yellowcake contains no Co60, just U and its daughter products.
Co60 has a half-life of a bit more than 5 years, so it is not found in nature ever. It would make an OK dirty bomb, though, because a gram emits lots and lots of gamma in its brief live. yellowcake, though, has a half-live of over 100,000 years, so a gram does not emit much energy at any given time (makes a lousy dirty bomb).
Try about 10,000 times that number.