Three shapes in one nucleus: the elusive state in Sulfur 43 found

Atomic nuclei are chameleons of the quantum world, in that they often assume different shapes. When nuclear states that exhibit very different shapes lie close in energy, the nucleus is said to exhibit shape coexistence.  Each of coexisting states can have a series of levels built on top – a band structure – in which the states have common properties. The excitations within such a band often involve collective vibrational or rotational motion. The excited states decay via gamma-ray emission to the lower-lying states and eventually to the ground state of the nucleus.

One of the most interesting candidates for shape coexistence is the neutron-rich nucleus sulfur-43 for which several theoretical studies have predicted triple shape coexistence, which involves prolate (a), oblate (b) and triaxial (c) shapes. Two such coexisting structures had in fact been found in previous experiments.

Based on the recent lifetime and high-resolution gamma-ray detection measurements performed with the state-of-the-art detection system GRETINA at NSCL, the third deformed structure has been firmly identified. Since it lies at almost the same excitation energy as another state, its discovery had to wait for the discussed sophisticated measurement that was capable of telling the two states apart by their different lifetimes.

This work is published in Phys. Rev. Lett. These results show that an intricate interplay of prolate, oblate, non-axial symmetric deformation occurs even in a light-mass nuclear system such as sulfur-43.