The pygmy dipole resonance has been studied in the proton-magic nucleus Sn124 with the ($\alpha${}, $\alpha${}$'${}$\gamma${}) coincidence method at E$\alpha${}=136\,\,MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in ($\alpha${}, $\alpha${}$'${}$\gamma${}) as well as in ($\gamma${}, $\gamma${}$'${}) reactions and a group of states at higher energies which is only excited in ($\gamma${}, $\gamma${}$'${}) reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.
The absolute cross section of the () reaction has been measured at to 8.0 MeV with an overall accuracy of 4%. The precision is needed to subtract reliably a background in the observation of geo-neutrinos, e.g. in the KamLAND detector.
1 February 1999 in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
The elastic scattering was studied at near-barrier energies, namely, 8, 8.5, 9, 10, 11, 13, 15, and , with the aim to map the real and imaginary part of the optical potential and therefore probe the threshold anomaly. Angular distributions were measured over a wide angular range of ( to ) for the lower energies and of ( to ) for the higher energies. The present data, together with previous ones on heavier targets ( and ) at near barrier energies, were analyzed by using optical potentials obtained in a double-folding framework. The results were compared with previous measurements of on the same targets. It was found that a striking difference occurs between the imaginary potentials of and , which, respectively, present an increasing and decreasing behavior approaching the barrier from higher to lower energies. On the other hand, this energy variation is not fully reflected to the real part of the potential, as it is described by dispersion relations. The strength of the real potential remains almost constant with a weak declining and uprising trend for the and , respectively. For a better understanding of our results, continuum-discretized-coupled-channel calculations were also performed and are discussed.