Yes, and even if you have a high-end alpha detector, ingested radioactive material cannot be detected externally if it's only emitting alpha. One would need to take a blood sample or something like that and do more complicated tests.

How did it get detected?

His symptoms were consistent with radiation poisoning and because he was a high profile dissident his case attracted enough attention that the Atomic Weapons Establishment tested his blood and urine.

They performed gamma ray spectroscopy but did not discover any strong signals, except for a very small spike at 803 keV. Some of the scientists were talking about the case and they were overheard by an older scientist who had worked on the UK's atomic weapons program back in the 50s. The early bombs relied on Po-210 and he recognized that 803 keV line as being characteristic of Po-210. Although the decay is almost exclusively via alpha particles, a very small fraction of decays happen via emission of a gamma ray at 803 keV.

Once they had the connection to Po-210 it was straightforward to test for its presence in his body.

> except for a very small spike at 803 keV. Some of the scientists were talking about the case and they were overheard by an older scientist who had worked on the UK's atomic weapons program back in the 50s. The early bombs relied on Po-210 and he recognized that 803 keV line as being characteristic of Po-210

I don't understand. So professionals from the AWE tried to identify the measured radiation lines from memory(?!), instead of consulting a publicly available database of spectral & decay lines? It's been a while since I've done any spectral analysis but I used to use ie.lbl.gov a lot (seems offline unfortunately) and they had a search function[0] that allowed you to filter their entire database of isotopes and decay chains by whatever line you were looking for.

[0]: https://web.archive.org/web/20150408041531/http://ie.lbl.gov...

Polonium-210 is a very rare example of near pure alpha-decay chain, radon decay products are short-lived and emit both beta and gamma particles.

Yes, this is why the idea to use Po-210 was a stroke of (evil) genius. Because of its unique decay profile it is extremely hard to detect. The FSB agents who did this could obtain a small but lethal dose of Po-210 in Russia and then carry it across borders dissolved in a vial of water. In that state it is not dangerous, so the agents had little risk to themselves by carrying it. And it is also undetectable by border security, even if they were monitoring for radiation.

Thanks for all the great insights! But why was the Po-210 not dangerous in that vial of water? It seems many people who got in touch with it got ill: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1767288/

According to another article the glass wall of the vial stops the radiation.

https://www.argonelectronics.com/blog/litvinenko-and-the-per...

That was after they opened it.

Also, from the article:

> The UK Health Protection Agency, which is advising authorities on technical aspects of the case, characterises the contamination in all 12 cases as “not significant enough to result in any illness in the short term,” while “any increased risk in the long term is likely to be very small.”

When alpha particles are stopped by anything, secondary x-rays are created. I wonder if airport x-ray machines are sensitive enough to pick that up.

It would probably just appear as faint noise, and then only if it happened to be at wavelengths that the sensors were tuned to detect, and which the machine's signal processors weren't trying to ignore, and if the image processing was designed to draw attention to it.

Reading more on Po-210, apparently it is used in anti-static objects.

"An antistatic fan made by NRD, of Grand Island, N.Y., contains 31,500 microcuries of polonium 210 — or, in theory, more than 10 lethal doses."

Source: https://www.nytimes.com/2006/12/03/weekinreview/03broad.html