Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A > ∼ 140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties, but calculations sensitively depend on nuclear physics. In particular the fission fragment yields determine the creation of 110 < ∼ A < ∼ 170 nuclei. Here we apply a new scission-point model, called SPY, to derive the fission fragment distribution (FFD) of all relevant neutron-rich, fissioning nuclei. The model predicts a doubly asymmetric FFD in the abundant A 278 mass region that is responsible for the final recycling of the fissioning material. Using ejecta conditions based on relativistic NS merger calculations we show that this specific FFD leads to a production of the A 165 rare-earth peak that is nicely compatible with the abundance patterns in the Sun and metal-poor stars. This new finding further strengthens the case of NS mergers as possible dominant origin of r-nuclei with A > ∼ 140.PACS numbers: 26.30. Hj,24.75.+i,26.60.Gj Introduction.-The rapid neutron-capture process (rprocess) of stellar nucleosynthesis explains the production of the stable (and some long-lived radioactive) neutron-rich nuclides heavier than iron that are observed in stars of various metallicities and in the solar system (see review of [1]). While r-process theory has made progress in understanding possible mechanisms that could be at the origin of the solar-system composition, the cosmic site(s) of the r-process has (have) not been identified yet and the astrophysical sources and specific conditions in which the r-process takes place are still among the most longstanding mysteries of nuclear astrophysics.

The constrained Hartree-Fock-Bogoliubov method is used with the Gogny interaction D1S to calculate potential energy surfaces of fissioning nuclei 226 Th and 256,258,260 Fm up to very large deformations. The constraints employed are the mass quadrupole and octupole moments. In this subspace of collective coordinates, many scission configurations are identified ranging from symmetric to highly asymmetric fragmentations. Corresponding fragment properties at scission are derived yielding fragment deformations, deformation energies, energy partitioning, neutron binding energies at scission, neutron multiplicities, charge polarization and total fragment kinetic energies. PACS numbers: 21.60.Jz, 24.75.+i, 27.90.+b II. SELF-CONSISTENT APPROACH TO SCISSION A. Constrained Hartree-Fock-Bogoliubov methodThe deformed states of the nuclei under study have been determined using the constrained Hartree-Fock-Bogoliubov (HFB) [24] theory based on the minimization principle of the energy functional, namely(1) whereĤ is the nuclear microscopic Hamiltonian,Q l0 a multipole operator, and λ N , λ Z , and λ l the Lagrange parameters associated to constraints on nucleon numbers

Background: Accurate knowledge of fission fragment yields is an essential ingredient of numerous applications ranging from the formation of elements in the r-process to fuel cycle optimization for nuclear energy. The need for a predictive theory applicable where no data is available, together with the variety of potential applications, is an incentive to develop a fully microscopic approach to fission dynamics.Purpose: In this work, we calculate the pre-neutron emission charge and mass distributions of the fission fragments formed in the neutron-induced fission of 239 Pu using a microscopic method based on nuclear density functional theory (DFT).Methods: Our theoretical framework is the nuclear energy density functional (EDF) method, where large amplitude collective motion is treated adiabatically using the time dependent generator coordinate method (TDGCM) under the Gaussian overlap approximation (GOA). In practice, the TDGCM is implemented in two steps. First, a series of constrained EDF calculations map the configuration and potential energy landscape of the fissioning system for a small set of collective variables (in this work, the axial quadrupole and octupole moments of the nucleus). Then, nuclear dynamics is modeled by propagating a collective wave packet on the potential energy surface. Fission fragment distributions are extracted from the flux of the collective wave packet through the scission line.Results: We find that the main characteristics of the fission charge and mass distributions can be well reproduced by existing energy functionals even in two-dimensional collective spaces. Theory and experiment agree typically within 2 mass units for the position of the asymmetric peak. As expected, calculations are sensitive to the structure of the initial state and the prescription for the collective inertia. We emphasize that results are also sensitive to the continuity of the collective landscape near scission.Conclusions: Our analysis confirms that the adiabatic approximation provides an effective scheme to compute fission fragment yields. It also suggests that, at least in the framework of nuclear DFT, three-dimensional collective spaces may be a prerequisite to reach 10% accuracy in predicting pre-neutron emission fission fragment yields.