An Interplay Between a Nuclear Halo and Deformation: Strong Electric Dipole Mode Found in 27Ne

For some phenomena, it is instructive to view the nucleus as made of proton and neutron fluids. In such a picture, the electric dipole mode can be viewed as an oscillation in which the proton and neutron fluids move against each other. In very neutron-rich nuclei, the valence neutrons are only weakly bound to remaining core nucleons and may engage in a ‘soft’ oscillation against the core when only small extra energy is supplied. Such a low-energy dipole mode had been suggested decades ago, and since then, a number of studies have been performed on halo nuclei such as ¹¹Li and ¹¹Be, where the halo neutrons move in spatially extended orbits. In particular, the strength of the soft vibration is an important quantity when assessing, in comparison to theoretical models, if the picture of a soft oscillation holds.

In a GRETINA lifetime measurement, recently published in Phys. Rev. Lett., a particularly enhanced low-energy electric dipole transition was found in the neutron-rich nucleus ²⁷Ne. This result is puzzling since one of excited states involved in the observed transition is not expected to favor the spatially extended wave function as found for lighter halo nuclei. A comparison with theoretical model, which accounts partly for the observed strength, suggests that the core of the ²⁷Ne system has to be excited or deformed. This result indicates that the dynamics of weakly-bound systems may be strongly affected by an interplay between nuclear halo and deformation effects.

Fig1: Schematic illustration of a deformed halo and its soft oscillation between a valence neutron and a deformed core