Spectral Structure of the Pygmy Dipole Resonance

Abstract

High-sensitivity studies of E1 and M1 transitions observed in the reaction 138 Bað; 0 Þ at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HIS facility. The electric dipole character of the so-called ''pygmy'' dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 ''spin-flip'' mode was observed for the first time in N ¼ 82 nuclei. DOI: 10.1103/PhysRevLett.104.072501 PACS numbers: 21.10.Hw, 21.60.Àn, 23.20.En, 24.70.+s In stable and unstable neutron-rich nuclei a resonancelike concentration of dipole strength is observed at excitation energies around the neutron-separation energy [1][2][3][4][5][6][7][8][9][10]. This clustering of strong dipole transitions has been named the pygmy dipole resonance (PDR). In hydrodynamic and collective approaches, it was suggested that an oscillation of a small portion of neutron-rich nuclear matter relative to the rest of the nucleus is responsible for the generation of pygmy resonances [11,12]. Further, in microscopic models based on the quasiparticle-random-phase approximation, relativistic (RQRPA) and nonrelativistic (QRPA), the position and the distribution of the PDR have been investigated [13][14][15][16]. From the analysis of transition densities, the unique behavior of the PDR mode is revealed, making it distinct from the well-known giant dipole resonance (GDR) [17]. The systematic studies of the PDR over isotonic and isotopic chains of nuclei indicate a correlation between the observed total BðE1Þ strength of the PDR and the neutron-to-proton ratio N=Z [5,8,14,15]. In addition, it has been suggested that the PDR is independent of the type of nucleon excess (neutron or proton) [13,15].The existence of the PDR mode near the neutron threshold has also important astrophysical implications. For example, the reaction rate of the (, n) and (n, ) reactions in explosive nucleosynthesis of certain neutron deficient heavy nuclei may be significantly enhanced by the PDR [18]. Furthermore, for very neutron-rich exotic nuclei, the PDR is an important topic of study at the new generation of radioactive ion beam facilities [19].In many cases the interpretation of the PDR excitation is based on the assumption of negative parity for the majority of the J ¼ 1 states. However, there has not been a systematic experimental verification that all the dipole states observed in the entire PDR region are indeed 1 À states. The parity was measured directly in certain energy intervals, e.g., off-axis bremsstrahlung or Compton polarimetry [20]. The advantage of using 100% linearly polarized photon beams for parity identification has been recently demonstrated [21][22][23][24], which opens new opportunities for unveiling the character of the nuclear dipole response.On the other hand, in heavy mass nuclei there should be M1 strength located in the same excitation energy region as the PDR, i.e., in the low-energy tail of the GDR [17]. A major ex...

Decay pattern of the Pygmy Dipole Resonance in130Te

“…To extract the total photoabsorption cross section σ tot = σ elast + σ inelast , the contribution of photo-excited states which directly decay to the ground state (σ elast ) and from cascade transitions (σ inelast ) have to be determined, following the idea presented in Refs. [8][9][10]. Even though the incoming mono-energetic photon beam exclusively excites states in a narrow energy range (∼ 10 2 keV), the decays of the first few 2 + excited states were observed.…”

“…In addition, the mono-energetic character of the HI γS photons allows for investigating the average decay properties of the dipole-excited states as a function of the excitation energy [8]. To extract the total photoabsorption cross section σ tot = σ elast + σ inelast , the contribution of photo-excited states which directly decay to the ground state (σ elast ) and from cascade transitions (σ inelast ) have to be determined, following the idea presented in Refs.…”

The high-efficiency spectroscopy setup at

“…see Refs. [19][20][21]) proving the excellent performance of the HIγS beam. This setup is extended by additional detectors in order to provide sufficient efficiency for γ-γ coincidence experiments.…”

Decay pattern of the Pygmy Dipole Resonance in140Ce

“…So far, measurements have been restricted mostly to ground-state transitions. Still, for a complete understanding of the decay pattern, cascading transitions involving low-lying excited states have to be investigated as it has been done in indirect measurements using a quasi-monochromatic photon beam [7,[15][16][17][18]. However, the branching ratios for primary transitions are often too small to be observed directly in standard NRF experiments due to the large amount of low-energy background and the corresponding sensitivity limit.…”

“…The properties of this phenomenon have been studied in stable (see e.g. [3][4][5][6][7][8][9][10][11]) and in a few radioactive nuclei [12][13][14], in particular in the Z=50 and N=82 mass region. The underlying nature of the PDR is still unclear, and strong experimental and theoretical effort is put into its investigation.…”