Synopsis: Nucleon attractions

High-precision mass measurements of an isotope of mercury will help us to understand the forces between nucleons in nuclei.

Comparing the masses of nuclei that differ by a few neutrons or protons can yield empirical values for the interaction strengths between the valence nucleons. By measuring the strength of these interactions, we can better extrapolate the structure of heavier, less stable nuclei to help answer such questions as “Do superheavy nuclei exist?” or “How are heavy elements created?”

One of the most useful nuclear regions in which to carry out these comparisons is around the doubly magic nucleus ${}^{208}\text{Pb}$, which has a well-understood shell structure. Unfortunately, the mass of ${}^{208}\text{Hg}$, which contains two fewer protons than ${}^{208}\text{Pb}$, has not been known to sufficient precision to make meaningful comparisons. A beautiful experiment appearing in Physical Review Letters has now filled in this missing piece. Using the unique capabilities available at the GSI near Darmstadt, Germany, an international team of scientists captured nuclear fragmentation products, including ${}^{208}\text{Hg}$, from a high-energy beam of ${}^{238}\text{U}$ into the Experimental Storage Ring (ESR). Sensitive detectors around the ring measured the circulation frequencies of the different species, which depend on the particle masses.

When combined with previous mass measurements, the new ${}^{208}\text{Hg}$ mass measurement allows for the first time an empirical determination of the average proton-neutron interaction for neutron orbits above the closed shell at $N=126$. The interaction strength decreases dramatically beyond the closed shell, confirming that it depends strongly on the spatial overlap of the valence orbits. These new results will help to refine predictions for nuclei far from stability. – Gene D. Sprouse

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