[en] Cross-sections were determined by measuring isotopic ratios before and after irradiation. The relative change of the isotopic ratio, and hence the accuracy of the result, is higher the better the enrichment of the isotope in question before irradiation. Since the enrichment of commercially available isotopes is insufficient, separated isotopes were prepared with the Seibersdorf inhomogeneous magnetic field isotope separator. Oxides were used as ion source charge material and vaporized by the chlorination method. To obtain optimum focusing conditions, the positions of the accelerating electrodes are adjustable during operation. The isotopes were collected in water-cooled graphite or aluminium pockets. After chemical processing, the isotopic ratios were determined by mass spectrometry. In a one-step separation, we obtained ¹⁵⁶Gd enriched to 99%, ¹⁵⁴Gd to 89%, ¹⁷⁰Yb to 96% and the heavier Yb isotopes to 99%. The samples were irradiated together with a cobalt flux monitor, with and without Co shielding. Afterwards the isotopic ratios were determined again. Cross-section data are given. (author)