Séminaire DRS/Phys. Nucl - "Recent developments in heavy-ion fusion reactions around the Coulomb barrier" (K. Hagino)

jeudi 29 sept. 2016, 10:30 → 11:30 Europe/Paris

Batiment 27

Heavy-ion fusion reactions at energies around the Coulomb barrier are strongly influenced by couplings of the relative motion of the colliding nuclei to several nuclear intrinsic motions. Here, multiple excitations to higher members of collective states, such as multi-phonon states and high-spin states in the ground state rotational band, often play an important role. In order to take into account those multiple excitations during fusion reactions, the coupled-channels approach has been a standard tool.In this approach, one often uses a simple macroscopic model for nuclear structure, that is, the harmonic oscillator model for vibrational nuclei and the rigid rotor model for deformed nuclei.In reality, however, most nuclei have neither a pure harmonic oscillator spectrum nor a pure rigid body rotational band, although the rigid rotor approximation works well for medium-heavy and heavy deformed nuclei. In this talk, we will discuss how one can incorporate nuclear structure calculations for general spectra into the coupled-channels approach for heavy-ion fusion reactions. To this end, we will employ the so called beyond-mean-field approach, which incorporates the quantum fluctuation of a mean-field wave function together with the angular momentum and particle number projections. This method provides transition strengths among collective states without resorting to the harmonic oscillator model or the rigid rotor model. We will then use them as inputs to coupled-channels calculations for subbarrier fusion reactions. We will apply this approach particularly to the 16O+208Pb system, for which previous coupled-channels calculations based on multi-harmonic-phonon excitations have failed to reproduce the observed energy dependence of fusion cross sections, and overestimated the height of the main peak in the fusion barrier distribution. We will demonstrate that our new approach largely resolves this long-standing unsolved puzzle by taking into account the ahnarmonicity of the octupole vibrational excitation in 208Pb [1]. We will also briefly discuss an extension of the Wong formula for fusion cross sections [2] as well as applications of the Bayesian statistics to fusion barrier distributions [3]. [1] "Anharmonicity of multi-octupole-phonon excitations in 208Pb: analysis with multi-reference covariant density functional theory and subbarrier fusion of 16O+208Pb", J.M. Yao and K. Hagino, Phys. Rev. C94 (2016) 011303(R). [2] "Investigating multi-channel quantum tunneling in heavy-ion fusion reactions with Bayesian spectral deconvolution", K. Hagino, Phys. Rev. C93 (2016) 061601(R). [3] "Examination of fusion cross sections and fusion oscillations with a generalized Wong formula", N. Rowley and K. Hagino, Phys. Rev. C91 (2015) 044617.