New Fission Fragment Distributions andr-Process Origin of the Rare-Earth Elements

Abstract

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.

Origin of r-Process Elements and Galactic Chemodynamical Evolution

“…Also, only ∼ 10% of the final yield came from the termination of the r-process path at N = 212 and Z = 111, while almost 90% of the A ≈ 160 bump shown in Figure 1 was from the fission of more than 200 different parent nuclei mostly via beta-delayed fission. This is in contrast to the yields of [31] that are almost entirely due to a few A ≈ 278 fissioning nuclei with a characteristic four hump FFD. It is important to keep in mind that many microscopic calculations of fission barriers are consistent with an early termination near A ≈ 280.…”

“…Recent studies [29][30][31][32]35] have indicated that the r process in NSMs produces a final abundance pattern that can be similar to the solar r-process abundances, but only for heavier A>130 nuclei.…”

“…[28][29][30][31][32][33][34][35]). The ejected matter from NSMs is very neutron-rich (⟨Z/A⟩ ≡ Y e ∼ 0.1).…”

“…The r-process path in the NSM calculations of [27] proceeds rather below the fissile region until nuclei with A ∼ 320, whereas the r-process path based upon microscopic calculations of fission barriers (such as [31]) terminates at A ≈ 278 [or for a maximum ⟨Z⟩ for [32]]. Also, only ∼ 10% of the final yield came from the termination of the r-process path at N = 212 and Z = 111, while almost 90% of the A ≈ 160 bump shown in Figure 1 was from the fission of more than 200 different parent nuclei mostly via beta-delayed fission.…”

Exploration of direct neutron capture with covariant density functional theory inputs

“…Here, high temperatures and neutron densities enable sequences of successive neutron captures to form nuclei near the neutron drip line which subsequently decay toward stability as the system expands and cools [1,2]. Core collapse supernovae [3] and merging neutron stars [4] are the two leading sites for the rprocess.…”