Determination of $^{170,172}$Yb($\alpha,n$)$^{173,175}$Hf reaction cross sections in a stacked-target experiment
The ytterbium isotopic chain offers multiple stable isotopes on which cross sections can be measured and insights into the evolution of the $\alpha$ optical-model potential with the neutron-to-proton ratio can be gained. It also includes the $p$ nucleus $^{168}$Yb, the abundance of which is significantly impacted by the $^{164,166}$Yb($\alpha,\gamma$) reactions. In order to study the $^{170,172}$Yb($\alpha,n$)$^{173,175}$Hf reaction cross sections and compare them with $^{168}$Yb($\alpha,n$)$^{171}$Hf cross sections, that have already been measured, the activation method was used. During irradiation the targets were arranged in stacks of four to reduce the required irradiation time. The average interaction energy inside each ytterbium layer was determined via Geant4 simulations. A manganese layer was used to verify the simulations by comparing the measured $^{55}$Mn($\alpha,(2)n$)$^{57,58}$Co reaction cross sections to previous results. For irradiation the 10 MV FN tandem accelerator located at the University of Cologne was used and the activation measurement was performed utilizing the Cologne Clover Counting setup. For the $^{170}$Yb($\alpha,n$) reaction seven cross sections at center-of-mass energies between 12.7 and 16.5 MeV were measured. For the $^{172}$Yb($\alpha,n$) reaction six cross sections for center-of-mass energies of 13.1 to 16.5 MeV could be determined with an additional upper limit at E$_{c.m.}$ = 12.3 MeV. Comparisons to theoretical models show that state-of-the-art $\alpha$-optical model potentials are able to reproduce the measured cross sections very well. The ratios of ($\alpha, n$) reaction cross sections in the ytterbium isotopic chain can be accurately reproduced as well.