Electric dipole strength distribution below the E1 giant resonance in N = 82 nuclei

Abstract

Abstract:In this study quasiparticle random-phase approximation with the translational invariant Hamiltonian using deformed mean field potential has been conducted to describe electric dipole excitations in 136 Xe, 138 Ba, 140 Ce, 142 Nd, 144 Sm and 146 Gd isotones. The distribution of the calculated E1 strength shows a resonance like structure at energies between 6-8 MeV exhausting up to 1% of the isovector electric dipole Energy Weighted Sum Rule and in some aspects nicely confirms the experimental data. It has been shown that the main part of E1 strength, observed below the threshold in these nuclei may be interpreted as main fragments of the Pygmy Dipole resonance. The agreement between calculated mean excitation energies as well as summed B(E1) value of the 1 − excitations and the available experimental data is quite good. The calculations indicate the presence of a few prominent positive parity 1 + States in heavy N = 82 isotones in the energy interval 6-8 MeV which shows not all dipole excitations were of electric character in this energy range.

Evidence for radiative coupling of the pygmy dipole resonance to excited states

“…The PDR includes partly a toroidal surface excitation [44,45] where the neutron skin may play a part in enhancing isoscalar-isovector mixing [46]. Theoretical calculations show that because the proton number is far from a magic number, the proton system in the ground state of 142 Nd is deformed, resulting in the PDR splitting into K=0 and K=1 parts (oscillations along and perpendicular to the proton symmetry access, respectively), with K=0 at lower energy [47,48]. This indicates that including deformation will be necessary to fully explain the properties of the PDR.…”

Comparission of E1 Response of 154Sm and 155Sm in the Pygmy Dipole Resonance (PDR) Region

“…The wave function of the odd mass deformed nuclei consists of a sum of single-quasiparticle and quasiparticle phonon terms (6) To obtain the K  excitation energies of electric dipole resonance for odd-mass nuclei, one has to solve the secular equation following secular equation.…”

“…The theoratical tool used in this paper based on selection of the suitable separable effective forces to restore the broken translational and Galilean invariances of the QPNM and QRPA hamiltonians for the description of the E1 excitations in odd-and even-mass nuclei, respectively. In our previous study, this method has been quite successful in explaining of the PDR in N=82 nuclei [6], low lying electric dipole excitations up to 4 MeV [7] and GDR mode in 235 U [8].…”

Investigation of pygmy dipole resonance in 120sn

“…There, by the selection of suitable separable effective forces, within the QRPA without introducing additional parameters translational and Galilean invariances restored for the description of the E1 excitations. In our previous study, this method has been quite successful in explaining of the PDRinN=82 nuclei (Guliyev et al, 2010) and low lying electric dipole excitations up to 4MeV (Kulievet al, 2010).Here, the strength of the electric dipole excitations will be investigated below particle threshold energy for 120 Sn nucleus.The contribution of K=0 and K=1 branches will be examined in PDR region and we are interested in not only the PDR resonance energy but also the contribution 1 -0 and 1 -1 states to none-energy weight sum rule of E1 excitations to PDR.Besides, we are also planning to investigate if PDR excitations overlap with the spin-flip M1 resonances or not.…”

“…Sn nuclei have been studied a wide range both experimentally and theoretically. Recently, It is discussed if the nuclei which their one of the numbers of nucleon are not magic number are deformed or not (Guliyev et al, 2010;Guliyev et al, 2002;Wood et al, 1992). However, while the global properties of E1 strengths are reasonably understood in regions of moderate to large deformations, the nature of the electric dipole mode is an open question in semi-magic nuclei with neutron numbers near mid-shell shell.…”