In a future police state, I need a way for investigators to determine who was present at a protest that turns violent.

My idea is that police use flashbang grenades to disperse the crowd that leave a radioactive trace on the participants. Agitators who wear disguises can be detected later at security checkpoints or through timely sweeps of the 'usual suspects'.

It needs to be:

  • mostly harmless but difficult to remove
  • a half-life of days (not years)
  • rapidly decays from the site, so that people who enter the area afterwards (20 minutes?) can be eliminated.

It functions as a plot-point from the POV of an investigator who eliminates an 'eyewitness' that actually arrived shortly after. It is explained in-story only to the level the reader understands how the investigator knows without a doubt the witness could not have been present as they claim.

What non-technical details can I drop to the reader (isotope, range, half-life) that indicate it's purpose as a temporary tracer (not a poison or kill-weapon)? Would some alternative to radioactivity (a chemical?) be a better choice?

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    $\begingroup$ You want smoke, not a flashbang. I mean, there's nothing wrong with a flash or a bang, but the smoke particles are what's going to stick to the participant's hair and skin and clothing. $\endgroup$ Feb 13, 2022 at 19:28
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    $\begingroup$ A frame challenge (I don't have enough information to put it on an answer, though): Use high pressure water hoses to disperse the crowd but add a special paint to the water that can only be removed via special means or is only visible under UV light $\endgroup$
    – Josh Part
    Feb 14, 2022 at 19:51
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    $\begingroup$ @smci, that has the problem that an officer can't test for it reliably and quickly in situ. And if you're going to throw enough hardware at it for all that, I think it might be easier to just build up CCTV and follow people screen by screen, which has the advantages of being harder to spoof and easier to convince a jury they have the right suspect. Supposing juries are a thing for OPs police state. $\endgroup$
    – user8827
    Feb 14, 2022 at 19:57
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    $\begingroup$ Clarification: Is the public aware of the tracer? Because if they are, it has to be something that can't be washed off with simple showering and laundering. Even if they're not aware, if you want it to last days you have to have a society that doesn't bathe much $\endgroup$ Feb 14, 2022 at 20:56
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    $\begingroup$ Why does it have to be radioactive? There are chemicals that can be detected by scanners, have the half-life properties you want, and aren't dangerous. The big problem with radioactive tracers is that short-half-life also tends to mean dangerous; it's half-life is short because it decays quickly and releases a lot of radiation in the process. $\endgroup$
    – Ryan_L
    Feb 15, 2022 at 18:22

14 Answers 14


A similar trick was used by the KGB to track possible defectors: they would spread radioactive dust on their door mat, and would simply scan with a Geiger counter all the vehicles, looking for the one which would set it off.

However the half life of an isotope doesn't depend from the place where it is. So, if it has an half-life of few days, it cannot decay from the site in 20 minutes.

A way for you to tell who was exposed to the flashbang from who went to the place afterwards is the location of the radioactivity: when John is exposed to the flashbang, the isotope will be located on all his clothes, skin and hair exposed to the dust produced by it; on the other hand Tina, who walked in the place half an hour later, will have the radioactive dust concentrated mostly on the sole of her shoes and around the bottom of her legs.

Wikipedia has a whole list of isotopes with half life of few days: gold 196 (6 days) seem a decent choice (considered the circumstances).

Also don't forget that an isotope dispersed as dust will inevitably be inhaled, and inhaled radioactive dust will hardly be harmless. Therefore I would slightly prefer gold or other biologically inert atoms above other elements, as it should not be absorbed by the organism like for example lead or calcium.

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    $\begingroup$ Considering gold 199 decays 100% into elemental mercury through beta radiation, I'd be fairly seriously concerned about the health effects of this. $\endgroup$
    – Daniel B
    Feb 13, 2022 at 17:29
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    $\begingroup$ @DanielB, I didn't look into the decay chain... good point... $\endgroup$
    – L.Dutch
    Feb 13, 2022 at 17:40
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    $\begingroup$ Re, "...inhaled..." The secret police probably don't feel deep concern for the long-term health of the people they oppress. Anybody who was at the event, instead of sitting at home watching state-run TV is a troublemaker who deserves whatever bad thing happens to them. $\endgroup$ Feb 13, 2022 at 19:32
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    $\begingroup$ For story purposes you could probably posit the existence of a radioactive element with a half life of a few days, where neither the element nor any of its decay products are readily absorbed by the body. The investigator doesn't know the exact elements and physics of how it works, so neither will the reader. The point about a later arrival being tagged on their feet and lower legs makes a nice story point then. The lower level cops believe the witness is plausible at first, because they show up on the scanner, until the investigator cleverly figures out the distribution is wrong. $\endgroup$
    – Ben
    Feb 14, 2022 at 0:53
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    $\begingroup$ @DanielB it's likely that a concentration needed to be picked up by a geiger counter is going to be small enough that there shouldn't be too much health risk. It's not like any amount of mercury is doing harm or you couldn't eat Tuna at all. $\endgroup$
    – Turksarama
    Feb 14, 2022 at 3:48

How about something like SmartWater?

It's a well established technology that uses polymer chains or similar information encodable chemistry (a lot of the technology seems to be a trade secret) that can be sprayed onto a person for the purposes of identification after a crime has been committed. The chemicals that SmartWater uses are designed to be difficult to remove and degrade quite slowly over months, but it's possible to use chemicals that degrade more quickly when exposed to heat or UV light, or have your police force hose down the area afterwards with an enzyme solution that breaks up the chemicals, leaving only trace amounts in the environment.


I have two ideas. One is high-tech, and the other is low-tech.

1 Spray the protestors with a non-lethal substance that only shows up under blacklight.

Basically, the grenade would contain a certain kind of dust that only glows under certain wavelengths of light, that way it is invisible to the protestors when they get home, but fully visible when the police officer comes to their house with a flashlight. This makes weeding out the people who were and were not at the protest significantly easier. All you have to do is check for the invisible paint.

The substance would have to be waterproof so they cannot simply shower it away but perhaps would eventually vanish after about two weeks or maybe a month.

This is not a fool-proof solution, but it would make the jobs of the police officers a lot easier. If a person is suspected of being at the protest site, just run them over with a blacklight and see if the grenade got them. If not, they're probably clear.

2 Depending on how advanced the technology of the era is, nanobots could be an option as well.

The grenades explode and leave hundreds of tiny little machines as traces on the people that were present at the protest. Each of the little robots gives off a certain signal that is capable of being traced by the machinery that the police have, so they can track every move of the protestors.

They do not even have to be microscopic trackers, necessarily, just devices that are small enough to latch onto the protesters and give off a signal that the police are able to trace through their devices.

I would imagine tiny machines that have sticky barbs. They latch onto clothing and are very hard to get off, or perhaps they can even latch onto bare skin. After a few days, they will fall off on their own, but until then they are there to stay.

There you have it, two ways that you can easily and non-lethally determine who was at the protest or not.

To explain how they come off, you could just say that the barbed machines deactivate once the culprit is caught, and the paint/glowing dust is simply temporary. No complex scientific jargon is required.

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    $\begingroup$ Your lowtech solution is a variation of the paint that security trucks currently use. If you open the secure container poof instant 360deg spray of glowinthedark paint. Lasts for days. $\endgroup$ Feb 14, 2022 at 11:57
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    $\begingroup$ If we imagine a society with a lot of cameras in public places (shocking thought, I know), they could be equipped with uv-lights, making it easier to spot “enemies of the state” $\endgroup$
    – Stefan
    Feb 14, 2022 at 17:17
  • $\begingroup$ Downside: lots of other things also glow in the dark under UV, creating massive potential for false positives. Cotton clothing, for instance. $\endgroup$
    – Vikki
    Feb 15, 2022 at 7:00
  • $\begingroup$ This (and the comments) is referring to UV flourescence. UV reflectance imaging with a UV-pass filter (cuts everything else) could be passive and probably not require a detectable UV light source (e.g. blacklight) for first-approximation screening. With properly selected dyes,the UV image could act as a mask for the image to help highlight sections of an image. UV reflectance can work with sunlight $\endgroup$
    – Yorik
    Feb 15, 2022 at 17:40

Taggants are already used or are proposed to be used in explosives. The idea is to leave behind traceable tags so the origin of the explosive can be traced. Some of these are are little microscopic pieces of plastic that can be coded like a bar code.

There have been other ideas of having codes in microscopic materials for counterfeiting applications. Some of these vary the reflected light over the spectrum or have the emission output at different wavelengths.

So essentially similar to glitter getting the tags out could be a problem. They don’t have to be obvious either since they can be small.

You could also engineer these to degrade or have a half-life without the disadvantages of being radioactive.


As Dutch said in his answer, if you use any amount of radioactive material that you could trace afterwards, that will hardly be harmless due to people inhaling dangerous radioactive dust.

Let me propose a non-radioactive alternative: sperm. Yes, you read it right. It can be chemically treated to be very thin. Once it makes it into people's clothes it can be found by smart usage of luminol. It takes a lot of washing to properly remove it from fabric, and won't stain stuff visibly as blood (which is what luminol is mostly used for) would. Once initial screening is done you can match perpetrators to flashbang grenades via DNA. Last but not least, this solution puts the bang in flashbang.

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    $\begingroup$ This invites outrageous fan fiction, however. $\endgroup$ Feb 14, 2022 at 1:07
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    $\begingroup$ I always knew all protestors were wankers $\endgroup$ Feb 14, 2022 at 12:44

Police already use specific colored ink for this purpose. Hong Kong police used blue ink in 2019 to identify protestors after the event.

In 2011, Montreal police used invisible ink for the same purpose.

  • $\begingroup$ It's also an upcoming trend in shops to use a paint-like substance in case of a robbery, mounted like a spray above the door. If the robber is arrested a few minutes later, they can't deny they were at the location. $\endgroup$
    – Mast
    Feb 16, 2022 at 9:01
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    $\begingroup$ @Mast: Interesting. Thanks for letting me know. That reminds me a of potential anti theft device I saw on a TV news item in the early to mid 1970s, where store owners could have what effectively was a closed & locked bucket attached to the top of a bench where they could place excess cash from the till via a slot in the top. If anyone tried to remove the container some type of ink cartridge would rupture covering the money & the thief with ink that didn't wash off. They had to wait weeks for new skin cells to replace the ink stained ones. $\endgroup$
    – user81881
    Feb 16, 2022 at 9:18

Use artificial DNA fragments instead.

These are already easily to manufacture in the laboratory and then packaged in microscopic cell like membranes which would then in turn be packed into a grenade. Each DNA 'packet' contains DNA encoded in such a way that the strands can be read as specific digits. Just type the string of digits (say 6) on the side of the grenade or their box and then when used you have a record of the which grenades were used at a particular riot or protest. The codes are machine readable via handheld devices and (if designed properly) hundreds if not thousands of microscopic particles would stick to the hair, skin or clothing of a protestor. They can even be made to fluoresce at a chosen light frequency and can be extracted from a person' using test strips that are simply inserted into a reader.

The code would tell you what incident the person was linked to and the density of coverage would tell you if they were directly involved or just have traces due to air drift or transient physical contact with a protestor.

Not perfect, there are chances of false positives but if a person is detained on suspicion of involvement and wasn't at the location of the incident at the time it occurred that can usually be determined afterwards.

Also you could reduce the chance of false IDs significantly if you switched from convention 'chemical grenades' to paint ball style weapons firing soft capsules filled with DNA Packets that burst on impact covering a specific target or possibly only those immediately to the right or left of the target as well rather than everyone for meters in all directions as could happen when a grenade explodes. This would put more particles on individual rioters with less chance of random scattering via the wind etc.


Frame challenge: wrong tool for the wrong job.

Flashbang grenades are more formally known as stun grenades. This should give you a pretty clear idea of their use: to stun, by use of a bright flash and loud noise. When being suddenly and violently blinded and deafened, you probably aren't going to run away very far, if at all. It isn't a weapon designed to disperse a crowd, it's designed to incapacitate before you run it and make an arrest.

You wouldn't use a stun grenade to "paint" a bunch of people for the simple and good reason it's not what stun grenades do. They don't fragment. They don't expel gas. They just make noise and sound. And you wouldn't use them for crowd control because it may cause non-temporary blindness or deafness, and can also set flammable material on fire. As far as I know, no riot control police unit uses stun grenades.

Tear gas grenades are better suited for crowd dispersal. They don't stun, rather they are designed to be extremely uncomfortable. They will impair vision (because tears), but they don't really incapacitate. It doesn't fragment, but it does emit a cloud of gas locally. Being a gas, it's wont to get into your eyes and lungs, so consider that if you don't want to be too harmful.

But even if tear gas is used for crowd dispersal, and you could conceivably use it to mark people, I think there is a much better alternative.

The water cannon. Water has a few advantages. It's readily available from fire hydrants. With the right amount of pressure, it's less-lethal but also not pleasurable, inciting people to disperse. Despite what you might read on the internet, dihydrogen monoxide is a non-lethal and very safe chemical. This is why it is also used in the real world.

The water alone can be used to identify people at a protest. If properly soaked, you wouldn't dry up before a few hours, so that would be a simple and decent, although easily defeated, tell.

You can mix a "colorant" with your water, preferably a non-toxic and food-safe colorant, and one that leaves a really-hard-to-clean mark. From there, you can quickly identify someone who was at the protest because they would be literally painted, and traces might remain for days or weeks. You can add a radioactive element to your paint, so the trace could remain longer. You could also use a UV or IR component so that it would be harder for the target to see if they've successfully removed it or not.

The colorant here could be a paint, or something that acts as a paint, in which case you have something similar in principle to dye packs.

It could also be a solid like glitter, and if you've experienced glitter you know it can be quite difficult to completely remove, but it's still harmless (except maybe to the eyes as a fashion choice).

Don't be afraid by the visible nature of the paint. Firstly, your system will become known after the first time you deploy it. Whether its effect is visible or not, protesters should assume they've been painted. Secondly, in the immediate aftermath, your security forces can perform the check with zero additional requirement. No blacklight, no scanner, just a pair of eyeballs.

This is all very low tech, very understandable by a reader, and even relatable. The obvious downside here is that it will also be difficult to remove when you paint the road and kerbs with it.

Here's another alternative and/or complement: Paintball. You may have played with those recreationally. You could easily tweak the paint to be harder to remove, but otherwise it's a non-lethal weapon as long as you don't aim for the head, but it's still quite painful enough to incite dispersal.

This could be used for more targeted painting, since this you can aim at specific people, for instance protest leaders. You could use a distinctively different marking so that they would be flagged differently at your security checkpoints.

While paintballs aren't used by police, similar weapons exist that throw little balls with tear gas, so it wouldn't be completely out of place.

  • $\begingroup$ +1 for Paintball. Police would enjoy a lot more crowd control :-) $\endgroup$ Feb 14, 2022 at 12:48
  • $\begingroup$ Paintball is going to be very risky if you use it for crowd control. There's a reason you have to wear a mask while playing. $\endgroup$
    – Erik
    Feb 16, 2022 at 9:10
  • $\begingroup$ @Erik I agree with that. Whether that stops a government from using them is another matter. For reference, a similar weapon in terms of lethality, which has been deployed in protests in the last decade by France. Some people have reported losing an eye to it, and we're a liberal democracy. I don't see this as a major obstacle for a police state. $\endgroup$ Feb 16, 2022 at 10:14
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    $\begingroup$ Give every agent a different colour, and you got a scoring system waiting to happen. $\endgroup$
    – vinzzz001
    Feb 16, 2022 at 10:54

Breathable chemical tracer

As other answers pointed out some kinds of tagging is already used. And a chemical tracer might be safer and more effective than a radioactive tracer.

If you want something new you could invent a chemical tracer that is sprayed around and settles in the mouth and in the lungs when people breath. Then they will exhale it slowly for few days afterwards. It could be picked by both chemical sensors like the ones for explosives used in the airports and by trained dogs.

After being sprayed it would take less of an hour to deposit on the ground, people passing later may kick some of it in the air, but they would not breath enough of it to trigger the sensors.

Could people get rid of it? They could try and wash their mouth, but removing it from the lungs would be challenging.

The only difficult detail will be to find out how it can pass through few layers of face masks.


Uranium 237 might be a potential answer. It has a half-life of just short of a week, 6.75 days. It is more dangerous than some due to its beta-decay. The decay can cause some burning, and is somewhat dangerous if ingested. The use of uranium-237 would be fairly detectable with geiger counters or the equivalent, and any ingested uranium would necessitate medical treatment, but could be passed in 1-2 weeks, which could be used by your authority if someone came in with mysterious internal burns.


I would advocate for a flashbang that has an all-caps FLASH. I.e. the protestors get flashed with neutrons.

This has the upside of..

  • the grenades keeping fresh (if you load them with a short-halflife element, they will stop being useful real quick),
  • the radioactive elements being created being part of the protestor's body, thus not simply off-washable
  • having an effective date-stamp (by using the relative concentrations of the decaying new elements), and possibly even recording their position in the crowd (exposure sites giving the angle, exposure depth giving the distance)
  • you get to call it 'going nuclear' on the protestors, police can also do a one-liner before firing, and then everyone goes 'oooh burn'
  • if you can influence the velocity of the neutrons, you can choose to set your phasers to kill, as well, depending on the situation.
  • nice psychological effect on the protestors: they cannot know whether the flash just now marks the end of their life, their reproductive facilities or 'just' their anonymity.

Iodine 131 is common (easy to get if you have the permits), has a half life of 8 days, and a high energy gamma (360 keV) that is easy to detect. As for mostly harmless, that is a matter of amount. About 1 MBq (27 uCi) will not do much harm (it will slightly increase cancer risk, if ingested).


A lot of radioactive isotopes are used in medical imaging... So if you use 4 of those isotopes in a barcode at 100 different levels relative to one another, perhaps in microplastic adhesive coatings which stick to the dudes/in oil...

Then you'd have 100,000,000 different barcode combinations of the 4 isotopes. Then you can track everything using some special reader system perhaps which reads a complex geiger reading, i dunno.



As @AdmiralPatate said, flashbangs are the wrong tool if you are looking for crowd dispersal. For one, they're really dangerous. For two, they're bad at dispersing crowds.

Instead, as he said, you're looking for something like tear gas. Tear gas is really unpleasant to be around because it's spicy. But tear gas is also relatively short lived: it makes staying here painful, and going away much more attractive. You can get a similar effect from various stinky compounds, with the added bonus that the stink can be detectable by a normal human nose for a long time.

Putrescine is one traditional example (IIRC, Stephenson wrote it into Zodiac with that exact use), butt there are a whole range of mostly sulfur-based compounds with truly astounding olfactory effects.


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