The gamma-ray strength function of $^{89}$Y and $^{90}$Y

TL;DR

This study provides new experimental data on the gamma-ray strength function of $^{89}$Y, resolving previous discrepancies and exploring low-energy enhancements, with implications for astrophysical nucleosynthesis modeling.

Contribution

The paper presents new measurements of the gamma-ray strength function of $^{89}$Y using multiple methods, including the Oslo method and ($ ext{γ}$,n) reactions, and compares these with theoretical calculations.

Findings

Identification of low-energy gamma-ray enhancement below 2.5 MeV.

Shell-model calculations attribute the enhancement to low-energy M1 transitions.

Calculated $^{89}$Y(n,γ) cross sections agree with experimental data and evaluations.

Abstract

In this work, we present new data on the $^{89}$Y($\gamma$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $\gamma$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'\gamma$)$^{89}$Y coincidences combined with the $^{89}$Y($\gamma$,n) data this providing experimental data for the $\gamma$-ray strength function of $^{89}$Y for $\gamma$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $\gamma$-rays below $\approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $\gamma$-ray strength function have been extracted from $^{89}$Y($d,p \gamma$)$^{90}$Y coincidences using the Oslo method. Using the ($\gamma,n$) and ($d,p\gamma$) data as experimental constraints, we have calculated the $^{89}$Y($n,\gamma$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$\gamma$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$\gamma)$ cross section is key to understanding how s-process stars produce heavy isotopes.