Elsevier

Nuclear Physics A

Volume 297, Issue 2, 13 March 1978, Pages 317-334
Nuclear Physics A

Fine structure in the giant resonance region and the collective dipole spin-flip excitation in 208Pb

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Abstract

Inelastic scattering of 40 and 45 MeV protons has been used to study the excitation region of 6–12 MeV in 208Pb. This region is found to be strongly structured with states of different multipolarity. Fine structure peaks between 8.5 and 10 MeV show mainly quadrupole and octupole structure. The “giant” M1 excitations observed recently are strongly excited and are described by microscopic form factors. Several strongly excited states between 6 and 8.5 MeV show angular distributions of characteristic dipole (L = 1) structure. Ground-state widths derived from our inelastic cross sections are many times larger than measured in γ-induced reactions, and this indicates a structure different from isovector dipole. These states may represent a collective dipole spin-flip excitation (L = 1, S = 1) which in a simple collective picture can be interpreted as a collective dipole oscillation of spin-up particles against spin-down particles. Two of these states, at 7.40 and 7.92 MeV, were identified in 180° electron scattering as 2 states. Derived B(M2) values from our data are in excellent agreement with the electron scattering results. The other two states at 6.26 and 8.37 MeV may represent collective 1 spin-flip excitations.

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    Present address: Institut für Kernphysik, Kernforschungsanlage Jülich, Germany.

    ††

    Present address: Institute of Experimental Physics, University of Warsaw, Warsaw, Poland.

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