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Title	Pseudospin Symmetry and Microscopic Origin of Shape Coexistence in the $^{78}$Ni Region: A Hint from Lifetime Measurements
Author(s)	Delafosse, C (Orsay, IPN) ; Verney, D (Orsay, IPN) ; Marević, P (Orsay, IPN ; CEA DAM) ; Gottardo, A (Orsay, IPN) ; Michelagnoli, C (GANIL) ; Lemasson, A (GANIL) ; Goasduff, A (INFN, Legnaro) ; Ljungvall, J (CSNSM, Orsay) ; Clément, E (GANIL) ; Korichi, A (CSNSM, Orsay) ; De Angelis, G (INFN, Legnaro) ; Andreoiu, C (Simon Fraser U., Dept. Chem.) ; Babo, M (GANIL ; Orsay, IPN) ; Boso, A (Padua U. ; INFN, Padua) ; Didierjean, F (Strasbourg, IPHC) ; Dudouet, J (Lyon, IPN) ; Franchoo, S (Orsay, IPN) ; Gadea, A (Valencia U., IFIC) ; Georgiev, G (CSNSM, Orsay) ; Ibrahim, F (Orsay, IPN) ; Jacquot, B (GANIL) ; Konstantinopoulos, T (CSNSM, Orsay) ; Lenzi, S  M (Padua U. ; INFN, Padua) ; Maquart, G (Lyon, IPN) ; Matea, I (Orsay, IPN) ; Mengoni, D (Padua U. ; INFN, Padua) ; Napoli, D  R (INFN, Legnaro) ; Nikšić, T (Zagreb U., Phys. Dept.) ; Olivier, L (Orsay, IPN) ; Pérez-Vidal, R  M (Valencia U., IFIC) ; Portail, C (Orsay, IPN) ; Recchia, F (Padua U. ; INFN, Padua) ; Redon, N (Lyon, IPN) ; Siciliano, M (INFN, Legnaro) ; Stefan, I (Orsay, IPN) ; Stezowski, O (Lyon, IPN) ; Vretenar, D (Zagreb U., Phys. Dept.) ; Zielinska, M (IRFU, Saclay) ; Barrientos, D (CERN) ; Benzoni, G (INFN, Milan) ; Birkenbach, B (Cologne U.) ; Boston, A  J (Liverpool U.) ; Boston, H  C (Liverpool U.) ; Cederwall, B (Royal Inst. Tech., Stockholm) ; Charles, L (Strasbourg, IPHC) ; Ciemala, M (Cracow, INP) ; Collado, J (Valencia U., Dept. Ing. Electronica) ; Cullen, D  M (U. Manchester (main)) ; Désesquelles, P (CSNSM, Orsay) ; de France, G (GANIL) ; Domingo-Pardo, C (Valencia U., IFIC) ; Eberth, J (Cologne U.) ; González, V (Valencia U., Dept. Ing. Electronica) ; Harkness-Brennan, L  J (Liverpool U.) ; Hess, H (Cologne U.) ; Judson, D  S (Liverpool U.) ; Jungclaus, A (CSIC, Madrid) ; Korten, W (IRFU, Saclay) ; Lefevre, A (GANIL) ; Legruel, F (GANIL) ; Menegazzo, R (Padua U. ; INFN, Padua) ; Million, B (INFN, Milan) ; Nyberg, J (Uppsala U. (main)) ; Quintana, B (U. Salamanca (main)) ; Ralet, D (CSNSM, Orsay) ; Reiter, P (Cologne U.) ; Saillant, F (GANIL) ; Sanchis, E (Valencia U., Dept. Ing. Electronica) ; Theisen, Ch (IRFU, Saclay) ; Valiente Dobon, J  J (INFN, Legnaro)
Publication	2018
Number of pages	7
In:	Phys. Rev. Lett. 121 (2018) 192502
DOI	10.1103/PhysRevLett.121.192502
Subject category	Nuclear Physics - Experiment
Abstract	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.
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