The shape of the nucleus inside an atom is rarely a perfect sphere. Investigating nuclear shape is essential to understand internal structure of nuclei, which impacts nuclear reactions and radioactive decay. This includes the production of elements in stars and the nuclear fission process, important for atomic energy application. The behavior of nuclei that have almost the same number of protons and neutrons, like ⁷⁰Se with 34 protons and 36 neutrons, is important for nuclear theory as the symmetry of the nuclear force (acting the same on protons and neutrons) is still being debated. In this work we study the shape of selenium nuclei by observing the way they spin. Because of quantum mechanics, nuclei can only spin at discreet speeds. After using ion beams to produce selenium isotopes with a lot of spinning energy, we watch them slow down by releasing energy as gamma-rays and electrons. With advanced detectors we measure the exact energies of these particle, which correspond to the allowed speed steps of that nucleus. In the case of ⁷⁰Se we could determine the pattern of allowed steps correspond to a shape with three different length axes, as shown in Fig. 1.
 While a perfectly spherical nucleus, like ²⁰⁸Pb, looks circular from any viewpoint (Fig. 1a), a typical deformed nucleus, like ²³⁸U, is stretched, having one longer axis, so it is only circular when viewed end-on (Fig. 1b). In ⁷⁰Se, no axes are equal and it does not have a circular profile whichever direction it is viewed from (Fig. 1c). Such an entirely non-spherical object tumbles in a particular way when spun, which we determined from the particle energies.

This work was supported by MEXT Leading Initiative for Excellent Young Researchers Grant Number JPMXS0320210193.