Branching Fractions for $\psi(2S) to \gamma \eta'$ and $\gamma \eta$
  We report the first measurements of the branching fractions: $B(\psi(2S) -> \gamma \eta') = (1.54 \pm 0.31 \pm 0.23) x 10^{-4}$ and $B(\psi(2S) -> \gamma \eta) = (0.53 \pm 0.31 \pm 0.08) x 10^{-4}$. The $\psi(2S)$ to $\gamma \eta'$ result is consistent with expectations of a model that considers the possibility of $\eta' - \eta$ mixing. The ratio of the $\psi(2S)$ to $\gamma \eta'$ and $\psi(2S)$ to $\gamma \eta$ rates is used to determine the pseudoscalar octet-singlet mixing angle. 
J. Z. Bai, et al. (BES Collaboration)