Nuclear Physics Division

Deformation in atomic nuclei has become a familiar topic in nuclear structure physics and axial- and reflection-symmetric quadrupole shapes are common across the chart of nuclides. There exists strong circumstantial evidence, however, that some combinations of protons and neutrons will give rise to octupole deformation, or a reflection-asymmetric pear-like shape.

It can be expected that the actinides will possess the strongest octupole collectivity, leading to the largest array of experimental evidence so far. But there's a problem‚ the nuclei in this region of the chart are unstable. This means that direct information on the electric-octupole transitions connecting excited states in these nuclei have not been accessible. So far, only ²²⁶Ra with its relatively long lifetime of 1600 years has had its octupole collectivity quantified.

With the advent of radioactive ion beams and in particular, the ground-breaking ability to post-accelerate the heavy elements radon and radium at REX-ISOLDE, we have recently overcome the challenges limiting our knowledge in this region. Coulomb excitation was successfully performed on ²²⁰Rn and ²²⁴Ra and the E1, E2 and E3 matrix elements connecting the lowest-energy states have been determined.

In this seminar I will briefly review octupole deformation in nuclei and discuss the results of these pioneering experiments as well as outlining plans for the future. The results are not only significant for nuclear structure, but also on the search for atomic electric-dipole moments (EDMs) which are predicted to be enhanced by octupole deformation. The consequence of our results on on-going experiments looking for EDMs to test the Standard Model will also be discussed.

http://www.nature.com/nature/journal/v497/n7448/full/nature12073.html