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

Abstract

In this work, we present new data on the ${}^{89}$Y(${\hat{I}}^3$,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 ${\hat{I}}^3$-ray strength function below threshold obtained by applying the Oslo method to ${}^{89}$Y($p,p^{\prime }{\hat{I}}^3$)${}^{89}$Y coincidences combined with the ${}^{89}$Y(${\hat{I}}^3$,n) data this providing experimental data for the ${\hat{I}}^3$-ray strength function of ${}^{89}$Y for ${\hat{I}}^3$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray strength function have been extracted from ${}^{89}$Y($d,p{\hat{I}}^3$)${}^{90}$Y coincidences using the Oslo method. Using the (${\hat{I}}^3,n$) and ($d,p{\hat{I}}^3$) data as experimental constraints, we have calculated the ${}^{89}$Y($n,{\hat{I}}^3$)${}^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,${\hat{I}}^3$) 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,${\hat{I}}^3)$ cross section is key to understanding how s-process stars produce heavy isotopes.

18 pages (including references) and 16 figures, to be submitted to PRC