Experimental constraints on the $\gamma$-ray strength function in ${}^{90}$Zr using partial cross sections of the ${}^{89}$Y(p,$\gamma$)${}^{90}$Zr reaction

Partial cross sections of the ${}^{89}$Y(p,$\gamma$)${}^{90}$Zr reaction have been measured to investigate the $\gamma$-ray strength function in the neutron-magic nucleus ${}^{90}$Zr. For five proton energies between $E_p=3.65$ MeV and $E_p=4.70$ MeV, partial cross sections for the population of seven discrete states in ${}^{90}$Zr have been determined by means of in-beam $\gamma$-ray spectroscopy. Since these $\gamma$-ray transitions are dominantly of $E1$ character, the present measurement allows an access to the low-lying dipole strength in ${}^{90}$Zr. A $\gamma$-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.