Ground State Decay of the Three-Proton Emitter $^{17}$Na Reveals Isospin Symmetry Breaking

Abstract

The spectrum of the exotic three-proton (3p) emitter $^{17}$Na has been studied by detecting all in-flight decay products. Derived from the measured angular correlations $^{14}$O+p+p+p, a resonant peak has been discovered at the 3p-decay energy of 2.24($^{+0.17}_{-0.25}$) MeV, which likely corresponds to the $^{17}$Na ground state. This decay energy value is significantly smaller than the previous experimental upper limit. Our measured $^{14}$O-p correlations stemming from the ground state decay have been quantitatively described by a sequential 1p-2p emission from a $^{17}$Na resonance via the intermediate $^{16}$Ne ground state, which allowed to derive the upper limit of $^{17}$Na ground-state width of 0.6 MeV. A dramatic systematic decrease in the mirror energy differences of mirror nuclei pairs has been observed at almost all 3p emitters with known proton separation energy (such as $^{31}$K, $^{20}$Al, and $^{17}$Na), in sharp contrast to the behavior in less exotic nuclei. Such a lowering effect indicates a general trend in evolution of nuclear structure for light to medium mass nuclei beyond the proton drip line, which is often associated with strong isospin symmetry breaking.

10 pages, 8 figures