Fragmentation of electric dipole strength inN=82isotones

Abstract

Fragmentation of the dipole strength in the N = 82 isotones 140 Ce, 142 Nd and 144 Sm is calculated using the second random-phase approximation (SRPA). In comparison with the result of the random-phase approximation (RPA), the SRPA provides the additional damping of the giant dipole resonance and the redistribution of the low-energy dipole strength. Properties of the low-energy dipole states are significantly changed by the coupling to two-particle-two-hole (2p2h) states, which are also sensitive to the correlation among the 2p2h states. Comparison with available experimental data shows a reasonable agreement for the low-energy E1 strength distribution.PACS numbers: 21.60. Jz, 24.30.Cz, 27.60.+j The low-energy dipole states, often referred to as the pygmy dipole resonance (PDR), have attracted recent experimental [1][2][3][4][5][6] and theoretical interests [7][8][9][10][11][12] (see also the recent review [13] and references therein). It is also of significant astrophysical interest, since the low-energy dipole strengths close to the neutron threshold strongly affect the astrophysical r-process nucleosynthesis [14].The quasiparticle random-phase approximation based on the Hartree-Fock-Bogoliubov ground state (HFB+QRPA) has been extensively used to study the PDR as well as the giant dipole resonances (GDR). Recent systematic calculations [15] for the Nd and Sm isotopes show that although the HFB+QRPA nicely reproduces characteristic features of the shape phase transition in the GDR, it fails to produce the low-energy dipole strengths at E x = 5.5 ∼ 8 MeV, observed in the N = 82 isotones, 142 Nd and 144 Sm [1,3]. The disagreement suggests that the coupling to complex configurations, such as multi-particle-multi-hole states, are required to study the PDR in these nuclei. In fact, the quasiparticle-phonon model (QPM), which takes into account coupling to multi-phonon states, successfully reproduces the low-energy dipole strengths in the N = 82 nuclei [2,4]. A similar approach based on the relativistic mean-field model has also been used to study the PDR in the tin and nickel isotopes [16]. These models assume the multi-phonon characters of the complex states and violate the Pauli principle. Thus, it is desirable to study properties of the PDR with a method complementary to these phonon-coupling approaches. In this work, we present studies for the dipole excitations in the N = 82 isotones, with the second random-phase approximation (SRPA) (Ref.[17] and references therein). The SRPA explicitly incorporates the two-particle-two-hole (2p2h) states instead of "two-phonon" states, and respects the Pauli principle in the 2p2h configurations. Recently, the low-energy dipole states in 40,48 Ca have been studied with the SRPA [18], which suggests that the coupling between one-particle-one-hole (1p1h) and 2p2h configurations enhances the electric dipole (E1) strength in the energy range from 5 to 10 MeV. We investigate whether a similar effect can be observed in the isotones of N = 82. Since there are many dipole states w...

Estimation of a 2p2h effect on Gamow-Teller transitions within the second Tamm-Dancoff approximation

“…The second is that SGII+Te1 significantly enhances the strengths in the energy region above about 60 MeV, while it suppresses them around 30-45 MeV than SGII. Figures 11 and 12 plot the results of 34 Si and 48 Ca at high energy regions, respec- ) The same results are seen for 34 Si as well. In Tab.…”

“…Finally, we adopt r box = 10 fm, E 2p2h cut = 100 MeV, and N = 13 for 24 O and N = 12 for 48 Ca, in this work. The same model space as 48 Ca is adopted for 34 Si. Under this condition, the dimensions N 1 +N 2 are about 9×10 4 , 1.4× 10 5 and 2.5 × 10 5 for 24 O, 34 Si, and 48 Ca, respectively.…”

Systematic investigation of low-lying dipole modes using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory

“…There are some calculations beyond the (Q)RPA as well [44][45][46][47][48][49]. However, even in the (Q)RPA level, different models often predict different properties.…”

Time-dependent density-functional description of nuclear dynamics

“…This approach is called time-dependent density matrix (TDDM) theory (Shunjin and Cassing, 1985). The extended RPA (Tohyama and Schuck, 2007) and the second RPA (Drozdz et al, 1990;Gambacurta et al, 2011Gambacurta et al, , 2012Tohyama and Nakatsukasa, 2012) can be derived as approximations to the small-amplitude limit of the TDDM theory (Tohyama and Gong, 1989), and have been used to the analysis of damping mechanisms of giant resonances and anharmonicities of low-frequency vibrations.…”