Complex sentence: At first, the mixture was used as-is, both barium and lanthanum together, but this led to unpleasant radioactive contamination that took long time to disappear, due to the 12.5 days of half-life of barium-140.
Split into simpler sentences: At first, the mixture was used as-is, both barium and lanthanum together. This however led to unpleasant radioactive contamination that took long time to disappear, due to the 12.5 days of half-life of barium-140.