Density functional approach to radiative neutron capture reactions on r-process nuclei*

Radiative neutron capture is one of the key reactions in the r-process nucleosynthesis, which takes place in the neutron star mergers and in the supernova explosions. Quantitative prediction of the cross section is required as the direct experimental measurement is difficult for very neutron-rich nuclei along the r-process path. The neutron capture reactions are usually described in terms of two different mechanisms, the compound nuclear (CN) process and the direct capture (DC) process. The CN process is assumed to proceed via compound states with high level density, as described by means of the Hauser and Feshbach statistical model. However, the statistical model may not be appropriate for neutron-rich nuclei along the r-process path due to small level density at entrance. The direct capture (DC) models assuming single-particle transition is alternatively adopted, but they neglect many-body correlations such as the pairing, the collective vibrations, and their influences.

 

Aiming at overcoming the above problems, we are developing a microscopic theory to calculate cross section of the radiative neutron capture reaction on neutron-rich nuclei using the continuum quasiparticle random-phase approximation(cQRPA) based on the time-dependent density functional theory (TDDFT). This new approach makes it possible to describe various excitation modes present in the doorway states of the (n,¥gamma) reaction, including soft dipole excitation, the giant resonances as well as  non-collective excitations and the single-particle resonances. Furthermore, it enables us to describe the reaction where the final states of the gamma transition are low-lying surface vibrational states. Using numerical results obtained for the neutron-rich Sn isotopes, e.g. ¹³⁹Sn (n,¥gamma) ¹⁴⁰Sn, we discuss various new features which are beyond the single-particle model; presence of narrow and wide resonances originating from non-collective and collective excitations and roles of the pairing, low-lying quadrupole and octupole vibrational states.