I don't get it. Here's what has been said so far:
1) The emitter is typically Americium.
2) It can be diffused into a metal carrier, from which it cannot easily
be removed.
3) There are strict exposure limits for this sort of radiation, and the
limits are stricter for pregnant persons.
To this I add a couple of physical facts:
4) Americium puts out alphas at 5.4 MeV.
5) The stopping range for alphas in this energy range is something like
0.04 mm of water (i.e. very small compared to the thickness of your skin).
So just what is the threat model here? Just how are these alphas going to
get to the fetus? Is there an exposure limit for ingested alpha emitters,
distinct from non-ingestible alpha emitters? Do the regulations spell out
to what lengths one must go to make something non-ingestible? Or am I
asking for rationality in a field where rationality is largely absent?