Measurement of (alpha,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions

The $gamma$-process in core-collapse and/or type Ia supernova explosions is thought to explain the origin of the majority of the so-called $p$ nuclei (the 35 proton-rich isotopes between Se and Hg). Reaction rates for $gamma$-process reaction network studies have to be predicted using Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of $alpha$-widths at astrophysical energies which are an essential input for the statistical model. It has an impact on the reliability of abundance predictions in the upper mass range of the $p$ nuclei. With the measurement of the $^{164,166}$Er($alpha$,n)$^{167,169}$Yb reaction cross sections at energies close to the astrophysically relevant energy range we tested the recently suggested low energy modification of the $alpha$+nucleus optical potential in a mass region where $gamma$-process calculations exhibit an underproduction of the $p$ nuclei. Using the same optical potential for the $alpha$-width which was derived from combined $^{162}$Er($alpha$,n) and $^{162}$Er($alpha$,$gamma$) measurement makes it plausible that a low-energy modification of the optical $alpha$+nucleus potential is needed.