Measuring the β-decay Properties of Neutron-rich Exotic Pm, Sm, Eu, and Gd Isotopes to Constrain the Nucleosynthesis Yields in the Rare-earth Region

Abstract

The β-delayed neutron-emission probabilities of 28 exotic neutron-rich isotopes of Pm, Sm, Eu, and Gd were measured for the first time at RIKEN Nishina Center using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The existing β-decay half-life (T 1/2) database was significantly increased toward more neutron-rich isotopes, and uncertainties for previously measured values were decreased. The new data not only constrain the theoretical predictions of half-lives and β-delayed neutron-emission probabilities, but also allow for probing the mechanisms of formation of the high-mass wing of the rare-earth peak located at A ≈ 160 in the r-process abundance distribution through astrophysical reaction network calculations. An uncertainty quantification of the calculated abundance patterns with the new data shows a reduction of the uncertainty in the rare-earth peak region. The newly introduced variance-based sensitivity analysis method offers valuable insight into the influence of important nuclear physics inputs on the calculated abundance patterns. The analysis has identified the half-lives of 168Sm and of several gadolinium isotopes as some of the key variables among the current experimental data to understand the remaining abundance uncertainty at A = 167–172.

Comparative study of neural network and model averaging methods in nuclear β-decay half-life predictions

“…For the nuclei whose half-lives from NUBASE2020 and new measurements deviate with each other, such as 165,166 Sm and 168 Eu, the NN-I4T better reproduces the half-lives from new measurements. For the nuclei whose half-lives are measured for the first time in [18], the NN-I4T better reproduces these newly measured data within the uncertainties, although the errors of new half-life measurements for these nuclei are generally large. This may indicate that the neural network approach can capture well the physics describing nuclear β decay half-lives, which illustrates that the NN-I4T has a reliable extrapolation ability of β decay half-life predictions, at least for the nuclei not far from the training region.…”

“…To test the extrapolation ability of the NN-I4T, figure 6 shows new measurements of nuclear β-decay half-lives from [18], which are not used to train the neural networks. The training data from NUBASE2020 are also for comparison.…”

“…Measurements of the β-decay half-lives have made great progress in recent years [14][15][16][17][18], however, many nuclei on the r-process path are still beyond the reach of experiments, especially for the nuclei around and above N = 126 [12]. Therefore, the theoretical predictions of β-decay half-lives are necessary for the r-process simulations.…”

Nuclear Uncertainties Associated with the Nucleosynthesis in Ejecta of a Black Hole Accretion Disk

“…The rare earth peak, located around mass number A ∼ 164, and particularly its left wing is found to be sensitive to unmeasured β-decay rates. Relative to masses, this is a weaker dependence, but notable nonetheless as it points to the requirement of half-life measurements in addition to precision mass measurements to solve the origin of this peak (Kiss et al 2022).…”

Nuclear data activities for medium mass and heavy nuclei at Los Alamos

“…The inclusion of measured β-decay properties has also been studied in the context of this pipeline. Kiss et al measured 28 new β-decay neutron emission probabilities and half-lives for important lanthanide species [39]. This study not only informed the impact of measurements in astrophysical environments, it was able to rank the importance of the measurements through novel Bayesian analysis.…”