Search for microsecond isomers at the dripline of the island of inversion*

The island of inversion around 32Mg, characterized by tensor-force-driven deformation, has

been well characterized in its north and west “shores.” The south-eastern “beaches” offer

intriguing physics where deformation and neutron dripline effects overlap, yet they remain

poorly known due to the difficulty in producing them in experimental facilities. In particular, if

standard ordering is restored at the dripline, it will suppress low-energy opposite-parity

intruders. Furthermore, inverting the parity order could result in microsecond isomers

between the same parity states. In this talk, I will present experimental work done at the

National Superconducting Laboratory as a part of the E19044 collaboration. A 48Ca beam

was fragmented to produce a cocktail beam of isotopes around Z=9, N=20 29F and

separated by mass using the A1900 spectrometer. The cocktail beam was implanted in a

YSO crystal, and the decay products were detected using 3 HPGe clovers for gamma rays

and 48 VANDLE bars for beta delayed neutrons. A comprehensive search for microsecond

isomers was performed on all Z>8 nuclei in the cocktail beam, using the delayed-gamma

beta-correlation technique. We propose a new state in 36Mg at 883 keV from observing a

168 keV isomeric transition. The isomer’s half-life, [120-400] ns at three standard

deviations, indicates it corresponds to an E2 transition feeding the first 2+ state. We

conclude it corresponds to a deformed second 0+ state, confirming shape coexistence

between the N=20 and N=28 islands of inversion.