Looks like it says “Co-60” (could be 66 but I really doubt it), but nowadays Cobalt-60 isn’t really used for diagnostics. Could be an old source; the half-life is around 5 years so rule-of-thumb would be 40 years to be fully depleted. Main use these days is for sterilizing materials I think.

E: wikipedia says medical sources that were used were ~2cm wide so, this looks like an old medical source.