Abstract
In stable and weakly bound neutron-rich nuclei, a resonance-like concentration of dipole states has been observed for excitation energies below the neutron-separation energy. This clustering of strong dipole states has been named the Pygmy Dipole Resonance (PDR) in contrast to the Giant Dipole Resonance (GDR) that dominates the E1 response. Understanding the PDR is presently of great interest in nuclear structure and nuclear astrophysics. High-sensitivity studies of E1 and M1 transitions in closed-shell nuclei using monoenergetic and 100% linearly-polarized photon beams are presented.
Highlights
During the last years the Pigmy Dipole Resonance (PDR) has attracted a great deal of experimental and theoretical effort. This mode of excitation was first observed in neutron-capture reactions [1]. Nowadays this mode has been found as a non-statistical enhanced dipole strength below the neutron-separation energy for isotopes ranging from light nuclei (8≤Z≤40) up to lead isotopes including the transitional nuclei [2–16]
Similar conclusion was drawn for deformed nuclei where the effect of deformation on the PDR has been studied in the xenon isotopic chain [15]
There are different theoretical mechanisms explaining the amplitude and the distribution of the nuclear dipole strength of PDR observed in the experiments [11,18,33]
Summary
During the last years the Pigmy Dipole Resonance (PDR) has attracted a great deal of experimental and theoretical effort. This mode of excitation was first observed in neutron-capture reactions [1]. Nowadays this mode has been found as a non-statistical enhanced dipole strength below the neutron-separation energy for isotopes ranging from light nuclei (8≤Z≤40) up to lead isotopes including the transitional nuclei [2–16]. Its property may provide a useful constraint on the energy density functional, to identify the equation of state (EOS) of the nuclear and neutron matters [11].
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