further made in adapting quantum transport theory to nuclear reactions. In the first step, dynamics of a simplified system of nuclear slabs was studied and approximations were explored of utility for realistic calculations of reactions. Quantum transport is pursued in different research areas, also e.g. for optical traps, and the knowhow gathered under this project is useful in these other contexts as well. In the context of direct reactions, the rate of fusion of three alpha particles into carbon-12 has been calculated, for a wide range of temperatures, including those for which nonresonant fusion processes dominate, by carrying out complete three-body quantal calculations of the fusion for the first time. The calculations helped to resolve ambiguities in the magnitude of the rates in the nonresonant region. The reaction is of critical importance for understanding the production of energy during star evolution and of production of elements in the Universe. Direct reaction theory is needed to extract nuclear structure information from measurements and to predict processes that cannot be measured (often of relevance to astrophysics). However, the importance and consequences of approximations within the theory, needed to make calculations feasible, is often not known. One way of assessing that importance is to compare results made with different approximations and without any of those when possible. Results for selected (d,p) transfer reactions, obtained with different approximations, have been compared at different energies with recommendations for the best regime where measurements should be made, which combine with reliable reaction calculations, for extracting structure information. Other theoretical efforts under this project concerned imaging of emission sources for nuclear reactions, from correlations of reaction products, extraction of nuclear viscosity from stopping data and improvements in the realism of transport simulations of central reactions. The validity of the mirror method for assessing unknown capture reactions was tested and confirmed. It has been further demonstrated that Coulomb dissociation data can be reliably used to extract information on neutron capture reactions. The last two efforts pertain to the methodology where processes that can be measured are used to learn on processes that are difficult to measure but are of importance in astrophysical circumstances. Two Theses, one Ph.D. and one M.Sc. have been defended based on results obtained under this project. Work towards several more Theses, initiated under this project, continues.
