Isovector and isoscalar dipole excitations inBe9andBe10studied with antisymmetrized molecular dynamics

Abstract

Isovector and isoscalar dipole excitations in 9 Be and 10 Be are investigated in the framework of antisymmetrized molecular dynamics, in which angular-momentum and parity projections are performed. In the present method, 1p-1h excitations on the ground state and large amplitude α-cluster mode are incorporated. The isovector giant dipole resonance (GDR) in E > 20 MeV shows the two peak structure which is understood by the dipole excitation in the 2α core part with the prolate deformation. Because of valence neutron modes against the 2α core, low-energy E1 resonances appear in E < 20 MeV exhausting about 20% of the Thomas-Reiche-Kuhn sum rule and 10% of the calculated energy-weighted sum. The dipole resonance at E ∼ 15 MeV in 10 Be can be interpreted as the parity partner of the ground state having a 6 He+α structure and has the remarkable E1 strength because of coherent contribution of two valence neutrons. The ISD strength for some low-energy resonances are significantly enhanced by the coupling with the α-cluster mode. The calculated E1 strength of 9 Be reasonably describes the global feature of experimental photonuclear cross sections consisting of the low-energy strength in E < 20 MeV and the GDR in E > 20 MeV.

Cluster and toroidal aspects of isoscalar dipole excitations in C12

“…The adopted effective interactions are the same as those used in the previous work [29]. The central force is the MV1 force [34] consisting of two-range Gaussian two-body terms and a zero-range three-body term.…”

“…The sAMD+GCM has been applied to ISM, ISD, and E1 excitations of light nuclei such as 10 Be, 12 C and 16 O [27][28][29]. A similar method has been recently applied to E1 and ISD excitations of 26 Ne by Kimura [30].…”

“…, 4). Finally, in the sAMD+GCM, we superpose all the basis wave functions of the sAMD and 3α wave functions in addition to the original VAP wave function (Φ AMD (Z 0 VAP )) as done in the previous work [29]. The final wave functions Ψ(J π k ) for the 0 + k and 1 − k states are expressed by superposition of the J π -projected wave functions with coefficients determined by diagonalizing the norm and Hamiltonian matrices.…”

“…, 8) are unit vectors for 8 directions explained in Ref. [29]. As for the intrinsic spin of the shifted single-particle wave function, the spin nonflip and flip states are adopted.…”

“…Even though cluster models are useful for 3α-cluster states of 12 C, the models a priori assume three α clusters, and therefore they are insufficient to study 1p-1h excitations and unable to describe GRs. In order to take into account coherent 1p-1h excitations for the GRs as well as the large amplitude cluster modes, one of the authors, Y. K-E., has extended the antisymmetrized molecular dynamics (AMD) [23][24][25][26] and constructed a new method, the shifted basis AMD combined with the generator coordinate method (GCM) with respect to the inter-cluster motion, which we named "sAMD+GCM" [27][28][29]. In the previous work [29], we applied the sAMD+GCM to calculate ISM and ISD transition strengths of 12 C and obtained significant LE-ISD strengths in the E = 10 − 15 MeV region well separating from the high-energy strengths for the ISGDR.…”