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B. S. O. Johansson

Publications and source records attributed to B. S. O. Johansson.

2 recordsLinked to original sources

Precision $β$-delayed charged-particle emission spectroscopy at FRIB: Proof of principle with the $β$-decay of $^{25}\mathrm{Si}$

We report on the $β$-delayed proton and $γ$-ray emission from $^{25}\mathrm{Si}$, measured at the Facility for Rare Isotope Beams (FRIB). Low-energy $^{25}\mathrm{Si}$ ions extracted from the Advanced Cryogenic Gas Stopper were implanted into a thin carbon foil surrounded by a compact, highly segmented array of silicon detector telescopes and two high-purity germanium detectors. This setup provides high-resolution charged-particle spectroscopy, establishing a proof of principle for precision stopped-beam decay studies at FRIB. We reconstruct the $^{25}\mathrm{Si}$ decay scheme, resolving new high-energy proton transitions and determining the feeding to excited states in $^{24}\mathrm{Mg}$. The observation of spectral interference patterns enables firm spin and parity assignments for highly excited states in $^{25}\mathrm{Al}$. The $^{25}\mathrm{Si}$ $β$-strength distribution is extracted and compared with large-scale shell-model calculations.

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A ground state $^{22}$Al halo is unlikely

We report the decisive resolution of the ground state spin and parity of the proton-dripline nucleus $^{22}$Al, a prime candidate for a proton halo. The resolution stems from the first $β$-delayed charged particle emission experiment in the Gas Stopping Area at the Facility for Rare Isotope Beams (FRIB), leveraging high-intensity, low-energy beams extracted from the Advanced Cryogenic Gas Stopper (ACGS). The pristine beam quality from FRIB and the ACGS enabled a sensitive particle identification technique using thin silicon detectors, allowing for the suppression of the dominant proton background and the first observation of the weak $β$-delayed $α$ transition from the Isobaric Analog State in $^{22}$Mg to the $^{18}$Ne ground state. This observation uniquely fixes the $^{22}$Al ground state as $4^+$. The valence proton is confined by a dominant $d$-wave centrifugal barrier which, combined with the Coulomb repulsion, hinders the tunneling required for halo formation despite the exceptionally low proton separation energy of $^{22}$Al.

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