I. V. Panov

Calculations of fission rates for r-process nucleosynthesis

Abstract Fission plays an important role in the r-process which is responsible not only for the yields of transuranium isotopes, but may have a strong influence on the formation of the majority of heavy nuclei due to fission recycling. We present calculations of beta-delayed and neutron-induced fission rates, taking into account different fission barriers predictions and mass formulae. It is shown that an increase of fission barriers results naturally in a reduction of fission rates, but that nevertheless fission leads to the termination of the r-process. Furthermore, it is discussed that the probability of triple fission could be high for A > 260 and have an effect on the formation of the abundances of heavy nuclei. Fission after beta-delayed neutron emission is discussed as well as different aspects of the influence of fission upon r-process calculations.

Neutron-induced astrophysical reaction rates for translead nuclei

Neutron-induced reaction rates, including fission and neutron capture, are calculated in the temperature range 10 8 ≤ T (K) ≤ 10 10 within the framework of the statistical model for targets with the atomic number 84 ≤ Z ≤ 118 (from Po to Uuo) from the neutron to the proton drip-line. Four sets of rates have been calculated, utilizing - where possible - consistent nuclear data for neutron separation energies and fission barriers from Thomas-Fermi (TF), Extended Thomas-Fermi plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and Hartree-Fock-Bogolyubov (HFB) predictions. Tables of calculated values as well as analytic seven parameter fits in the standard REACLIB format are supplied. We also discuss the sensitivity of the rates to the input, aiming at a better understanding of the variations introduced by the nuclear input.

THE ROLE OF FISSION IN NEUTRON STAR MERGERS AND ITS IMPACT ON THE r-PROCESS PEAKS

M. Eichler, A. Arcones, A. Kelic, O. Korobkin, K. Langanke, T. Marketin, G. Martinez-Pinedo, I. Panov, T. Rauscher, S. Rosswog, C. Winteler, N. T. Zinner, and F. K. Thielemann, ‘The role of fission on neutron star mergers and its impact on the r-process peaks’, in proceedings CETUP* 2015 – Workshop on Dark Matter, Neutrino Physics and Astrophysics PPC 2015 – IXth International Conference on Interconnections between Particle Physics and Cosmology. Deadwood, South Dakota, USA. 15-17 July 2015. Barbara Szczerbinska, Rouzbeh Allahverdi, Kaladi Babu, Baha Balantekin, Bhaskar Dutta, Teruki Kamon, Jason Kumar, Farinaldo Queiroz, Louis Strigari, and Rebecca Surman eds., ISBN 9780735414006. Available online at doi: http://dx.doi.org/10.1063/1.4953296. Published by AIP Publishing.

The r-Process in the region of transuranium elements and the contribution of fission products to the nucleosynthesis of nuclei with A ≤ 130

We discuss the influence of nuclear masses and mass distributions of fission products on the formation of heavy elements at the final stages of the r-process recycled through fission on long duration timescales. The fission recycling is of great importance in an environment with a high density of free neutrons (e.g., in neutron star merger scenarios), when the r-process duration is long enough for most of the seed nuclei to be transformed into actinoids. The fission products of transuranium elements are again drawn into the r-process to produce the abundance curve beyond the iron peak. In this case, to explain the abundances of the A ∼ 130 peak elements, not only the nuclear masses, fission barriers, and reaction rates, but also the fission product mass distribution must be predicted. Our r-process calculations using new nuclear masses and fission barriers and reaction rates based on them have shown that the simple two-fission-fragment model used previously in r-process calculations cannot describe adequately the position of the second peak in the observed abundance curve. We show that agreement between calculations and observations can be achieved only when we properly consider the mass distribution of fission products by taking into account the emission of instantaneous fission neutrons.

Have Superheavy Elements been Produced in Nature

We discuss the possibility whether superheavy elements can be produced in Nature by the astrophysical rapid neutron capture process. To this end we have performed fully dynamical network r-process calculations assuming an environment with neutron-to-seed ratio large enough to produce superheavy nuclei. Our calculations include two sets of nuclear masses and fission barriers and include all possible fission channels and the associated fission yield distributions. Our calculations produce superheavy nuclei with

Superheavy elements and r-process

\ensuremath A\approx 300

Probabilities of delayed processes for nuclei involved in the r-process

that however decay on time scales of days.

Contribution of fission to heavy-element nucleosynthesis in an astrophysical r-process

The probability for the production of superheavy elements in the astrophysical r-process is discussed. The dependence of the estimated superheavy-element yields on input data is estimated. Preliminary calculations revealed that the superheavy-element yields at the instant of completion of the r-process may be commensurate with the uranium yield, but the former depend strongly on the models used to forecast the properties of beta-delayed, neutron-induced, and spontaneous fission. This study is dedicated to the 80th anniversary of V.S. Imshennik’s birth.

Nucleosynthesis of heavy elements: Computational experiment

Delayed fission, along with induced and spontaneous fission, is responsible for the suppression of the production of superheavy elements both during the r-process and after its completion. Beta-decay strength functions are required for calculating delayed fission. In the present study, respective strength functions are calculated by relying on the theory of finite Fermi systems and by predominantly employing nuclear masses and fission barriers predicted by a generalized Thomas-Fermi model. The probabilities for delayed fission and for the emission of delayed neutrons are calculated for a number of isotopes. On the basis of calculations performed in order to determine the probabilities for delayed processes, it is shown that some of the delayed-fission probabilities calculated thus far were substantially overestimated. The application of these new results to calculating the r-process may change substantially both the r-process path and the yields of superheavy nuclei.

Influence of spontaneous fission rates on the yields of superheavy elements in the r-process

During the formation of heavy elements in the neutron star merger (NSM) scenario with a fairly long duration of the r-process, most of the seed nuclei rapidly burn out at the initial stage. The nucleosynthesis wave rapidly reaches the region of actinoids, where beta-delayed, neutron-induced, and spontaneous fission are the main reaction channels. The fission products of transuranium elements are again drawn into the r-process as new seed nuclei to form the yields of elements with mass numbers A > 100. The contribution from the various types of fission to the formation of heavy and superheavy nuclei is investigated. The proposed r-process model applied to the NSM scenario describes well the observed abundances of chemical elements, which confirms the formation of the main r-process component in the NSM scenario. Simple extrapolations of the spontaneous fission half-lives are shown to be inapplicable for the region of nuclei with N ∼ 184, because the formulas do not reflect the increase in half-life when the shell structure changes as the number of neutrons approaches 184. The formation of superheavy elements in the r-process is possible, but their survival depends to a large extent on how reliable the predictions of nuclear parameters, including the half-lives of the forming nuclei from the island of long-lived isotopes, are. The contributions from various types of fission—neutron-induced, beta-delayed, and spontaneous one—to the formation of heavy elements in the main r-process have been determined.