Synopsis: The limits of a closed shell

The noble gas atoms, like xenon and argon, are inert because their electrons form a closed shell. Similarly, the filling of proton or neutron shell states in atomic nuclei has a stabilizing effect. New data are, however, showing that even a closed shell structure is a fragile boundary, especially for weakly bound, exotic nuclei far from the valley of stability.

Lead is an attractive element to study these effects. With 82 protons, lead has a closed shell structure for the protons. Lead also has many accessible isotopes, which allows experimentalists to measure how the binding effects of a closed shell structure weaken in nuclei with progressively fewer neutrons.

One reason that the closed shell weakens in neutron-deficient elements is that attractive interactions between valence protons and neutrons in spatially overlapping orbits lower the energy of certain proton excitations. A light lead nucleus with these proton excitations has a nonspherical, or deformed, shape that is different than the normal states of the nucleus. To see this “shape coexistence,” however, requires highly sensitive spectroscopy of the nuclear states.

Now, in a Rapid Communication appearing in Physical Review C, a collaboration between Finland, the UK, France, and Belgium reports a gamma-ray spectrum of 180Pb—the most neutron deficient isotope of Pb yet studied with spectroscopy. They first detected the gamma rays from a variety of nuclear reactions and then identified those gamma rays coming from 180Pb nuclei. To do this, they measured the characteristic alpha decay of 180Pb at the focal plane of a magnetic spectrometer. The experiment was a true tour-de-force as the cross section for producing 180Pb is exceptionally small (of order 10 nanobarns).

The team’s results will provide valuable constraints on femtoscopic models of nuclear structure near the proton drip line. – Rick Casten

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