Pseudospin Symmetry and Microscopic Origin of Shape Coexistence in the $^{78}$Ni Region: A Hint from Lifetime Measurements

C. Delafosse (1) , D. Verney (1) , P. Marević (1, 2) , A. Gottardo (1) , C. Michelagnoli (3) , A. Lemasson (3) , A. Goasduff , J. Ljungvall (4) , E. Clément (3) , A. Korichi (4) , G. de Angelis , C. Andreoiu , M. Babo (3, 1) , A. Boso , F. Didierjean (5) , J. Dudouet (6) , S. Franchoo (1) , A. Gadea , G. Georgiev (4) , F. Ibrahim (1) , B. Jacquot (3) , T. Konstantinopoulos (4) , S.M. Lenzi , G. Maquart (6) , I. Matea (1) , D. Mengoni , D.R. Napoli , T. Nikšić , L. Olivier (1) , R.M. Pérez-Vidal , C. Portail (1) , F. Recchia , N. Redon (6) , M. Siciliano , I. Stefan (1) , O. Stezowski (6) , D. Vretenar , M. Zielinska (7) , D. Barrientos , G. Benzoni , B. Birkenbach , A.J. Boston , H.C. Boston , B. Cederwall , L. Charles (5) , M. Ciemala , J. Collado , D.M. Cullen , P. Désesquelles (4) , G. de France (3) , C. Domingo-Pardo , J. Eberth , V. González , L.J. Harkness-Brennan , H. Hess , D.S. Judson , A. Jungclaus , W. Korten (7) , A. Lefevre (3) , F. Legruel (3) , R. Menegazzo , B. Million , J. Nyberg , B. Quintana , D. Ralet (4) , P. Reiter , F. Saillant (3) , E. Sanchis , Ch. Theisen (7) , J.J. Valiente Dobon

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

Lifetime measurements of excited states of the light $N=52$ isotones $^{88}$Kr, $^{86}$Se, and $^{84}$Ge have been performed, using the recoil distance Doppler shift method and VAMOS and AGATA spectrometers for particle identification and gamma spectroscopy, respectively. The reduced electric quadrupole transition probabilities $B(E2;2^+ \rightarrow 0^+)$ and $B(E2;4^+ \rightarrow 2^+)$ were obtained for the first time for the hard-to-reach $^{84}$Ge. While the $B(E2;2^+ \rightarrow 0^+)$ values of $^{88}$Kr, $^{86}$Se saturate the maximum quadrupole collectivity offered by the natural valence $(3s, 2d, 1g_{7/2}, 1h_{11/2})$ space of an inert $^{78}$Ni core, the value obtained for $^{84}$Ge largely exceeds it, suggesting that shape coexistence phenomena, previously reported at $N \lesssim 49$, extend beyond $N=50$. The onset of collectivity at $Z=32$ is understood as due to a pseudo-SU(3) organization of the proton single-particle sequence reflecting a clear manifestation of pseudospin symmetry. It is realized that the latter provides actually reliable guidance for understanding the observed proton and neutron single particle structure in the whole medium-mass region, from Ni to Sn, pointing towards the important role of the isovector-vector $\rho$ field in shell-structure evolution.

Mots clés: Nuclear Physics

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

C. Delafosse, D. Verney, P. Marević, A. Gottardo, C. Michelagnoli, et al.. Pseudospin Symmetry and Microscopic Origin of Shape Coexistence in the $^{78}$Ni Region: A Hint from Lifetime Measurements. Physical Review Letters, 2018, 121 (19), pp.192502. ⟨10.1103/PhysRevLett.121.192502⟩. ⟨hal-01922075⟩