A team of experimental physicists has identified a resonant peak that likely marks the ground state of the very exotic nucleus 17Na. By measuring all decay fragments in flight, they found a three‑proton (3p) decay energy of 2.24(+0.17/−0.25) MeV. This value is much lower than the previous experimental upper limit of about 4.85 MeV and points to unexpected changes in nuclear structure for proton‑rich nuclei.

The experiment was done at GSI in Germany. A high‑energy beam of 24Mg produced a secondary beam that was used to make 17Na in a beryllium target. The escaping particles from the unstable 17Na — a 14O nucleus and three protons — were tracked in coincidence by an array of four large double‑sided silicon microstrip detectors (DSSD). These detectors recorded hit positions with high precision, which let the team reconstruct the full decay geometry and the angles between 14O and each proton.

To read out the decay energy, the researchers used angular correlations and a derived kinematic variable called ρ3, which combines the three proton‑to‑14O angles and reflects how the decay energy is shared. They compared the measured ρ3 distribution with detailed simulations. By selecting events with proton‑to‑14O angles between 20 and 42 milliradians — the range typical for the ground state of the intermediate nucleus 16Ne — they isolated a decay pattern that matches a sequential decay: one proton emitted from 17Na into 16Ne, followed by a two‑proton emission from 16Ne. From this analysis they obtain the 3p decay energy 2.24(+0.17/−0.25) MeV and place an upper limit of 0.6 MeV on the 17Na ground‑state width.

The result matters because it changes how mirror nuclei behave when you compare a proton‑rich nucleus with its neutron‑rich partner. The authors report a systematic drop in mirror energy differences (MED — defined as the neutron separation energy of the neutron‑rich nucleus minus the proton separation energy of its proton‑rich mirror) for nearly all known 3p emitters such as 31K, 20Al and 17Na. This strong lowering is different from what is seen in less exotic nuclei and is commonly associated with isospin symmetry breaking. Isospin symmetry is a basic idea that the nuclear force treats protons and neutrons similarly; violations can come from the proton‑proton electric repulsion (the Coulomb force), the proton‑neutron mass difference, or small charge‑dependent parts of the nuclear force.