Identifying the spin-trapped character of the 32Si isomeric state

The properties of a nanosecond isomer in 32Si, disputed in previous studies, depend on the evolution of proton and neutron shell gaps near the island of inversion. We have placed the isomer at 5505.2(2) keV with J7` = 5-, decaying primarily via an E3 transition to the 2+1 state. The E3 strength of 0.0841(10) W.u. is unusually small and suggests that this isomer is dominated by the (vd3/2)-1 circle times (vf7/2)1 configuration, which is sensitive to the N = 20 shell gap. A newly observed 4+1 state is placed at 5881.4(13) keV; its energy is enhanced by the Z = 14 subshell closure. This indicates that the isomer is located in a yrast trap, a feature rarely seen at low mass numbers.