There’s an establishing scene where the composite character "Ulana Khomyuk” takes a wipe and swipes a sample of dust on the outside of her window, then runs it though what I assume is some type of gamma-ray spectrograph, and quickly reports back that it’s Xenon-135, so that means the fallout was from a reactor, not a bomb.
But - why? Is this realistic?
Xenon-135 is a fission product of both U-235 and PU-239. One thing I can think of is the relative purity of the spectra. If it was a bomb, would the fallout be a huge mess of radionuclides? Since Xenon is a gas, maybe the fact that it was “pure” points to a leak, rather than an air-burst?
The Wikipedia article on Xenon states that both reactors and bombs produce Xenon-135, but only the reactor produces Xenon-136.
One could therefore assume that the script writers tried to make that scene short and boring so it would appeal to the ignorant masses, and got it slightly wrong.
The real question here is why you would swipe some dust to analyze the presence of a gas…
A slight nitpick: 135Xe is a beta emitter with an energy of 1.168 MeV and a half life of t1/2 ~ 548 minutes. Because it is a neutron absorber (often referred to as a “neutron poison”) it can only build up to certain levels in a reactor and be emitted at very low rates from the reactor when it is functioning normally. Large amounts of 135Xe would be indicative of a massive excursion, and if detected outside of the reactor cell (Chernobyl #4 did not have a containment dome) would be indicative of a massive radiation escape.
I can’t speak to why detection of 135Xe would be specific to a reactor breach rather than the detonation of a fission weapon but measuring the relative ratio of 135Xe and 135Cs would give (with a half life of t1/2 ~ 15 minutes) would give a window on when the excursion occurred. A nuclear detonation would produce a lot of secondary radionuclides from the tamper and casing that would not be present in a reactor. Upon review I see that @Humbagger identified that 136Xe is the result of neutron absorption by 135Xe that would only occur in the moderated fission process in a reactor but I don’t know that it would be produced in quantities sufficient to make a verifiable detection from a single sample.
It could be entrained in dust residue but again probably not at quantities identifiable from a single swipe. Naturally occurring 136Xe is a “primordial nuclide” produced by stellar nucleosynthesis processes and with a half life about eleven orders of magnitude longer than the current age of the universe so amounts greater than those naturally occurring in the atmosphere but I think you’d have to get a large sample. Airborne detection of above ground nuclear testing requires sampling thousands of cubic feet of air, and again they are typically looking for casing-derived radionuclides. This kind of seems like something a screenwriter thought would sound good rather than the actual detection methods used on-site but I haven’t seen the series so I do not have the context for the scene.
Actually, if this is the scene you are talking about, she says “Iodine 131”. That at least makes sense as far as dust goes. But again, it would be the ratio between two isotopes that matters.
The part that makes it really hard to use in detection is of course the really low decay rate. In practice, Xe-135 and Xe-133 seem to be used, see this figure from a research paper.