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Nuclear field theory (Bohr-Mottelson, II). Equivalent to RPA with dipole-dipole interaction. Shifts of the lines are smaller than the uncertainty of the 2 qp energies. M1?. E1. B(E1)[arb. units]. screening. enhancement. Transitional (X(5)). deformed. 88. 90. Two-neutron

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## Nuclear field theory (Bohr-Mottelson, II)

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**Nuclear field theory (Bohr-Mottelson, II)**Equivalent to RPA with dipole-dipole interaction**Shifts of the lines are smaller than the uncertainty of the**2 qp energies.**M1?**E1**B(E1)[arb. units]**screening enhancement**Transitional (X(5))**deformed**88**90**Two-neutron**transfer crossections**Shape coexistence analysis by Zielinska et al.**Collective degrees of freedom by IBA Good fit to energies, reasonable fit to E2 matrixelements Strong mixing**2**shell correction, zero pairing 1 3 4**1 and 2**merge into a state with 56 neutrons protons 42**3,4**3 1 4**Two-state mixing**Origin of mixing: pair scattering Size of the coupling: few 100 keV strong mixing E1 operator cannot move 2 particles**Two-level mixing model**strong branching very different M very different mixing**i no mixing**experiment 3:1 Zielinska et al 1.33:1**nb**pb b nb Pair scattering blocked: b partially blocked: pb not blocked: nb**2qp octupole**collective octupole Collective dipole Sensitive to deformation yes no yes no no Relation between the E1 matrix elements B(E1,low):B(E1,8 MeV) experiment: 1:10 2qp- calculations: <1:1000 Coupling to other degrees of freedom Soft octupole mode**Relation between M1 matrix elements**M1 matrix elements are weakly deformation dependent**2**1 3 4**Conclusions**Origin of low-lying dipole strength? M1 or E1 by coupling to octupole Dipole branch to second 0+ new evidence for shape coexistence Occurrence for only few states is a consequence of Pauli Principle. Information about the microstructure. Derivation of mixing amplitudes problematic

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