Low-energy electric dipole response in 120Sn

A M Krumbholz ,H Sakaguchi,Y Shimbara,M Yosoi,Y Shimizu,T Kawabata,J Zenihiro,R Neveling,Y Fujita,H Okamura,J Simonis,I Poltoratska,A Tamii,H Fujita,K Hatanaka,T Adachi,P Von Neumann-Cosel,A Richter,A Krugmann,Nguyen Tuan Khai ,Dirk Martin ,C A Bertulani ,Hiroaki Matsubara ,H J Ong ,Tomokazu Suzuki ,G Süsoy ,Toshikazu Hashimoto ,F D Smit ,E Ganioğlu ,J H Thies ,V Yu Ponomarev ,Chihiro Iwamoto

doi:10.1016/j.physletb.2015.03.023

Abstract

The electric dipole strength distribution in 120Sn has been extracted from proton inelastic scattering experiments at Ep=295 MeV and at forward angles including 0°. It differs from the results of a Sn120(γ,γ′) experiment and peaks at an excitation energy of 8.3 MeV. The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and is more than three times larger than what is observed with the (γ,γ′) reaction. This implies a strong fragmentation of the E1 strength and/or small ground state branching ratios of the excited 1− states.

Highlights

The low-energy electric dipole strength in neutron-rich nuclei, commonly termed Pygmy Dipole Resonance (PDR), is currently a topic of great interest [1]. It occurs at energies well below the isovector Giant Dipole Resonance (GDR) and exhausts a considerable fraction of the total E1 strength in nuclei with a large neutron-to-proton ratio [2,3,4,5]
Dipole strength in the vicinity of the neutron threshold may lead to significant changes of neutron-capture rates in the astrophysical r-process [14,15,16]
While the increase of the E1 strength due to the statistical model corrections can be large in more deformed nuclei [33,51], in the semimagic nucleus 120Sn it does not exceed 40% and cannot explain the orders-of-magnitude difference observed at excitation energies > 7 MeV

Summary

Introduction

The low-energy electric dipole strength in neutron-rich nuclei, commonly termed Pygmy Dipole Resonance (PDR), is currently a topic of great interest [1]. Considered to be a single-particle effect [17], many microscopic models nowadays favor an explanation of the PDR as an oscillation of a neutron skin – emerging with an increasing N/Z ratio – against an approximately isospin-saturated core This conclusion is based on the analysis of theoretical transition densities which differ significantly from those in the GDR region. Possible branching ratios to excited states are often neglected, but statistical model calculations of the branching ratios suggest potentially large corrections of the deduced E1 strength [33] This uncertainty has an impact on the determination of the E1 polarizability, which has been established as a measure of the neutron skin and the density dependence of the symmetry energy [10,34].

Experiment

Multipole decomposition

Low-energy E1 strength

Comparison with model calculations

Findings

Conclusions and outlook