Collective Excitations and Pairing Effects in Drip-Line Nuclei

Abstract

We discuss novel features of a new continuum RPA formulated in the coordinate-space Hartree-Fock-Bogoliubov framework. This continuum quasiparticle RPA takes into account both the one-and two-particle escaping channels. The theory is tested with numerical calculations for monopole, dipole and quadrupole excitations in neutron-rich oxygen isotopes near the drip-line. Effects of the particle-particle RPA correlation caused by the pairing interaction are discussed in detail, and importance of the selfconsistent treatment is emphasized. §1. IntroductionCollective excitation in unstable nuclei is one of the most attractive subjects since the exotic structures in the ground state, such as halo, skin, and the presence of loosely bound nucleons, may cause new features in the excitations, e.g. the lowenergy dipole mode that is being discussed extensively. The random phase approximation (RPA) or the linear response theory is one of the most powerful framework to investigate such problems microscopically. Indeed the continuum RPA theory in the coordinate-space representation 1), 2) has played major roles so far since it can describe the continuum states crucial for nuclei near drip-line.The pairing correlation is another key feature of drip-line nuclei. 6) -8) To treat the coupling of the continuum states as well as the density dependence of the pairing correlation, the Hartree-Fock-Bogoliubov (HFB) theory formulated in the coordinate-space representation 6), 9) has been developed while the conventional BCS approximation has inherent deficiency.It is therefore important to combine the continuum RPA and the coordinate space HFB in a consistent way in order to describe the excitations in unstable nuclei near drip-line, especially when the pairing correlation play crucial roles. We have recently shown that a new quasiparticle RPA (QRPA) satisfying this requirement is indeed possible. 10) In the present paper, we discuss characteristic features of the theory and analyze excitations of near-drip-line nuclei, focusing on effects of the pairing, by using numerical calculations performed for the monopole, dipole and quadrupole excitations in neutron-rich oxygen isotopes. The previous continuum QRPA approaches employ the BCS approximation. 11) -14) Other QRPA approaches applied to unstable nuclei neglect the escaping effects since some use the BCS quasiparticle basis, 15) -17) and other adopt the coordinate-space HFB but use the discretized canonical basis. 18) The present formalism provides the first consistent continuum * )

Di-neutron correlation in monopole two-neutron transfer modes in the Sn isotope chain

“…In the present study, we adopt the Skyrme-Hartree-Fock-Bogoliubov model to describe the pair correlated ground states, and apply a linear response formulation of the quasiparticle random phase approximation (QRPA) to describe the excitation modes [28,44,45,49]. The framework is essentially the same as what we have adopted in the preceding study of the two-neutron transfer in neutron-rich Sn isotopes [44,45].…”

“…In the preceding paper [45], we adopted the continuum version of the linear response QRPA, i.e. the continuum QRPA [49], in which the unperturbed response function is constructed in terms of the HFB Green's function in place of the spectral representation. The continuum QRPA is appropriate to describe the response at energies above the particleemission threshold.…”

Green's function method for single-particle resonant states in relativistic mean field theory

“…In Ref. [55], Matsuo applied this Green's function in the quasi-particle random-phase approximation (QRPA), with which the collective excitations coupled to continuum states can be described [56][57][58][59][60][61][62]. In Ref.…”

Search for neutron decoupling inO22via the (d,d'γElekes1, Dombrádi2, Aoi3 et al. 2006Phys. Rev. C180160View full textAdd to dashboardCite

“…According to theoretical calculations, the most specific excitation mode of nuclei with a thick neutron skin is the isoscalar quadrupole excitation of the skin, which is a large-amplitude motion decoupled from the rest of the nucleus [4,5]. Recently, as a result of incorporation of the continuum coupling into the quasiparticle random phase approximation, calculations predicted a surface neutron quadrupole mode associated with a bound 2 + state [6][7][8]. The strength of the neutron contribution is expected to be proportional to the number of neutrons in the neutron skin [6,7].…”

“…Recently, as a result of incorporation of the continuum coupling into the quasiparticle random phase approximation, calculations predicted a surface neutron quadrupole mode associated with a bound 2 + state [6][7][8]. The strength of the neutron contribution is expected to be proportional to the number of neutrons in the neutron skin [6,7]. In self-consistent calculations, where the effect of the shell closure is also taken into account [8], a bell-shaped strength distribution as a function of mass is predicted similar to the mass dependence of the quadrupole electromagnetic transition probability, but the neutron strength is calculated to be several times larger than the electromagnetic one for the oxygen isotopic chain.…”