$\beta$-decay half-lives of 55 neutron-rich isotopes beyond the $N=82$ shell gap

The $\beta$-decay half-lives of 55 neutron-rich nuclei ${}^{134-139}\mathrm{Sn}$, ${}^{134-142}\mathrm{Sb}$, ${}^{137-144}\mathrm{Te}$, ${}^{140-146}\mathrm{I}$, ${}^{142-148}\mathrm{Xe}$, ${}^{145-151}\mathrm{Cs}$, ${}^{148-153}\mathrm{Ba}$, and ${}^{151-155}\mathrm{La}$ were measured at the Radioactive Isotope Beam Factory employing the projectile fission fragments of ${}^{238}\mathrm{U}$. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.

Figure 1

Particle identification of the nuclides and the well separated hydrogen-like charge state [${}^{A-3}{Z}^{(Z-1)+}$] events accompanied closely on the left side of the fully stripped (${}^A{Z}^{Z+}$) isotopes as illustrated in the figure [22]. The isotopes included by the green shaded area were already measured in previous experiments. The nuclei tagged by red circles are the most exotic isotopes measured in this experiment for each element.

Figure 2

Time distribution of ${}^{142}\mathrm{Te}$ $\beta$-decay events fitted to the sum of activities of several components: parent nuclei (solid green line), daughter nuclei (solid magenta line), granddaughter nuclei (solid orange line), $\beta$-delayed daughter nuclei (dashed magenta line), $\beta$-delayed granddaughter nuclei (dashed orange line), as well as a constant background (solid blue line).

Figure 3

Systematic trends of the $\beta$-decay half-lives from this work (solid circles) and from previous measurements (open triangles) [23, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38]. The measurements are compared to predictions of five theoretical models: FRDM$+$QRPA (2003) [39] (brown), FRDM$+$QRPA (2019) [40] (green), KTUY$+$GT2 [41, 42] (red), RHB$+pn$-RQRPA [43] (blue), and DF$+$CQRPA [44, 45] (magenta).

Figure 4

Ratio between theoretical calculations and measured half-lives plotted versus experimental half-life and neutron number for the four models: FRDM$+$QRPA (2003) [39] (brown), FRDM$+$QRPA (2019) [40] (green), KTUY$+$GT2 [41, 42] (red), and RHB$+pn$-RQRPA [43] (blue), with the previous measurements (empty dots) and 13 newly measured half-lives (solid dots).

Figure 5

Gamow-Teller strength functions in the daughter states following $\beta$-decay of ${}^{141}\mathrm{Te}$ calculated by the FRDM$+$QRPA(2019) model, based on the odd particle located in level 45 (a) and level 46 (b). The thin vertical arrow indicates the ${\mathrm{Q}}_{\beta }$ value, the slightly thicker arrow indicates the neutron separation energy.

Figure 6

The calculated $r$-process abundance with (red) and without (blue) the 13 newly measured half-lives by employing the five distinct astrophysical trajectories ((1) low entropy ($s$ $\approx$ 50) and initial electron fraction ($Y_e=0.27$) [51], (2) high entropy ($s$ $\approx$ 200), initial electron fraction ($Y_e=0.3$) [52], (3) low entropy ($s$ $\approx$ 30) and initial electron fraction (artificially reduced to produce a main $r$ process, $A>120$) [53], (4) low entropy ($s$ $\approx$ 10) and initial electron fraction ($Y_e=0.05$) with fission recycling [54], and (5) low entropy ($s$ $\approx$ 10) and initial electron fraction ($Y_e=0.19$) [51]). The experimental data were input if they are known; otherwise, the theoretical properties were employed (nuclear mass (FRDM-2012 [47]) $+$ $T_{1/2}$ and $P_n$ (FRDM$+$QRPA (2019) [40]) $+$ ($n,\gamma$) rates (calculated based on FRDM-2012 [47]). The black points represent the solar $r$-process abundance [55].