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Researchers at the University of Tokyo and their collaborators have measured for the first time the atomic masses of very neutron-rich calcium isotopes having more neutrons than the newly proposed magic number 34, and confirmed conclusively the magic nature at the neutron number 34 through the systematic analysis of those masses.

The atomic nucleus consists of protons and neutrons and possesses a stronger than normal stability for particular numbers of those particles. These numbers are called the “magic numbers” of nuclei. The values of the magic numbers are determined by the nature of the nuclear forces between protons, neutrons, or proton and neutron. For unstable nuclei that exist for a very short time only and decay into more stable nuclei, it has been observed that the conventional magic numbers disappear or that new magic numbers emerge. For the neutron number 34, circumstantial evidence of its magic nature was reported; however, the atomic mass measurements of calcium isotopes with more than 34 neutrons were needed as conclusive evidence.

In the current study, the research group measured the atomic masses of calcium-55, 56 and 57, which contain more neutrons than calcium-54 (20 protons, 34 neutrons), and confirmed conclusively that 34 is a magic neutron number. The magicity of the number 34 was verified by the two-neutron separation energies, which are the energies that separate two neutrons from a nucleus. The measurements revealed the magic nature at the neutron number 34 with a large decrease of the separation energy for neutron numbers larger than 34.

The newly measured masses which quantify toughness of the emergent magic number 34 lead to improve our understanding of the nuclear structure of unstable, as well as stable, nuclei. This information will also help to better understand the nucleosyntheses in supernovae, neutron-star mergers, and the nuclear structure of superheavy elements.

This study was conducted in collaboration with RIKEN Nishina center, RCNP Osaka University, Tokyo City University, Kyoto University, Kyushu University, Rikkyo University, Tokyo University of Science, University of Notre Dame, and Michigan State University.

Figure: Two-neutron separation energy of neutron-rich calcium isotopes. Two-neutron separation energy is constant for nuclei up to the conventional magic numbers, but the two-neutron separation energy of nuclei with neutron numbers larger than the magic numbers suddenly decreases. In calcium isotopes, the neutron numbers 20, 28 and 32 are known as magic numbers. The present study revealed that 34 also becomes a magic number in calcium isotopes.

From the website of The School of Science, The University of Tokyo

To learn more about this research, please view the full research report entitled "Magic Nature of Neutrons in 54Ca: First Mass Measurements of 55-57Ca" at this page of Physical Review Letters.

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