Weighing exotic calcium nuclei

Naturally occurring calcium is mostly ⁴⁰Ca, a stable isotope with 20 neutrons and 20 protons. Twenty is a magic number in the shell model of nuclear structure. So the ‘doubly magic’ character of ⁴⁰Ca makes the nucleus exceptionally stable and spherical. But known isotopes range from ³⁵Ca to ⁵⁷Ca, and the extreme proton- and neutron-rich ones have lifetimes measured in milliseconds. Precise mass measurements are important for understanding how binding energies vary far away from the nuclide chart’s region of stability. Gamma spectroscopy of ⁵²Ca has hinted at a new magic number (32 neutrons) emerging in the calcium sequence. But there were no adequate mass measurements beyond ⁵²Ca to determine the variation of binding energies. Now a collaboration at CERN’s ISOLDE radioactive ion-beam facility has reported the first precision measurements of the masses of ⁵³Ca and ⁵⁴Ca. Their short lifetimes and minuscule production rates required the team to develop a multireflection time-of-flight detector (shown here) for mass spectrometry. With hundreds of reflections creating a very long flight path, the spectrometer determines the binding energies of the two exotic species with a precision of 0.01%. Showing an abrupt drop in binding strength beyond ⁵²Ca, the results verify 32 as a new magic number in that excess-neutron region. And they agree well with the team’s innovative theoretical predictions, which invoke two- and three-nucleon interactions based only on fundamental particle theory. The three-nucleon interaction turns out to be essential in reproducing the new results. (F. Wienholtz et al., Nature 498, 346, 2013 .)—Bertram Schwarzschild