Gamma decay of the possible 1- two-phonon state in 140Ce excited via inelastic scattering of 17O

The γ decay from the low-lying dipole states of 140Ce excited via inelastic scattering of 17O at bombarding energy of 340 MeV was measured using the high resolution AGATA-Demonstrator array in coincidence with scattered ions detected in two segmented ΔE-E silicon detectors of the TRACE array. Particular attention is here given to the decay of the first 1- state at 3643 keV which is considered to be of two-phonon character. The gamma-gamma coincidence method was applied to select desired decay branch. No direct decay from this state was observed to 2+ and 3- phonon states which would be the proof of the pure harmonic coupling. The comparison between experimentally obtained differential cross sections and analysis with distorted wave Born approximation (DWBA) allowed to conclude that the first 1- state has a different nature than higher-lying pygmy dipole states. This was possible using the form factor obtained by folding a microscopically calculated transition density.