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

C Delafosse,F Legruel,F Ibrahim,F Didierjean,M Siciliano,A Goasduff,C Michelagnoli,V González,F Recchia,J Collado,J Ljungvall,D Ralet,Ch Theisen,D Mengoni,A Boso,J Dudouet,A Gottardo,P Reiter,A Jungclaus,B Jacquot,S Franchoo,A Lemasson,H Hess,J Eberth,P Désesquelles,D Verney,L Charles,L Olivier,C Andreoiu,G De France,A Gadea,R Menegazzo,C Portail,A Korichi,J Nyberg,B Cederwall,D Barrientos,G De Angelis,G Benzoni,I Stefan,M Babo,B Quintana,E Clément,T Konstantinopoulos,F Saillant,N Redon,B Birkenbach,B Million,D S Judson ,M Ciemała ,A Lefèvre ,Petar Marević ,D R Napoli ,E Sanchís ,D M Cullen ,L J Harkness-Brennan ,Tamara Nikšić ,I Matéa ,Guillaume Maquart ,C Domingo‐Pardo ,O Stézowski ,M Zielińska ,Dario Vretenar ,Г Георгиев ,H C Boston ,R M Pérez-Vidal ,A J Boston ,W Körten ,S M Lenzi ,J J Valiente Dobón

doi:10.1103/physrevlett.121.192502

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^{+}→0^{+}) and B(E2;4^{+}→2^{+}) were obtained for the first time for the hard-to-reach ^{84}Ge. While the B(E2;2^{+}→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≲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 ρ field in shell-structure evolution.

Highlights

While the BðE2; 2þ → 0þÞ values of 88Kr, 86Se saturate the maximum quadrupole collectivity offered by the natural valence (3s, 2d, 1g7=2, 1h11=2) space of an inert 78Ni core, the value obtained for 84Ge largely exceeds it, suggesting that shape coexistence phenomena, previously reported at N ≲ 49, extend beyond
The onset of collectivity at Z 1⁄4 32 is understood as due to a pseudo-SU(3) organization of the proton single-particle sequence reflecting a clear manifestation of pseudospin symmetry
Thanks to the superior resolving power of AGATA and precise gamma-emission angle determination offered by the advent of gamma tracking, the recoil distance Doppler shift (RDDS) lifetime measurement of excited states in light N 1⁄4 52 isotones could be extended down to the exotic 84Ge

Summary

Published by the American Physical Society

The latter provides 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 ρ field in shell-structure evolution. Nucleons experience two almost equivalently large potentials, the short-range repulsive vector potential V ≈ 350 MeV and medium-range attractive scalar potential S ≈ −400 MeV, leading to Δ ≈ 750 MeV of the order of magnitude of the nucleon mass (m ≈ 940 MeV) itself, explaining η ≈ 1 and the large nuclear SO coupling The counterpart of this QCD-emerging quasiequality S ≈ −V is that the net binding potential Σ 1⁄4 V þ S is inherently small, leading to the far-reaching consequence that pseudospin symmetry (PSS) is approximately realized in nuclei [3,4,5]. A total of ≈1.5 × 107 fissionfragment events were identified out of which the 88Kr, 192502-2

This work

Our data suggest for the first time a shape transition from