Investigation of low-lying isovector octupole excitations in spherical nuclei

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Aim of the project is to conduct experiments with neutron-induced reactions to determine the spectroscopic properties of recently proposed first candidates for low-lying isovector octupole excitations in spherical nuclei. Due to historical reasons the analogue low-lying isovector excitations in the quadrupole sector are commonly referred to as mixed-symmetry states. For a firm assignment of the nature of the states proposed as candidates, it is mandatory to determine the B(M1) value of the transition connecting the candidate with the first excited 3- state, the isoscalar octupole phonon, with a high level of accuracy. Therefore, it is necessary to measure for this transition the multipole mixing ratio, the relative intensity and the radiation character which has been done already in the project with the EXILL spectrometer. In addition, for the absolute B(M1) value the lifetime is needed which will be measured with the GAMS spectrometer at ILL as it is suited best for the expected lifetime in the femtosecond range. Establishing this new kind of low-lying collective mode in spherical nuclei offers several theoretical models, such as, for example, the Quasiparticle Phonon Model (QPM) or modern beyond mean-field models, the opportunity to fix the coupling constant of theisovector part of proton-neutron residual interaction in the octupole part of the multipole expansion of the effective nuclear force.