Abstract
The magic neutron number N = 20 arises after 2 quanta of the harmonic oscillator and persists, albeit weakened, after considering a more realistic potential including the spin-orbit interaction. The N = 20 shell gap can be found between the 0d3/2 and 0f7/2 orbitals.
It has become clear that magic numbers are not a fixed property throughout the nuclear chart, but can evolve as a function of Z and N due to (residual) proton-neutron interactions, which can lead to a quenching of certain magic numbers and the appearance of others. The archetypical example of these very rapid changes in nuclear structure is the so-called 'island of inversion', comprising the very neutron-rich nuclei near N ∼ 21 and Z ∼ 11.
Historically it was found that these nuclei are more bound than expected and subsequent experimental observations pointed toward an abrupt onset of collectivity with deformed ground states. The most striking observations are highly collective E2 transitions between low-lying states, in particular the low energy of the first Jπ = 2+ state of 32Mg and the very large B(E2; 0+gs → 2+1). These properties can best be explained by the dominance of intruder configurations, i.e. configurations outside the sd-shell.
The interest in this region of the nuclear chart has grown enormously over time, due to the ever better accessibility of these nuclei at RNB facilities. I will review the current experimental understanding of these nuclei on a few selected examples and will give an outlook on the experimental progress to be expected in the near future.
Furthermore an outlook and future plans concerning in-beam γ-ray spectroscopy at RIKEN is given.
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