Experimental constraints on the γ-ray strength function in 90Zr using partial cross sections of the Y89(p,γ)Zr90 reaction

Partial cross sections of the Y89(p,γ)Zr90 reaction have been measured to investigate the γ-ray strength function in the neutron–magic nucleus 90Zr. For five proton energies between Ep=3.65 MeV and Ep=4.70 MeV, partial cross sections for the population of seven discrete states in 90Zr have been determined by means of in-beam γ-ray spectroscopy. Since these γ-ray transitions are dominantly of E1 character, the present measurement allows an access to the low-lying dipole strength in 90Zr. A γ-ray strength function based on the experimental data could be extracted, which is used to describe the total and partial cross sections of this reaction by Hauser–Feshbach calculations successfully. Significant differences with respect to previously measured strength functions from photoabsorption data point towards deviations from the Brink–Axel hypothesis relating the photo-excitation and de-excitation strength functions. Keywords: γ-ray strength function, Nuclear astrophysics, Pygmy Dipole Resonance, Statistical model, γ-ray spectroscopy, Measured cross sections