From my understanding, the new CT machines are able to characterise material composition using dual-energy X-ray, and this is how they were able to relax the rules.

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jandrewrogers 4 hours ago | parent [-]

I am not up-to-date on the bleeding edge but that explanation doesn’t seem correct? The use of x-rays in analytical chemistry is for elemental analysis, not molecular analysis. (There are uses for x-rays in crystallography that but that is unrelated to this application.)

At an elemental level, the materials of a suitcase are more or less identical to an explosive. You won’t easily be able to tell them apart with an x-ray. This is analogous to why x-ray assays of mining ores can’t tell you what the mineral is, only the elements that are in the minerals.

FWIW, I once went through an airport in my travels that took an infrared spectra of everyone’s water! They never said that, I recognized the equipment. I forget where, I was just impressed that the process was scientifically rigorous. That would immediately identify anything weird that was passed off as water.

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wyldfire 3 hours ago | parent | next [-]

Here's an article that talks about Dual-energy CT [1]. And another one talking about material discrimination using DECT [2].

[1] https://en.wikipedia.org/wiki/Spectral_imaging_(radiography)

[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC2719491/

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jandrewrogers 2 hours ago | parent [-]

Neither of those articles seem to support the idea that you can do molecular analysis with x-rays. They are all about elemental analysis, which is not useful for the purpose of detecting explosives.

	▲	littlecranky67 an hour ago \| parent \| next [-]
		Not sure if they use dual-energy x-ray as in [0], but you don't need to if you take x-ray shot from different angles. Modern 3D reconstruction algorithms you can detect shape and volume of an object and estimate the material density through its absorption rate. A 100ml liquid explosive in a container will be distinguishable from water (or pepsi) by material density, which can be estimate from volume and absorption rate. https://en.wikipedia.org/wiki/Dual-energy_X-ray_absorptiomet...
	▲	don_esteban an hour ago \| parent \| prev [-]
		Hm, isn't it enough to just detect water and flag everything else as suspicious? If your liquid is 80%+ water (that covers all juices and soft drinks), it is not going to be an explosive, too much thermal ballast.

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palata an hour ago | parent | prev [-]

> FWIW, I once went through an airport in my travels that took an infrared spectra of everyone’s water! They never said that, I recognized the equipment. I forget where, I was just impressed that the process was scientifically rigorous. That would immediately identify anything weird that was passed off as water.

Something like 10 years ago, I had my water checked in a specialised "bottle of water checker" equipment in Japan. I had to put my bottle there, it took a second and that was it. I have been wondering why this isn't more common ever since :-).

No idea if it was an "infrared spectra machine" of course.