$^{96}_{36}$Kr$_{60}$–Low-Z Boundary of the Island of Deformation at N =60

J. Dudouet (1) , A. Lemasson (2) , G. Duchêne (3) , M. Rejmund (2) , E. Clément (2) , C. Michelagnoli (2) , F. Didierjean (3) , A. Korichi (2, 4) , G. Maquart (1) , O. Stezowski (1) , C. Lizarazo , R.M. Pérez-Vidal , C. Andreoiu , G. De Angelis , A. Astier (4) , C. Delafosse (5) , I. Deloncle (4) , Z. Dombradi , G. de France (2) , A. Gadea , A. Gottardo , B. Jacquot (2) , P. Jones , T. Konstantinopoulos (4) , I. Kuti , F. Le Blanc (3) , S.M. Lenzi , G. Li , Radomira Lozeva (3, 4) , B. Million , D.R. Napoli , A. Navin (2) , C.M. Petrache (4) , N. Pietralla , D. Ralet (4) , M. Ramdhane (6) , N. Redon (1) , C. Schmitt (2) , D. Sohler , D. Verney (5) , D. Barrientos , B. Birkenbach , I. Burrows , L. Charles (3) , J. Collado , D.M. Cullen , P. Désesquelles (4) , C. Domingo Pardo , V. González , L. Harkness-Brennan , H. Hess , D.S. Judson , M. Karolak (7) , W. Korten (7) , M. Labiche , J. Ljungvall (4) , R. Menegazzo , D. Mengoni , A. Pullia , F. Recchia , P. Reiter , M.D. Salsac (7) , E. Sanchis , Ch. Theisen (7) , J.J. Valiente-Dobon , M. Zielinska (7)

1 IPNL - Institut de Physique Nucléaire de Lyon
2 GANIL - Grand Accélérateur National d'Ions Lourds
3 IPHC - Institut Pluridisciplinaire Hubert Curien
4 CSNSM - Centre de Sciences Nucléaires et de Sciences de la Matière
5 IPNO - Institut de Physique Nucléaire d'Orsay
6 LPSC - Laboratoire de Physique Subatomique et de Cosmologie
7 IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers

Prompt γ-ray spectroscopy of the neutron-rich $^{96}$Kr, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the VAMOS++ spectrometer. A second excited state, assigned to $J^π$ = $4^+$, is observed for the first time, and a previously reported level energy of the first 2+ excited state is confirmed. The measured energy ratio R4/2 = E($4^+$)/E($2^+$) = 2.12(1) indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N = 60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z = 36, of similar amplitude to that observed at N = 60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g9/2, d5/2, and s1/2 orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R4/2 as a function of the neutron number up to N = 60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N = 60 isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at N = 60 and Z = 36.

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

J. Dudouet, A. Lemasson, G. Duchêne, M. Rejmund, E. Clément, et al.. $^{96}_{36}$Kr$_{60}$–Low-Z Boundary of the Island of Deformation at N =60. Physical Review Letters, 2017, 118, pp.165201. ⟨10.1103/PhysRevLett.118.162501⟩. ⟨in2p3-01518704⟩