I'm writing something where the characters are looking for someone among a large set of people. They don't know who this person is and they don't know what they're looking for, so their best bet is to look for anything unusual about this person. I already have a method by using which they'll eventually find him, but I want them to muse over possible, reasonable alternatives. One that comes to mind is looking for any radioactive tracer that might have been injected in the subject. However, for this to make sense within the context of the plot, such tracer should be long-lived enough to be detectable at least for years, and safe enough not to appreciably increase the subject's risk of death over short timespans. (If he's more likely to get cancer at age 90, tough luck, but if the tracer will kill him within five, that's not good.) Any ideas?
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$\begingroup$ I'm not sure. I'm looking for an isotope that exists (if it does) in the real world. I'm afraid I would get more fanciful answers over there, which is not what I'm looking for. $\endgroup$– NicolaMar 26, 2021 at 7:58
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1$\begingroup$ Uranium 238 has a half life of 4.5 billion years. You could look here and find something suitable en.wikipedia.org/wiki/List_of_radioactive_nuclides_by_half-life $\endgroup$– WaylanderMar 26, 2021 at 7:59
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$\begingroup$ @Nicola So the chemical aspect of the question is to suggest "a radioisotope that with a long enough half-life to be detectable in a person after years, from some unknown but supposedly appreciable distance and not so high to kill them by radiation exposure and in quantities similar to radioactive tracers." $\endgroup$– user79161Mar 26, 2021 at 8:05
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$\begingroup$ The tracer also needs to be something that is absorbed by the body and not excreted $\endgroup$– WaylanderMar 26, 2021 at 8:17
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1$\begingroup$ @user79161 something like that. Mostly, my problem is that I don't know if all safe tracers are short-lived, or if there is one that is long-lived. How the characters plan to detect it is their problem, we don't even need to know. I'll look into the list suggested by Waylander, I was looking for something like that but couldn't find it. $\endgroup$– NicolaMar 26, 2021 at 8:18
2 Answers
Good candidates for your research would be the fallout residues of atomic bombs having exploded in the world between $1945$ and $1964$. These bombs were made by USA and the former USSR (Russia today) during the "cold war". Each time one of these superpowers had developed a "more powerful" bomb, the other one managed to create a still more powerful one. And so on for about $20$ years. This stupid course stopped in $1964$, because of the level of the fallout radioactivity measured all over the world. In $1964$, the radioactivity from the fallout had much increased all over the world. The whole planet had been contaminated by the winds to $10$% of the limit value where all food would have to be decontaminated before being eaten. So both Russians and Americans knew about this danger. They managed to meet in Geneva and decided to stop their nuclear rivalry. All that were made for "in the interest of the peace in the world". It was magnificent. As a result the radioactivity slowly decreased all over the world. At the end of the century it was back again to the level $1945$.
One of the isotopes present in the fallout is the strontium-$90$, who is assimilated like calcium, and milk contains much calcium. So $\ce{^{90}Sr}$ was found in the teeth of the newborns overall. Its half-life is about $28$ y. It was followed in teeth all over the world from $1964$ to today. It was observed that this isotope disappears from the body with much less than $28$ y. Apparently calcium (and strontium) are naturally and continuously renewed in the body.
Another isotope that can be interesting for you is the radium-$226$ who was deposited on the watch dials before $1964$. Like strontium, radium is assimilated by the body like calcium. Workers that were painting the dials with radium used brushes, and have been contaminated by this isotope. So they are radioactive. They emit constantly gamma rays. They can be detected with a Geiger counter. The level of the radioactivity is not dangerous. But they emit rays. Is it what you want ?
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$\begingroup$ The OP is requesting something that can be used to identify a particular individual, so you would need a radioisotope not commonly found in the general population. The fact that fallout residues from atomic bombs are ubiquitous means they would not be good candidates to meet the OP's needs. $\endgroup$– theoristMar 27, 2021 at 0:05
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$\begingroup$ @theorist Strontium-90 has disappeared today from the emerged surface of the Earth. And the OP's story should happen now. So why not Sr-90 ? $\endgroup$– MauriceMar 27, 2021 at 8:02
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$\begingroup$ My reasoning was that, since that radioisotope was considered ubiquitous in the early 1960's, and since it's only been two $\ce{^{90}Sr}$ half-lives since the above-ground nuclear test ban, the background levels hadn't sufficiently decreased to enable it to be a unique signature for the person being tracked. I've also read that the only mechanism by which $\ce{^{90}Sr}$ disappears from the general environment is by nuclear decay. But if that's not the case, and the background levels are instead, say, 100-fold less than they were in the 1960's, then it might work. $\endgroup$– theoristMar 27, 2021 at 18:14
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$\begingroup$ @theorist The background is much smaller than 100 times less than the situation in 1965. Because all the strontium-90 has been washed away by the rains all over the planet. These nuclides are now diluted in the oceans. $\endgroup$– MauriceMar 27, 2021 at 21:33
One possibility is Technetium-99m . According to wikipedia it is the most commonly used medical radioisotope in the world, so get your person involved in an appropriate medical procedure and you should be sorted.
But you have to be a little careful what you are looking for, Technetium-99m itself decays in around 6 hours by emitting a gamma ray. But the result, Technetium-99, has a half life of ~210,000 years. So given that Tc is so rare in the environment any raised levels of it on the body might be enough for the sign you are looking for.
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$\begingroup$ OK, on reading the comments above and the wikipedia article in full I see "In the body, technetium quickly gets converted to the stable TcO4- ion, which is highly water-soluble and quickly excreted" so this probably doesn't tick the box. But I'll leave it in case it is of interest. $\endgroup$– Ian BushMar 26, 2021 at 13:30
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1$\begingroup$ Tc-99m is applied as pertechnetate. Tc-99m and Tc-99 are removed from the body faster than by their physical half-life alone. After 7 d, only 0.000000051 % of the initial Tc-99m is left in the body, after 100 d, only 0.23 % of any initial Tc-99 is left in the body. You will probably not be able to detect elevated levels of Tc-99 in the body later (Tc-99 is hard to detect anyway). $\endgroup$– LoongMar 26, 2021 at 16:14