Study of shell evolution and shape coexistence in neutron rich krypton isotopes
Final Report Abstract
In summary, the project started well in 2017 according to schedule. We even joined a new collaboration, which allowed for a complementary high-resolution gamma-spectroscopy measurement of the nuclei of interest. In 2018, we performed measurements at ALTO, IPN Orsay, France and at HIE-ISOLDE, CERN. From which the latter suffered from reduced 96Kr beam intensity and huge contamination of stable 96Mo beam, yielding no new nuclear physics results on the 96Kr. On the other hand, the Nuball fission data on neutron-rich Kr isotopes showed new results. More specifically we reported on prompt and delayed γ -ray spectroscopy of the neutron-rich 94Kr, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction 238U(n, f ) in combination with the nu-Ball array, a novel hybrid γ spectrometer for energy and lifetime measurements. Several new yrast and non-yrast transitions in 94Kr were observed for the first time, extending the previously known level scheme. In 94Kr we also observed a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock- Bogoliubov calculations suggests a (9−) spin and an oblate deformation for this isomer corresponding to a twoquasi-neutron state, indicating an isomeric structure very similar to the neighboring isotones 96Sr and 92Se. At the same time the rich neutron-induced fission dataset measured during the Nuball fission campaign in 2018, including our active participation, allowed to extract new experimental results on the mechanism of angular momentum generation in nuclear fission. These were published in a Nature article led by Dr. J.N.Wilson (IJCLab Orsay, France; 2021). After some years of attempting to measure complementary high-resolution gamma-ray data on the neutronrich 94-96Kr isotopes with partial success, we came back to the low-resolution SEASTAR 2015 data on 94-96Kr. With additional support from Dr. Pieter Doornenbal (RIKEN Nishina Center, Wako, Japan) on the experimental analysis, as well as collaborations with two theoretical groups, one on mapped interacting boson model calculations and one on five-dimensional collective Hamiltonian, the new results were published. Our article “γ-ray spectroscopy of low-lying yrast and non-yrast states in neutron-rich 94,95,96Kr” reported, as the name says, on gamma spectroscopy of neutronrich 94,95,96Kr isotopes measured as a part of the “Shell Evolution And Search for Two-plus energies At RIBF” (SEASTAR) campaign at the RIKEN. Excited yrast and non-yrast states were observed, and half-lives extracted via GEANT4 simulations. In 94,96Kr candidates for the first excited 3− state were identified. For 95Kr, the prompt SEASTAR data were combined with delayed spectroscopic data measured with the EURICA array to observe transitions on top of the known (7/2)+ isomer at a level energy of 195.5(3) keV. The comparison of the new experimental results with the above-mentioned theoretical approaches, both using the Gogny D1M interaction, suggests an oblate-prolate shape coexistence already in 96Kr with neutron number N=60. Even after the end of project funding we continue the studies of these and neighboring neutron-rich Kr isotopes as there are still unsolved nuclear structure questions.
Publications
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Prompt and delayed γ spectroscopy of neutron-rich Kr94 and observation of a new isomer. Physical Review C, 102(6).
Gerst, R.-B.; Blazhev, A.; Warr, N.; Wilson, J. N.; Lebois, M.; Jovančević, N.; Thisse, D.; Canavan, R.; Rudigier, M.; Étasse, D.; Adamska, E.; Adsley, P.; Algora, A.; Babo, M.; Belvedere, K.; Benito, J.; Benzoni, G.; Boso, A.; Bottoni, S. ... & Ziliani, S.
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Angular momentum generation in nuclear fission. Nature, 590(7847), 566-570.
Wilson, J. N.; Thisse, D.; Lebois, M.; Jovančević, N.; Gjestvang, D.; Canavan, R.; Rudigier, M.; Étasse, D.; Gerst, R.-B.; Gaudefroy, L.; Adamska, E.; Adsley, P.; Algora, A.; Babo, M.; Belvedere, K.; Benito, J.; Benzoni, G.; Blazhev, A.; Boso, A. ... & Ziliani, S.
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γ-ray spectroscopy of low-lying yrast and non-yrast states in neutron-rich Kr94,95,96. Physical Review C, 105(2).
Gerst, R.-B.; Blazhev, A.; Moschner, K.; Doornenbal, P.; Obertelli, A.; Nomura, K.; Ebran, J.-P.; Hilaire, S.; Libert, J.; Authelet, G.; Baba, H.; Calvet, D.; Château, F.; Chen, S.; Corsi, A.; Delbart, A.; Gheller, J.-M.; Giganon, A.; Gillibert, A. ... & Xu, Z.
