N = 28: Shell and Shapes

Abstract : The Segré chart, from hydrogen (Z = 1) to the heaviest elements (Z = 118 observed so far), is usually represented with a particular focus on magic numbers. Within a naive shell model picture, magic nuclei are associated with particle configurations where orbits are fully occupied and for which a large energy gap exists at the Fermi surface. For these reasons correlations are hindered in magic nuclei that are characterized by (i) an high excitation energy of their first excited state, (ii) a low transition probability from the ground state to the first excited state, and (iii) a spherical shape. Appearing for the nucleon numbers 2, 8, 20, 28, 50, 82, and 126, magic numbers are the pillars supporting the chart of nuclei usually represented from the proton to the neutron drip lines. However, this picture originating from our knowledge on stable nuclei has to be refined: the spherical magic numbers are now known not to be a valid concept from drip line to drip line. This experimental fact has been first established in the N = 20 neutron rich nucleus, 32Mg, in the early 1980s [1].