Electric dipole response of neutron-rich calcium isotopes in relativistic quasiparticle time blocking approximation

Abstract

New results for electric dipole strength in the chain of even-even Calcium isotopes with the mass numbers A = 40 -54 are presented. Starting from the covariant Lagrangian of Quantum Hadrodynamics, spectra of collective vibrations (phonons) and phonon-nucleon coupling vertices for J ≤ 6 and natural parity were computed in a self-consistent relativistic quasiparticle random phase approximation (RQRPA). These vibrations coupled to Bogoliubov two-quasiparticle configurations (2q⊗phonon) formed the model space for the calculations of the dipole response function in the relativistic quasiparticle time blocking approximation (RQTBA). The calculations in the latter approach were performed for the giant dipole resonance (GDR) and compared to those obtained in RQRPA and to available data. Evolution of the dipole strength with neutron number is investigated for both high-frequency GDR and low-lying strength. The development of a pygmy resonant structure on the low-energy shoulder of GDR is traced and analyzed in terms of transition densities. A dependence of the pygmy dipole strength on the isospin asymmetry parameter is extracted.

Influence of complex configurations on the properties of the pygmy dipole resonance in neutron-rich Ca isotopes

“…The filled triangles indicate the PPC effects, which can be compared with QRPA results indicated by the open triangles. The 48 Ca(γ, γ ′ ) experiments give f P DR = 0.33±0.04% [12], while the calculations with the PPC effects lead to 0.28% (to be compared with the RQTBA result of 0.55% [17]). The QRPA calculations predict that the visible f P DR kink can be identified at the magic neutron number N = 28.…”

“…This is not the PDR where the neutron skin oscillates against the inner core. The next fairly collective gion for the neutron-rich Ca isotopes has been found in the framework of other theoretical approaches [6,17,71]. Let us now discuss the low-energy E1 strength.…”

“…The PPC calculations reproduce the observed trend in light Ca isotopes, although the theoretical value of B(E1) for 44 Ca underestimates the experimental value by a factor of 2. It is worth men- tioning that the relativistic quasiparticle time blocking approximation (RQTBA) [17], the value of B(E1) is also substantially less than the experimental one. As shown in Fig.…”

“…The results were generally consistent with the theoretical prediction regarding the shifting of dipole strength to lower energies, see, e.g., Refs. [6,[15][16][17]. Moreover, recent experimental studies indicate N = 32 as a new magic number in Ca isotopes due to the high energy of the first 2 + state in 52 Ca [18] and the trend obtained for the two-neutron separation energies [19].…”

Isospin dynamics in nuclear structure

“…The extended versions include like-particle superfluid pairing [15] and quasiparticle-vibration coupling (QVC), two-phonon coupling [16,17] and "two-quasiparticles plus N phonons" (2q⊗Nphonon) configurations [37]. The method has been applied successfully to excitation spectra of various closed and open-shell medium-mass spherical nuclei [15,[18][19][20][21][22][23][24]. It has described well both high and lowenergy parts of the spectra including the giant multipole resonances, the isospin splitting of the pygmy dipole mode [25][26][27][28], isoscalar dipole modes [29,30], and stellar reaction rates of the r-process nucleosynthesis [31].…”