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

TL;DR

This study measures partial cross sections of the $^{89}$Y(p,$$)$^{90}$Zr reaction to investigate the low-lying dipole $$-ray strength function in $^{90}$Zr, revealing deviations from the Brink-Axel hypothesis.

Contribution

It provides new experimental data on the $$-ray strength function in $^{90}$Zr and compares it with previous photoabsorption data, challenging existing theoretical assumptions.

Findings

Extracted $$-ray strength function describes reaction cross sections successfully.

Observed significant differences with photoabsorption data.

Indications of deviations from the Brink-Axel hypothesis.

Abstract

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.