Take a very elongated nucleus of strontium-98, remove two protons... and it becomes a sphere. Although this abrupt shape transition, observed for the first time at GANIL, remains unexplained, a deeper understanding of this phenomenon will allow the physicists to know a little more about the complex organization of nucleons in the nucleus.
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.
Contacts: Antoine Lemasson (Ganil), Jérémie Dudouet (IPNL) Gilbert Duchêne (IPHC)
9636Kr60 –Low-Z Boundary of the Island of Deformation at N=60, J. Dudouet et al., Phys. Rev. Lett. 118, 162501