Large-scale evaluation ofβ-decay rates ofr-process nuclei with the inclusion of first-forbidden transitions

Abstract

Background: R-process nucleosynthesis models rely, by necessity, on nuclear structure models for input. Particularly important are beta-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:To provide a table of β-decay half-lives and β-delayed neutron emission probabilities, including firstforbidden 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 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, both for even-even, odd-A and odd-odd nuclei, in particular for short-lived nuclei. 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 beta-decay half-lives, and beta-delayed neutron emission probabilities have a noticeable impact on the results of heavy element nucleosynthesis models.

94 β -Decay Half-Lives of Neutron-Rich Cs55 to Wu1, Nishimura2, Lorusso3 et al. 2017Phys. Rev. Lett.

“…The measured half-lives are reported in the Table (see Supplemental Material [29]). Figure 3 shows the systematic trends of β-decay half-lives as functions of neutron number N. Experimental results are compared with previous measurements, and the predictions of three theoretical models: FRDM þ QRPA [26], KTUY þ GT2 [27,28], and the relativistic Hartree-Bogoliubov (RHB) with the proton-neutron relativistic quasiparticle random phase approximation (pn-RQRPA) [30]. Our measurements are in very good agreement with the literature values, while discrepancies with theoretical predictions in some cases reach 1 order of magnitude.…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…This causes a very strong r-process with fission cycling. The utilized mass model, beta-decay half-lives, fission barriers, and fission yield prescriptions [12,18,29,38,42,55,65,78] have a strong effect on the final abundance distribution. While during the r-process (when reactions are still in n, γ − γ, n equilibrium) the 2nd and 3rd r-process peak are exactly at the right position.…”

“…One option to remedy this effect are variations in beta-decay rates. Shorter half-lives [38] of heavies release neutrons from fission earlier, when n, γ − γ, n equilibrium is still in place and can avoid or strongly reduce the late shift of the 3rd peak [12] (see Fig.3a). This effect is also seen with the HFB mass model (see also [18]), while Mendoza-Temis [42] analyzed additional nuclear uncertainties and showed especially an improvement with the Duflo-Zuker mass model.…”

“…a. r-process abundance distribution in dynamical ejecta of neutron star mergers, utilizing different mass models, fission fragment distributions, and half-lives [12]. The effect of improved half-lifes [38], in comparison to the original FRDM based ones, is shown. b.…”

Measurement of the heaviestβ-delayed 2-neutron emitter:136Sb

“…In the neutron-rich region, β − -decay is the dominant mode, however β-delayed one-neutron emission, discovered by Roberts et al [1] in 1939, becomes the dominant decay channel for very neutronrich isotopes. The importance in measuring the decays of neutron-emitting nuclei in certain regions of the chart of nuclides has been emphasized by international data evaluators experts [2,3] and by theoreticians, who need more realistic inputs for levels above the neutron separation energy, S n , for their nuclear structure models, and to reproduce astrophysical environments where the r -process nucleosynthesis could take place [4][5][6][7][8]. In order to obtain new experimental β-delayed neutron emission probabilities, P n -values, a new high efficiency 4π neutron detector named BELEN [9,10] has been designed and a e-mail: roger@baeturia.com b e-mail: dillmann@triumf.ca developed.…”