Theoretical Estimate of the Half-life for the Radioactive ¹³⁴Cs and ¹³⁵Cs in Astrophysical Scenarios

Abstract

We analyze the 55 134 Cs → 56 134 Ba and 55 135 Cs → 56 135 Ba β − decays, which are crucial production channels for Ba isotopes in asymptotic giant branch (AGB) stars. We calculate, starting from relativistic quantum mechanics, the effects of multichannel scattering onto weak decays, including nuclear and electronic excited states (ESs) populated above ≃10 keV, for both parent and daughter nuclei. We find increases in the half-lives for T > 108 K (by more than a factor of 3 for 134Cs) as compared to previous works based on systematics. We also discuss our method in view of these previous calculations. An important impact on half-lives comes from nuclear ES decays, while including electronic temperatures yields further increases of about 20% at energies of 10–30 keV, typical of AGB stars of moderate mass (M ≲ 8 M ⊙). Despite properly considering these effects, the new rates remain sensitively lower than the Takahashi & Yokoi values, implying longer half-lives at least above 8–9 keV. Our rate predictions are in substantial accord with recent results based on the shell model, and strongly modify branching ratios along the s-process path previously adopted. With our new rate, nucleosynthesis models well account for the isotopic admixtures of Ba in presolar SiC grains and in the Sun.

GEANT4 simulations for γ-ray detector array design for inplasma β-decays studies of nuclear astrophysics interest

“…However the rate of 134 Cs decay estimated from [1] seems currently unsatisfactory [20], as it inhibits the proper reproduction of the two s-only isotopes 134 Ba, 136 Ba in suitable proportions. In particular, in [5] one needs to assume that the temperature dependence of the decay rate is less steep by around a factor of 8 (as modelled in [21]) to level the production of 134 Ba and 136 Ba. Hence, in-plasma measurements could shed light on the role played by the ionization state affecting the β−decay and a simulated virtual experiment was carried out to assess the feasibility and sensitivity of the PANDORA setup and experimental approach to be adopted.…”

News on the slow neutron capture process in AGB stars

“…In particular, the mainstream (MS) ones, which account for about 90% of all SiC grains, are believed to have originated from carbon AGB stars and show their nucleosynthetic signature [22]. Therefore, isotope ratios of heavy elements measured in MS SiC grains are commonly used to study the s-process occurring in AGB stars and provide constraints on AGB stellar models [23][24][25][26]. Most of the presolar MS SiC grains have been shown to originate from AGB stars with M ∼ 2 M ⊙ and close-to-solar metallicity [27,28].…”

“…In these objects, the main neutron source is the 13 C(α, n) 16 O reaction [3,4], while the 22 Ne(α, n) 25 Mg reaction is only marginally activated during the TPs [5]. For the 13 C(α, n) 16 O reaction to effectively operate, some 13 C must be present in the He-rich zone located below the H-rich convective envelope.…”

Presolar grain isotopic ratios as constraints to nuclear physics inputs for s-process calculations

“…However to reproduce the galactic abundances of the 2 s-only nucleus 135 Ba and 136 Ba the temperature dependence of the decay rate for 134 Cs has to be assumed about 8 times less steep than suggested by [17] or by changing upward the cross section of 134 Cs by more than a factor of two [7] . Recently, thanks to an approach based on the solution of the Dirac-Hartree Fock equations, Taioli et al (2022) [20] found the temperature dependence of the 134 Cs and the 135 Cs β−decay rates to smaller than so far assumed. In particular, the half-lives of 134 Cs estimated by the authors is longer than the one by [17] by a factor ranging from 2.5 (near 8-10 keV) to more than 30 (at 30 keV).…”