Spin-vibrational 1 + States in 140 Ce

The properties of collective I π =1 + states in double even semi-magic nucleus 140 Ce are investigated in random phase approximation (RPA). The calculation of the B(M1) strength, elastic γ-scattering cross section σ γ (ω) and γ-decay width Γ γ (ω i) show that in 140 Ce the isovector spin-spin interactions concentrate the main strength of M1 transition at energy 7.89 MeV where the isovector spin-flip magnetic resonance may appear. These predictions are consistent with the observed M1 strength in 140 Ce which is seen to be concentrated within a range of about 0.8 MeV centered at an excitation of 7.95MeV. During the last few years great success has been achieved in measurements of the spin-flip magnetic dipole resonance in nuclei [1]. These experiments show that in heavy spherical and deformed nuclei there exists a very broad M1 resonance at energies between 7 and 11 MeV centred at energy on the order of 44A-1/3 MeV. Since long ago some experimental evidence for unusual concentration of M1 strength in nucleus 140 Ce at energy between 7 and 11 MeV has been obtained by inelastic (e,e') [2], (p,p') [3], (n,γ) [4] and tagged photon scattering [5]. Indeed because of the limited energy resolution no individual state could be discerned in the existence scattering experiments in the resonance energy region. Recent technical improvements of the photon scattering experiments [6] and a new method proposed to investigate the scattering reactions with photons on fully ionised nuclei, namely FEL γ-nucleus colliders [7,8] open the way for such studies up to excitation energies of about 10 MeV. Therefore the study of the magnetic dipole giant resonance in heavy nuclei has attractive considerably interest. By now there are several papers dealing with spin-vibrational 1 + excitations in 140 Ce [9,10]. In these papers, however, either rather schematic calculations are performed, or on pays special attention only to the collective 1 + state, which should appear at relatively high excitation energy. In connection with the increasing interest of investigating the M1 transitions to highly-excited states, it is interesting to study in the microscopic approach the properties of spin-YLEUDWÕRQDO + states generated by the isovector spin-spin interactions and estimate their energy, M1-strength and elastic photon scattering cross section in 140 Ce. In this paper collective 1 + states are treated as vibrational states in the framework of quasi-particle random phase approximation (QRPA). Supposing the collectivisation of states being …