The β--decay half-lives of extremely neutron-rich nuclei are important for understanding nucleosynthesis in the r-process. However, most of their half-lives are unknown or very uncertain, leading to the need for reliable calculations. In this study, we updated the coefficients in recent semi-empirical formulae using the newly updated mass (AME2020) and half-life (NUBASE2020) databases to improve the accuracy of the half-life prediction. In particular, we developed a new empirical model for better calculations of the β--decay half-lives of isotopes ranging in Z = 10 – 80 and N = 15-130. We examined the β--decay half-lives of the extremely neutron-rich isotopes at and around the neutron magic numbers of N = 50, 82, and 126 using either five different semi-empirical models or finite-range droplet model and quasi-particle random phase approximation (FRDM+QRPA) method. The β--decay rates derived from the estimated half-lives were used in calculations to evaluate the impact of the half-life uncertainties of the investigated nuclei on the abundance of the r-process. The results show that the half-lives mostly range in 0.001 < T1/2 < 100 s for the nuclei with a ratio of N/Z < 1.9; however, they differ significantly for those with the ratio of N/Z > 1.9. The half-life differences among the models were found to range from a few factors (for N/Z < 1.9 nuclei) to four orders of magnitude (for N/Z > 1.9). These discrepancies lead to a large uncertainty, which is up to four orders of magnitude, in the r-process abundance of isotopes. We also found that the multiple-reflection time-of-flight (MR-TOF) technique is preferable for precise mass measurements because its measuring timescale applies to the half-lives of the investigated nuclei. Finally, the results of this study are useful for studies on the β-decay of unstable isotopes and astrophysical simulations.

In this paper, we examined the β--decay half-lives of 94 extremely neutron-rich isotopes with Z = 26 − 57 close to the neutron drip line, which are important for the r-process calculations. The half-lives were calculated using four semi-empirical models and compared to those based on the FRDM+QRPA approach and available measured data. The impact of the difference in the models on the half-life predictions was investigated. We found that theoretical calculations for the β-decay half-life have a large deviation, up to 60%, which is mostly similar to that in measurements. The half-lives of the investigated nuclei are ranging from a few to hundreds of milliseconds. The r-process abundances in various astrophysical scenarios were calculated by using the predicted half-lives. The half-life uncertainty due to different models results in a large deviation in the isotopic abundance, specially for the isotopes in the mass range of A > 210. The shell closures in 76Fe is still a doubt due to the discrepancy in the trends of the half-life and paring gap while a closed-shell at N = 82 in 127Rh is possible. The results of this study also notice that it is a challenge for measuring precisely the masses of 106Rb, 116,117Nb, 122Tc, and 128Rh because of their short half-lives.

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Energy loss straggling was found to be critical in evaluating the energy reaction using heavy-ion beams during the early stage of experiments at accelerator facilities. Despite a significant attempt in simulating this quantity using computer codes such as LISE++ and SRIM, there still exists a discrepancy between experimental data and computed results. In this study, we provide a greatly improved precision of estimations using the LISE++ code by evaluating the energy loss straggling of the alpha particles at 5.486 MeV in Tb, Ta, and Au materials. After comparing with the observables, it was found that the ratio of the energy loss straggling computed by the LISE++ code to that measured in experiments has a fairly large range of 1.5 - 3.0. For this reason, the so-called modified LISE++ calculation is constructed by adding the adjusting parameters into the original estimation to minimize the uncertainty of the straggling prediction. The modified calculation has shown dramatic improvements in computed energy loss straggling, which are almost similar to those obtained in the measurements, of 5.486-MeV alphas in the aforementioned materials with the atomic numbers in a range of Z = 65 – 79.