Si 25 β+ -decay spectroscopy

Background: β-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. Purpose: To search for new β-delayed protons and γ rays of Si25 to investigate the properties of Al25 excited states. Method: Si25β decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and γ rays emitted in the decay were detected simultaneously. A Monte Carlo method was used to model the Doppler broadening of Mg24γ-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal sd-shell Hamiltonians were performed. Results: The most precise Si25 half-life to date has been determined. A new proton branch at 724(4) keV and new proton-γ-ray coincidences have been identified. Three Mg24γ-ray lines and eight Al25γ-ray lines are observed for the first time in Si25 decay. The first measurement of the Si25β-delayed γ-ray intensities through the Al25 unbound states is reported. All the bound states of Al25 are observed to be populated in the β decay of Si25. Several inconsistencies between the previous measurements have been resolved, and new information on the Al25 level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of Na25 and the shell-model calculations. Conclusions: The measured excitation energies, γ-ray and proton branchings, log ft values, and Gamow-Teller transition strengths for the states of Al25 populated in the β decay of Si25 are in good agreement with the shell-model calculations, offering gratifyingly consistent insights into the fine nuclear structure of Al25.