Two-phonon octupole excitation in ²⁰⁸Pb

Experimental evidence for identification of the 5241-keV 0⁺ state in ²⁰⁸Pb as the lowest-spin member of the 3⁻ ⊗ 3⁻ quartet is reported. This state, observed previously in neutron transfer reactions and inelastic proton scattering, has been populated with the inelastic neutron scattering reaction. Its decay to the 3⁻ octupole phonon in ²⁰⁸Pb is confirmed by γ-γ coincidence measurements, and the spin-0 assignment is substantiated by the population cross section. The cascade of two E3 transitions thus established is an expected signature of a two-phonon octupole excitation.