Publication

Collectivity in “Spherical” 143Eu and 144Eu

Nuclear physics

Physical Review C, Rapid Communication, 52 (1995), Number 1.

Authors:

M. Piiparinen, A. Atac, S.J. Freeman, R. Julin, S. Juutinen, A. Lampinen, T. Lönnroth, D. Müller, J. Nyberg, G. Sletten, P. Tikkanen, S. Törmänen, A. Virtanen, C.T. Zhang, R. Wyss

Summary

The publication investigates the unexpected emergence of collective nuclear excitations in the near-spherical europium isotopes ¹⁴³Eu (Z=63, N=80) and ¹⁴⁴Eu (Z=63, N=81), located close to the N=82 neutron shell closure. Using γ-ray spectroscopy following heavy-ion-induced fusion-evaporation reactions, the authors observed low-lying quadrupole vibrational states and enhanced E2 transition strengths in both nuclei, indicative of residual collectivity despite their proximity to shell-stabilized spherical shapes.

Key findings:

  1. ¹⁴³Eu exhibits a 2⁺₁→0⁺₁ transition with a B(E2) value of ~40 W.u., significantly exceeding single-particle estimates and suggesting collective proton-neutron correlations.
  2. ¹⁴⁴Eu displays a rotational-like band built on a 5⁻ state, interpreted as a mixed-symmetry configuration involving coupled proton-particle and neutron-hole excitations.
  3. Comparisons with the Interacting Boson Model (IBM-2) reveal discrepancies, particularly in the description of ¹⁴⁴Eu, highlighting the interplay between single-particle and collective degrees of freedom near closed shells.
The study challenges assumptions about purely spherical behavior in nuclei near magic numbers, demonstrating that weak collectivity persists due to valence nucleon interactions and core-polarization effects. These results bridge the gap between vibrational and rotational models in transitional nuclei.

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