Low-energy electric and magnetic dipole excitations in Er

Shizuma, Toshiyuki*; Omer, M.   ; Hajima, Ryoichi*; Koizumi, Mitsuo ; Takahashi, Tone; Tsunoda, Yusuke*; Otsuka, Takaharu*

Low-energy dipole excitations in Er were investigated via nuclear resonance fluorescence using a quasi-monoenergetic, linearly polarized photon beam produced at the High Intensity -ray Source (HIS) facility at Duke University. The parity of the excited states was determined from the azimuthal asymmetry in the intensities of resonantly scattered rays with respect to the polarization plane of the incident beam. Electric and magnetic dipole strengths were extracted for excitation energies between 2.2 and 3.5 MeV. The results are interpreted in terms of the nuclear scissors mode by comparing with large-scale shell-model calculations based on the Qasiparticle Vacua Shell Model. In addition, the characteristics of the E1 excitations are examined in comparison with those of neighboring rare-earth nuclei to explore their relations to collective octupole--vibrational two-phonon excitations. This work is a contribution of the Japan Atomic Energy Agency (JAEA) to the International Atomic Energy Agency (IAEA) under the agreement of the coordinated research program (CRP), J02015 (Facilitation of Safe and Secure Trade Using Nuclear Detection Technology - Detection of RN and Other Contraband). This work was a part of the study of NRF phenomenon aiming at nuclear security and safeguards applications, being supported by the subsidiary for "promotion of strengthening nuclear security or the like" of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.