Why aren't "radioactive items" allowed on flights? I've done physics, I completely understand what nuclear radiation is and why it is harmful. What I can't seem to figure out are the rules surrounding radioactive emitters/items on flights. Surely it's not just any radioactive items, because everything is at least slightly radioactive.

You get about 12 times the sieverts per hour at 35,000 feet than you generally do at sea level (3 μSv/hr in the air, 0.25 μSv/hr at sea level). And even then it's essentially harmless.

What if I had a strong radiation source, but encased in thick lead shielding? Or a strong emitter of only alpha particles? Do they measure it in Sieverts per hour, or Becquerels? Or Gray? All of it makes a difference.

Something that measures high in Gray or Becquerels could be damn near close to undetectable in Sieverts per hour - arguably the only relevant unit in terms of radiation effects on human health.

There's not enough info available to nerds like me, so insight would be greatly appreciated.
Radioactive substances are potentially hazardous to people. 

It is correct that many radioactive items would be perfectly safe to carry onboard, e.g. a key hanger with an alpha or beta particle emitter cast in acrylic, or a radioactive source for scientific purpose, if that source is properly packaged.


Glowing key hanger containing tritium. Image source: Nite GlowRing

Travelers usually do not carry radioactive items with them. If there is the need or wish to bring a radioactive item along, there is normally no need to have it in the hand luggage. This makes radioactive items different from lithium ion batteries. While potentially hazardous, there is a strong demand for lithium ion batteries on board. Banning them would essentially ban mobile phones and laptops.

Airport security staff as well as airline ground and cabin crew are neither qualified nor equipped to inspect and judge the safety of radioactive items. Opening the safe packaging of radioactive items might be dangerous. An item that cannot be opened cannot be checked if something else is hidden inside.

Finally, radioactivity stirs an irrational fear in many (if not most) people.

Given the need to restrict the transport of radioactive material somehow, and the difficulties in controlling it in a more sophisticated manner, the ban of such material from hand luggage seems comprehensible.