I wasted my high school years on physics and maths, and mostly slept through chemistry, but I believe barium is used as a contrast agent (because it's dense and blocks x-ray energy) rather than a radioactive source in medical imaging. There are radioactive isotopes of barium but I don't think they're used in medicine. And there are radioactive isotopes (iodine, fluorodeoxyglucose) that are ingested for medical scans. (Yeah, I'm an expert after 15 minutes on the inter-tubes.
)
All the same, assuming you can obtain a radioactive source, why drink it at all?
Just carry it in an 8 ounce drink bottle in your carry-on. It's just as likely (I assume) to set off a radiation detector as if ingested.
And for bonus points you can "voluntarily surrender" the bottle at the checkpoint. @:-) That'll teach 'em to throw all the dangerous non-dangerous dangerous liquids in the regular trash can.