Abstract
The excitation of the Pygmy Dipole Resonance (PDR) in the 68Ni nucleus, above the neutron emission threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a 68Ni beam, obtained by the fragmentation of a 70Zn primary beam at INFN-LNS, impinged on a 12C target. The γ-ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The 68Ni isotope as well as other heavy ion fragments were detected using the FARCOS array. The population of the PDR was evidenced by comparing the detected γ-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68Ni channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative γ-ray angular distribution. The measured γ-ray angular distribution confirms the E1 character of the transition. The γ decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error.
Highlights
Much relevance has been given to the collective states in neutron-rich nuclei
We report on the first observation of the Pygmy Dipole Resonance (PDR), above the neutron emission threshold, in the 68Ni nucleus, using an isoscalar probe, obtained in an experiment carried out at INFNLNS of Catania
In summary we have observed, for the first time, the γ -ray decay of the pygmy resonance populated by an isoscalar probe in the 68Ni
Summary
Much relevance has been given to the collective states in neutron-rich nuclei. In a pioneering work [4], it has been shown that, as soon as the number of neutrons increases, the isovector and isoscalar dipole responses show a small bump at low energy in the strength distribution, which is well separated from the well known peak of the Giant Dipole Resonance (GDR). The comparison between the excitation of these low dipole states induced by either alpha particles, 17O nuclei or real photons reveals a particular property which is known as “isospin splitting” [2,3,17,18,19,20,21,22,23,24,25]: only the low energy region of the PDR is populated by both isoscalar and isovector probes, while the higher part is excited mainly via electromagnetic interaction. This is a kind of standard procedure when one has no experimental information about the elastic scattering cross section
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