Tomislav Marketin

Large-scale evaluation of β-decay rates of r-process nuclei with the inclusion of first-forbidden transitions

Background: r-process nucleosynthesis models rely, by necessity, on nuclear structure models for input. Particularly important are β-decay half-lives of neutron-rich nuclei. At present only a single systematic calculation exists that provides values for all relevant nuclei making it difficult to test the sensitivity of nucleosynthesis models to this input. Additionally, even though there are indications that their contribution may be significant, the impact of first-forbidden transitions on decay rates has not been systematically studied within a consistent model. Purpose: Our goal is to provide a table of β-decay half-lives and β-delayed neutron emission probabilities, including first-forbidden transitions, calculated within a fully self-consistent microscopic theoretical framework. The results are used in an r-process nucleosynthesis calculation to asses the sensitivity of heavy element nucleosynthesis to weak interaction reaction rates. Method: We use a fully self-consistent covariant density functional theory (CDFT) framework. The ground state of all nuclei is calculated with the relativistic Hartree-Bogoliubov (RHB) model, and excited states are obtained within the proton-neutron relativistic quasiparticle random phase approximation (pn-RQRPA). Results: The β-decay half-lives, β-delayed neutron emission probabilities, and the average number of emitted neutrons have been calculated for 5409 nuclei in the neutron-rich region of the nuclear chart. We observe a significant contribution of the first-forbidden transitions to the total decay rate in nuclei far from the valley of stability. The experimental half-lives are in general well reproduced for even-even, odd-A, and odd-odd nuclei, in particular for short-lived nuclei. The resulting data table is included with the article as Supplemental Material. Conclusions: In certain regions of the nuclear chart, first-forbidden transitions constitute a large fraction of the total decay rate and must be taken into account consistently in modern evaluations of half-lives. Both the β-decay half-lives and β-delayed neutron emission probabilities have a noticeable impact on the results of heavy element nucleosynthesis models.

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.

Inclusive charged-current neutrino-nucleus reactions calculated with the relativistic quasiparticle random-phase approximation

Inclusive neutrino-nucleus cross sections are calculated using a consistent relativistic mean-field theoretical framework. The weak lepton-hadron interaction is expressed in the standard current-current form, the nuclear ground state is described with the relativistic Hartree-Bogoliubov model, and the relevant transitions to excited nuclear states are calculated in the relativistic quasiparticle random-phase approximation. Illustrative test calculations are performed for charged-current neutrino reactions on 12C, 16O, 56Fe, and 208Pb, and results compared with previous studies and available data. Through the use of the experimental neutrino fluxes, the averaged cross sections are evaluated for nuclei of interest for neutrino detectors. We analyze the total neutrino-nucleus cross sections and the evolution of the contribution of the different multipole excitations as a function of neutrino energy. The cross sections for reactions of supernova neutrinos on 16O and 208Pb target nuclei are analyzed as functions of the temperature and chemical potential.

Benchmarking nuclear models for Gamow-Teller response

A comparative study of the nuclear Gamow–Teller response (GTR) within conceptually different state-of-the-art approaches is presented. Three nuclear microscopic models are considered: (i) the recently developed charge-exchange relativistic time blocking approximation (RTBA) based on the covariant density functional theory, (ii) the shell model (SM) with an extended “jj77” model space and (iii) the non-relativistic quasiparticle random-phase approximation (QRPA) with a Brueckner G-matrix effective interaction. We study the physics cases where two or all three of these models can be applied. The Gamow–Teller response functions are calculated for 208Pb, 132Sn and 78Ni within both RTBA and QRPA. The strengths obtained for 208Pb are compared to data that enable a firm model benchmarking. For the nucleus 132Sn, also SM calculations are performed within the model space truncated at the level of a particle–hole (ph) coupled to vibration configurations. This allows a consistent comparison to the RTBA where ph⊗phonon coupling is responsible for the spreading width and considerable quenching of the GTR. Differences between the models and perspectives of their future developments are discussed.

β-decay rates of r-process nuclei in the relativistic quasiparticle random phase approximation

The fully consistent relativistic proton-neutron quasiparticle random phase approximation (PNRQRPA) is employed in the calculation of �-decay half-lives of neutron-rich nuclei in the N≈50 and N≈82 regions. A new density-dependent effective interaction, with an enhanced value of the nucleon effective mass, is used in relativistic Hartree-Bogoliubov calculation of nuclear ground states and in the particle-hole channel of the PN-RQRPA. The finite range Gogny D1S interaction is employed in the T = 1 pairing channel, and the model also includes a proton-neutron particleparticle interaction. The theoretical half-lives reproduce the experimental data for the Fe, Zn, Cd, and Te isotopic chains, but overestimate the lifetimes of Ni isotopes and predict a stable 132 Sn.

Fragmentation of spin-dipole strength in 90Zr and 208Pb

An extension of time-dependent covariant density functional theory that includes particle–vibration coupling is applied to the charge-exchange channel. Spin-dipole excitation spectra are calculated an compared to available data for 90Zr and 208Pb. A significant fragmentation is found for all three angular-momentum components of the spin-dipole strength as a result of particle–vibration coupling, as well as a shift of a portion of the strength to higher energy. A high-energy tail is formed in the strength distribution that linearly decreases with energy. Using a model-independent sum rule, the corresponding neutron skin thickness is estimated and shown to be consistent with values obtained at the mean-field level.

Calculation of β-decay rates in a relativistic model with momentum-dependent self-energies

The relativistic proton-neutron quasiparticle random phase approximation (PN-RQRPA) is applied in the calculation of β-decay half-lives of neutron-rich nuclei in the Z ≈ 28 and Z ≈ 50 regions. The study is based on the relativistic Hartree-Bogoliubov calculation of nuclear ground states, using effective Lagrangians with density-dependent meson-nucleon couplings, and also extended by the inclusion of couplings between the isoscalar meson fields and the derivatives of the nucleon fields. This leads to a linear momentum dependence of the scalar and vector nucleon self-energies. The residual QRPA interaction in the particle-hole channel includes the π + ρ exchange plus a Landau-Migdal term. The finite-range Gogny interaction is employed in the T = 1 pairing channel, and the model also includes a proton-neutron particle-particle interaction. The results are comparedwithavailabledata,anditisshownthatanextensionofthestandardrelativisticmean-fieldframeworkto include momentum-dependent nucleon self-energies naturally leads to an enhancement of the effective (Landau) nucleon mass, and thus to an improved PN-QRPA description of β − -decay rates.

First Measurement of Several β-Delayed Neutron Emitting Isotopes Beyond N=126

The β-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with β-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb, and Bi in the mass region N≳126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the β-decay strength distribution. This provides important constraints on global theoretical models currently used in r-process nucleosynthesis.

Large-scale calculations of supernova neutrino-induced reactions in Z=8-82 target nuclei

Background: In the environment of high neutrino fluxes provided in core-collapse supernovae or neutron star mergers, neutrino-induced reactions with nuclei contribute to the nucleosynthesis processes. A number of terrestrial neutrino detectors are based on inelastic neutrino-nucleus scattering and modeling of the respective cross sections allow predictions of the expected detector reaction rates. Purpose: To provide a self-consistent microscopic description of neutrino-nucleus cross sections involving a large pool of Z=8–82 nuclei for the implementation in models of nucleosynthesis and neutrino detector simulations. Methods: Self-consistent theory framework based on relativistic nuclear energy density functional is employed to determine the nuclear structure of the initial state and relevant transitions to excited states induced by neutrinos. The weak neutrino-nucleus interaction is employed in the current-current form and a complete set of transition operators is taken into account. Results: We perform large-scale calculations of charged-current neutrino-nucleus cross sections, including those averaged over supernova neutrino fluxes, for the set of even-even target nuclei from oxygen toward lead (Z=8–82), spanning N=8–182 (OPb pool). The model calculations include allowed and forbidden transitions up to J=5 multipoles. Conclusions: The present analysis shows that the self-consistent calculations result in considerable differences in comparison to previously reported cross sections, and for a large number of target nuclei the cross sections are enhanced. Revision in modeling r-process nucleosynthesis based on a self-consistent description of neutrino-induced reactions would allow an updated insight into the origin of elements in the Universe and it would provide the estimate of uncertainties in the calculated element abundance patterns.

Uncertainties in modeling low-energy neutrino-induced reactions on iron-group nuclei

Charged-current neutrino-nucleus cross sections for 54, 56Fe and 58, 60Ni are calculated and compared using frameworks based on relativistic and Skyrme energy-density functionals and on the shell model. The current theoretical uncertainties in modeling neutrino-nucleus cross sections are assessed in relation to the predicted Gamow-Teller transition strength and available data, to multipole decomposition of the cross sections, and to cross sections averaged over the Michel flux and Fermi- Dirac distribution. By employing different microscopic approaches and models, the decay-at-rest (DAR) neutrino-56Fe cross section and its theoretical uncertainty are estimated to be 〈σ〉th=(258±57)×10−42 cm2, in very good agreement with the experimental value 〈σ〉exp=(256±108±43)×10−42 cm2.