Ground State Decay of the Three-Proton Emitter 17Na Reveals Isospin Symmetry Breaking
Abstract
The spectrum of the exotic three-proton (3p) emitter 17Na has been studied by detecting all in-flight decay products. Derived from the measured angular correlations 14O+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 17Na ground state. This decay energy value is significantly smaller than the previous experimental upper limit. Our measured 14O-p correlations stemming from the ground state decay have been quantitatively described by a sequential 1p-2p emission from a 17Na resonance via the intermediate 16Ne ground state, which allowed to derive the upper limit of 17Na 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 31K, 20Al, and 17Na), 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.
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