Neutron-rich tin (Sn) isotopes play a pivotal role in the rapid neutron-capture process (r-process), a key mechanism for synthesizing heavy elements in extreme astrophysical environments. The interplay of slower $\beta$-decay rates relative to neutron-capture rates - driven by shell effects near $Z = 50$ - delays the r-process flow, forcing material to undergo successive neutron captures until a breakout point is reached, where sufficiently short half-lives enable further nucleosynthesis [1]. This threshold, potentially near $^{140}$Sn ($N = 90$), depends on whether the neutron density of the astrophysical site outpaces the r-process timescale [2]. Systematic studies of half-lives of neutron-rich Sn isotopes are thus critical to inferring the neutron-density threshold and constraining r-process conditions. Recent sensitivity studies further underscore the importance of $^{140}$Sn half-lives, demonstrating their significant impact on final r-process abundances [3].
Theoretically, the half-lives of neutron-rich Sn isotopes are supposed to be sensitive to the deformation effects [4]. Some models predict a new subshell closure at $N = 90$ [5, 6], yet experimental evidence remains sparse [7, 8]. Comparisons to the $^{78}$Ni region, where magicity influences half-lives, highlight the need for direct measurements [9].
We propose an experiment focusing on the $\beta$-decay half-lives of neutron-rich $^{140}$Sn and $^{141}$Sn (and vicinity isotopes in cocktail beam) within the post-EURICA project at RIBF. Building on prior statistics in other $\beta$-decay experiments [7, 10], projected beam intensities, LISE++ simulations [11], and recent BigRIPS machine studies, these isotopes are now experimentally accessible. Beyond half-lives, isomer and $\beta$-$\gamma$ spectroscopy will probe low-lying states, elucidating shell evolution near $N = 90$. These measurements will provide critical benchmarks for nuclear structure models and r-process simulations, advancing our understanding of heavy-element formation.
[1] S. Shibagaki et al., ``Relative Contributions of the Weak, Main, and Fission-Recycling r-Process,'', ApJ 816, 79 (2016).
[2] J. Van Schelt et al., ``First Results from the CARIBU Facility: Mass Measurements on the r-Process Path,'', Phys. Rev. Lett. 111, 061102 (2013).
[3] M. R. Mumpower et al., ``The Impact of Individual Nuclear Properties on r-Process Nucleosynthesis,'', Prog. Part. Nucl. Phys.. 86 (2016).
[4] P. Moller et al., ``Nuclear properties for astrophysical and radioactive-ion-beam applications (II),'', ADNDT 125, 1 (2019); P. Moller, private communication.
[5] S. Sarkar and M. Saha Sarkar, ``New Shell Closure for Neutron-Rich Sn Isotopes,'', Phys. Rev. C 81, 064328 (2010).
[6] A. R. Vernon et al., ``Nuclear Moments of Indium Isotopes Reveal Abrupt Change at Magic Number 82,'', Nature 607, 260 (2022); PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-611360/v1].
[7] G. S. Simpson et al., ``Yrast 6+ Seniority Isomers of $^{136,138}$Sn,'', Phys. Rev. Lett. 113, 132502 (2014).
[8] A. Jungclaus et al., ``Position of the Single-Particle 3/2$^-$ State in $^{135}$Sn and the $N=90$ Subshell Closure,'', Phys. Lett. B 851, 138561 (2024).
[9] Z. Y. Xu et al., ``$\beta$-Decay Half-Lives of $^{76,77}$Co, $^{79,80}$Ni, and $^{81}$Cu: Experimental Indication of a Doubly Magic $^{78}$Ni,'', Phys. Rev. Lett. 113, 032505 (2014).
[10] V. H. Phong et al., ``$\beta$-Delayed One and Two Neutron Emission Probabilities South-East of $^{132}$Sn and the Odd-Even Systematics in r-Process Nuclide Abundances,'', Phys. Rev. Lett. 129, 172701 (2022).
[11] V. H. Phong et al., ``Mass measurements beyond $^{132}$Sn reaching the r-process path,'', NP2212-RIBF216 accepted proposal at RIBF (2022).
