Teams at CERN’s ISOLDE facility and at RIKEN in Japan have found evidence that an exotic isotope of the metallic element nickel (78Ni) is doubly magic, opening a new vista on an important region of the nuclear-stability chart.
Like electrons in an atom, protons and neutrons in a nucleus have a penchant for configurations that offer extra stability, called magic numbers. Nuclei that have magic numbers of both protons and neutrons are of particular interest for understanding how nucleons bind together. Examples are 16O, containing eight protons and eight neutrons, and 40Ca (20 protons and 20 neutrons), both of which are stable nuclides.
One of the main efforts in modern nuclear physics is to create systems at the extremes of nuclear stability to test whether these magic numbers, and the nuclear shell model from which they derive, are still valid. Two usual suspects are 132Sn (with a half-life of 40 s) and 78Ni (0.12 s). Sn (tin) is the element with the highest number of stable isotopes (10), attesting to the magic nature of its 50 protons.
The next magic number is 82, corresponding to the number of neutrons in 132Sn. Nickel has a magic number of 28 protons but the recipe for adding the magic 50 neutrons to make 78Ni has proven challenging for today’s radioactive beam factories. CERN’s ISOLDE facility has now got very close, taking researchers to the precipice via nickel’s nuclear neighbour 79Cu containing 50 neutrons and 29 protons.
Andree Welker of TU Dresden and collaborators used ISOLDE’s precision mass spectrometer ISOLTRAP to determine the masses and thus binding energies of the neutron-rich copper isotope 79Cu, revealing that this next-door neighbour of 78Ni also exhibits a binding-energy enhancement. To probe the enhancement, Ruben de Groote of KU Leuven and collaborators used another setup at ISOLDE called CRIS to measure the electromagnetic moments of the odd-N neighbour 78Cu, providing detailed information about the underlying wave functions. Both the ISOLTRAP masses and the CRIS moments were compared with large-scale shell-model calculations involving the many relevant orbitals. Both are in excellent agreement with the ISOLDE results, suggesting that the predictions for the neighbouring 78Ni can be taken with great confidence.
An independent study of 79Cu carried out by Louis Olivier at the IN2P3–CNRS in France and colleagues based on a totally different technique has reached the same conclusion. Using in-beam gamma-ray spectroscopy of 79Cu at the Radioactive Isotope Beam Factory at RIKEN in Japan, the team produced 79Cu via proton “knockout” reactions in a 270 MeV beam of 80Zn. No significant knockout was observed in the relevant energy region, showing that the 79Cu nucleus can be described in terms of a valence proton outside a 78Ni core and affirming nickel’s doubly magic character.