I do not have the biosafety data for neutron radiation nearby (there are some nice calculators for beta and gamma emitters online), but I strongly suspect that

I do not have the biosafety data for neutron radiation nearby (there are some nice calculators for beta and gamma emitters online), but I strongly suspect that 18g of californium lie somewhere in between of "instant death" and "certain death in a few hours" (closer to the "instant death"). Anyway, it is also an alpha emitter, so the entire scenario is impossible -- it would be white hot in such amounts. Regarding that airport thriller -- you can't bring any nuclear materials (particularly neutron emitters) through any major airport, they are all equipped with radiation detectors and neutron detectors, specifically to prevent that. I do know a thing or two about nuclear forensics; unfortunately, I do not have any talent for writing fiction, so this thriller is not coming anytime soon. :)