Isospin splitting of giant E1 excitations in 90Zr

Abstract

The radiative proton capture reaction has been used to study the distribution of the E1 strength in 90Zr from E x = 13.7 to 25.2 MeV. The reaction 89Y(p, γ 0) 90Zr displays the “normal” T < giant dipole resonance (GDR) at E x = 16.5 MeV with a width Γ = 4 MeV. For this resonance the measured γ 0 angular distributions are nearly constant ( a ̄ 2 = −0.34) and are consistent with a predominantly d 3 2 p 1 2 1̄ configuration. Several analogue T > dipole levels are observed superimposed on the broad T < GDR and interfere with it in the characteristic manner. Resonance parameters for the T > E1 levels have been derived from a two-resonance interference analysis and the characteristic dipole enhancement for the highest T > levels is observed. A comparison of the measured and calculated values of {Γ} γo indicates the usual hindrance factor for the low-lying s 1 2 p 1 2 −1 and d 3 2 p 1 2 T > levels. The dominant d above the d 3 2 resonance observed in elastic proton scattering. The integrated 90Zr(γ, p 0) strength is separated into its T > and T < components and compared to 90Zr( γ, xn) and to theoretical predictions based on isospin sum rules. Good agreement between theory and experiment is found. The reaction 89Y(p, γ) 90Zr ∗ leading to unresolved excited states also reveals both a “normal” T < GDR and analogue dipole levels, of which the highest ones appear to form a T > GDR. The shift of this T < GDR relative to the T < GDR for γ 0 is consistent with the average excitation energy of the low-lying states in 90Zr. The T > GDR's for both γ 0 and the γ's leading to excited states are shifted above the corresponding T < GDR's by approximately 4 MeV which leads to a value for the collective potential strength in the Lane potential of V 1 = 57±4 MeV.