Speaker
Dr.
Nikolay Arsenyev
(Bogoliubov Laboratory of Theoretical Physic, Joint Institute for Nuclear Research)
Description
Using radioactive beams to explore nuclei far from the $\beta$-stability line has sparked intensive experimental and theoretical studies of neutron-rich nuclei during recent years [1]. In addition an attention has been devoted to effects of varying the ratio between the proton $Z$ and neutron $N$ numbers on different nuclear structure characteristics of nuclei deviated from their valley of $\beta$-stability. One of the phenomena associated with the change in $N/Z$ ratios is the pygmy dipole resonance (PDR). The PDR leads to the enhancement of dipole strength below the region of the isovector giant dipole resonance. The structure and dynamics of the PDR is one of the hot topics in nuclear physics. There is a special structure of the PDR which appears as a new collective motion in neutron-rich nuclei [2]. The other reason is the role of the PDR in nucleosynthesis. The PDR also induces noticeable effects on $(\gamma,n)$ cross section and on the r- process [3].
The quasiparticle random phase approximation (QRPA) with a self- consistent mean-field derived from Skyrme energy density functionals (EDF) is one of the most successful methods for studying the low-energy dipole strength, see e.g., Ref. [2]. Due to the anharmonicity of vibrations there is a coupling between one- phonon and more complex states. The main difficulty is that the complexity of calculations beyond standard QRPA increases rapidly with the size of the configuration space, and one has to work within limited spaces. Using a finite rank separable approximation for the residual particle-hole interaction derived from the Skyrme forces one can overcome this numerical problem [4-6]. As an illustration, we study the properties of the low-lying dipole states in the even-even nuclei $^{40-58}$Ca. Using the same set of the EDF parameters we describe available experimental data for $^{40,44,48}$Ca and give the prediction for $^{50-58}$Ca [7]. In particular, there is an impact of the coupling between one- and two-phonon states on low-energy $E1$ strength of $^{40,44,48}$Ca. We predict a strong increase of the summed $E1$ strength below 10 MeV, with increasing neutron number from $^{48}$Ca.
[1] D. Savran, T. Aumann, A. Zilges, Prog. Part. Nucl. Phys. 70, 210 (2013).
[2] N. Paar, D. Vretenar, E. Khan, G. Colò, Rep. Prog. Phys. 70, 691 (2007).
[3] M. Arnould, S. Goriely, K. Takahashi, Phys. Rep. 450, 97 (2007).
[4] Nguyen Van Giai, Ch. Stoyanov, V. V. Voronov, Phys. Rev. C57, 1204 (1998).
[5] A. P. Severyukhin, V. V. Voronov, Nguyen Van Giai, Phys. Rev. C77, 024322 (2008).
[6] A. P. Severyukhin, V. V. Voronov, Nguyen Van Giai, Eur. Phys. J. A22, 397 (2004).
[7] N. N. Arsenyev, A. P. Severyukhin, V. V. Voronov, Nguyen Van Giai, Phys. Rev. C95, 054312 (2017).
Primary author
Dr.
Nikolay Arsenyev
(Bogoliubov Laboratory of Theoretical Physic, Joint Institute for Nuclear Research)
Co-authors
Dr.
Alexey Severyukhin
(Bogoliubov Laboratory of Theoretical Physic, Joint Institute for Nuclear Research)
Prof.
Nguyen Van Giai
(Institut de Physique Nucleaire, CNRS-IN2P3, Universite Paris-Sud)
Prof.
Victor Voronov
(Bogoliubov Laboratory of Theoretical Physic, Joint Institute for Nuclear Research)
