The shape of the nucleus is a reflection of the internal organization of its nucleons (Z protons and N neutrons). It usually evolves gradually with Z or N. Yet between strontium-98 (38 protons, 60 neutrons) and krypton-96 (36 protons, 60 neutrons), nuclear physicists were surprised to observe an abrupt shape transition that their current theories fail to explain.

The scientists produced nuclei of strontium-98 and krypton-96 (among hundreds of other particles) by nuclear fission, induced by a beam of uranium-238 ions on a target of beryllium-9. They identified them through the gamma radiation they emit by "de-excitation." They observed two very different emission spectra that are characteristic of an elongated and a spherical shape, respectively.

These measurements were performed using two state-of-the-art spectrometers: AGATA (Advanced GAmma-ray Tracking Array) and VAMOS (VAriable MOde magnetic Spectrometer). The latter is a magnetic spectrometer for the identification of exotic nuclei with the greatest acceptance to date. AGATA is a European gamma spectrometer consisting of 32 germanium detectors, which currently cover π radians and will ultimately cover 4π.