β-decay spectroscopy of 27S

Abstract

Background: β-decay spectroscopy provides valuable nuclear physics input for accurate modeling of nova and x-ray burst observables and a stringent test for shell-model theories far from the stability line. The decay scheme of S27 is complicated and far from being understood due to a lack of experimental data prior to this work. Purpose: We aim to experimentally constrain the thermonuclear Si26(p,γ)P27 reaction rate and to probe the possible mirror asymmetry in S27 and Na27 decays. Method: The S27 ions were collected by double-sided silicon strip detectors operating in conjunction with high-purity germanium detectors, so the positrons, protons, and γ rays emitted in the decay were measured simultaneously. Results: The precise resonance energy and the ratio between γ and proton partial widths of the key 3/2+ resonance were obtained, thereby determining the Si26(p,γ)P27 reaction rate based mainly on experimental constraints. The half-life of S27, the excitation energies, β-feeding intensities, log ft values, and Gamow-Teller transition strengths for the states of P27 populated in the β decay of S27 were determined. A more complete S27β-decay scheme was constructed and compared to the β decay of mirror nucleus Na27 and to the shell-model calculations with the universal sd (USD) Hamiltonian taking into account the shift of single-particle energies and the reduction of two-body matrix elements. Conclusions: This work yields a new Si26(p,γ)P27 reaction rate two orders of magnitude lower than the rate recommended in the reaction rate libraries at around 0.1 GK. Experimental evidence for the observation of mirror asymmetries for the Gamow-Teller transitions of S27 and Na27 is also provided. The shell-model calculations using a modified USD interaction related to the weakly bound proton 1s1/2 orbit give a reasonable description of the decay properties of S27.