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
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
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