Jormungandr, the Snakebot of Doom, is sitting in the ruins of New York City, busily extracting iron and steel from the rubble in order to turn it into railgun slugs - and also extracting any survivors it finds.
So far, it appears to be ignoring the helicopters buzzing around, ferrying in relief supplies to the survivors in Central Park, and picking up the survivors that it exhumes.
This is in distinct contrast to its reaction to an incoming airstrike, an artillery strike and Chinese ICBMs.
Some important people may start to wonder just why Jormungandr ignores helicopters, and may contemplate slipping a nuke aboard one of them, getting it in close, and detonating it.
However, Jormungandr knows more than is realised. It has practically every bird and bat on the the planet bugged, and if someone was seen by a bird loading anything suspicious onto a helicopter, Jormungandr's reaction would be quite different.
Jormungandr and its flying spies have radiation detectors of a sensitivity far beyond the best of those made by humans, but while they can isolate radiation sources weaker than the background, there is still an uncomfortably short passive detection range. However, there are active methods of detecting atomic weapons, mostly involving bombarding the suspicious package with neutron beams and detecting characteristic gamma emissions as tbe neutrons cause the fissionable material to fission.
The problem with this is that plutonium and uranium are still hard to detect even when bombarded with neutrons.
So, Jormungandr is likely to be interested in any helicopter that is carrying cargo that could be a concealed atomic weapon.
The question: What cargoes that might be transported to a disaster relief area by helicopter would be of sufficient size and would emit radiation similar to that which an atomic weapon might emit, and hence could be mistaken for an atomic weapon by Jormungandr?
Modern atomic weapons do not simply contain Uranium or Plutonium as their only radioactive substance. There may well be deuterium and/or tritium even for a pure fission weapon, and a fusion weapon is practically guaranteed to contain Deuterium and Tritium. Also, the krytron or sprytron switches used for detonation pulse synchronisation often contain radioactive substances such as Nickel-63, usually β-emitters.