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Shape Coexistence in Krypton Isotopes

An international collaboration led by IRFU and Riken Nishina Center (Japan) has performed the first spectroscopy of very neutron-rich krypton nuclei 98Kr and 100Kr, using the Radioactive Isotope Beam Factory accelerator in Riken. These experiments complement the results obtained on rubidium and zirconium and deepen the knowledge on a shape transition that occurs in these three nuclei beyond 60 neutrons. 
Published on 27 July 2017

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.

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