If one googles "island of inversion" there are about a thousand results almost all of which refer to a group of neutron-rich nuclei in a region of the nuclear chart centered around 31Na (11 protons and 20 neutrons). The name goes back to a paper by Warburton, Becker and Brown [1] where the unusual features for nuclei in this mass region were studied in the nuclear shell model. The shell-model single-particle energies for the neutrons near the Fermi surface in this region are shown in the upper left-hand side of the figure. Two configurations are shown for 20 neutrons; the closed-shell configuration (A), and configuration (B) consisting of two neutrons excited from the 0d3/2 and 1s1/2 orbitals into the 0f7/2 and 1p3/2 orbitals across the N=20 shell gap. Near the shell gaps, configurations of type (B) usually appear as excited states. The energy of these states is lower than twice the shell gap due to the pairing correlations between the particles and between the holes.

The unusual feature of nuclei inside the island of inversion (shown by the red circle above "20" in the figure) is that the total (correlated) energy of configuration (B) comes below that of (A). The change is sudden, with (B) forming an excited state in 34Si (14 protons) and then becoming the ground state for 32Mg (12 protons). The reason for this sudden change is due to two factors; a gradual reduction in the spherical N=20 shell gap as one approaches the neutron drip line at fluorine (9 protons), and the configuration for the protons suddenly changing from "closed shell" in 34Si to "open shell" in 32Mg. leading to stronger proton-neutron correlations and deformation.

Many experiments have studied the states in the island of inversion with configurations similar to (B). In a recent experiment [2] two neutrons are transferred to 30Mg with 18 neutrons leaving 32Mg and a proton which is detected. The shell-model configuration for N=18 is shown in the figure. When two neutrons are added to make a Jp=0+ final state they can go into the 0d3/2,1s1/2 orbitals making state (A) or into the 0f7/2,1p3/2 orbitals making state (B). Two 0+ states were observed, the ground state and an excited state at 1.06 MeV. The energy of the excited state which is presumed to correspond to configuration (A) is lower than any of the theoretical predictions discussed in this paper. The simplest estimate based on the theoretical extrapolation for the energy of (A) together with the measured energy of the ground state (B) taken from Fig. 5 in [3] is 2.5 MeV. The cross sections for the population of these two states give indirect information on the details of their structure. It is inferred that there is mixing between (A) and (B), and that the 1p3/2 component of (B) is larger than expected. This may be a signal that the single-particle energy of the 1p3/2 orbital which is about 1.5 MeV above that of 0f7/2 orbital in 34Si is dropping relative to 0f7/2 as one approaches the neutron drip line. This is a feature of loosely bound orbitals with l=1 (see Fig. 4 in [4] important for understanding why the neutron drip line suddenly changes from 16 neutrons for the oxygen isotopes (8 protons) to greater than 22 neutrons for the fluorine isotopes (9 protons).

The island of inversion is actually part of an archipelago of islands related to the breaking of magic numbers, starting with an islet near 11Li (N=8) and extending to a larger island centered around 64Cr (N=40); the red circles in the figure. All of these islands are in neutron-rich regions of the nuclear chart where traditional shell gaps are reduced relative to stable nuclei, leading to magic numbers that change rapidly with proton number. Experimental and theoretical advances for the properties of these neutron-rich nuclei are crucial for the nuclear many-body problem as well as for understanding how nuclei are made in the astrophysical processes of rapid neutron capture. This highlight is based upon a Physics Viewpoint article [5].


[1] E. K. Warburton, J. A. Becker and B. A. Brown, Phys. Rev. C41, 1147 (1990). link to paper

[2] K. Wimmer et al., Phys. Rev. Lett. 105, 252501 (2010). link to journal

[3] B. A. Brown and W. A. Richter, Phys. Rev. C 74, 034315 (2006). link to journal

[4] I. Hamamoto, Phys. Rev. C 76, 054319 (2007). link to journal

[5] B. A. Brown, Islands of Insight in the Nuclear Chart, Physics 3, 104 (2010). link to paper