Adding an extra nucleon (proton or neutron) to an atomic nucleus may be enough to trigger a full rearrangement. This could translate into a different nucleus shape, and sometimes, several shapes can even coexist at very low nucleus excitation energy. This "quantum phase transition" can then be identified by measuring the energy of the first excited levels in the nucleus.

This phenomenon is observed in krypton, strontium and zirconium isotopes, which are very neutron-rich (with up to 64 neutrons). Although Zirconium (Zr) and Strontium (Sr) display an abrupt transition in their first energy level starting at 60 neutrons, until now, there had not been experimental data on krypton isotopes with more than 60 neutrons.

With this in mind, the collaboration performed measurements on krypton ⁹⁸Kr and ¹⁰⁰Kr, two nuclei with 62 and 64 neutrons, respectively. These showed a smoother transition than in the case of Zirconium and Strontium. Moreover, this experience has highlighted a state of low excitation energy in the ⁹⁸Kr nucleus, which would correspond to a shape differing from that of the fundamental state, thus indicating the coexistence of two shapes.

For these measurements, the researchers used the "in-flight fission" of a first high-energy uranium beam ²³⁸U to produce a secondary ion beam. After magnetic separation, the ions are sent on a liquid hydrogen cryogenic target developed at IRFU. Following the reaction where a proton is pulled out of the projectile, the products were analyzed using a time projection chamber developed and produced at IRFU, and a gamma photons spectrometer composed of 186 sodium iodide oscillators, to reconstruct the energy levels of  ^98 ,100Kr.