Accessing nuclear structure beyond the proton dripline

Kyle Brown (Michigan State University, USA)

Two-nucleon decay is the most recently discovered nuclear-decay mode.  For proton-rich nuclei, most multi-proton decays occur via sequential steps of one-proton emission. Direct two-proton (2p) decay was believed to occur only in even-Z nuclei beyond the proton-drip line where one-proton decay is energy forbidden. This has been observed for the ground states of around a dozen nuclei including ⁶Be, the lightest case, and ⁵⁴Zn, the heaviest case. Direct 2p decay has also recently been observed for isobaric-analog states where all possible 1p intermediates are either isospin allowed and energy forbidden, or energy allowed and isospin forbidden.

For light proton emitters (A < ~30), the lifetimes are short enough that the invariant-mass technique is ideal for measuring the decay energy, intrinsic width, and, for multi-proton decays, the momentum correlations between the fragments. I will describe recent measurements of proton emitters using the invariant-mass technique with the silicon arrays like the High-Resolution Array (HiRA). I will present a new, high-statistics measurement on the decay of the ground and excited states in ¹²O and ¹⁹Mg. By measuring the momentum correlations between the decay fragments, one can observe how the decay transitions from direct to sequential as the decay energy increases. I will also present the first observation of ¹¹O, the mirror of the well-known halo nucleus ¹¹Li, as well as the first observation of ¹⁸Mg.