It is possible to use radiation to detect a person at a distance.
At the annual Christmas tree-lighting party in New York City’s
Rockefeller Center in November, police pulled six people aside in the
crowd and asked them why they had tripped sensors.
“All six had recently had medical treatments with radioisotopes in
their bodies,” Richard Falkenrath, the city’s deputy commissioner for
counterterrorism, told a Republican governors’ meeting in Miami
recently. “That happens all the time.”
Radioisotopes are commonly used to diagnose and treat certain cancers
and thyroid disorders, to analyze heart function, or to scan bones and
I have read accounts of people who were treated with radioactive iodine getting pulled over in the Holland tunnel where there have been radiation detectors since 9/11. I am not sure this sort of detection would work at great distance but detecting an individual in a car speeding past is pretty good. Also: these radiation detectors are around in some places, just like security cameras. You could hack into their feeds and use them to track your target.
Different radioisotopes decay differently. Radioactive iodine gives off gamma rays and beta particles, and a sensitive detector could distinguish between them. Other radiopharmaceuticals (for example the fluorine used in an FDG PET scan) give off just beta particles. Phosphorus-32 used to be used medically and it decays giving off beta particles but more energetic ones than fluorine. A mix should have characteristics of both.
So you could distinguish between radioisotopes on your target by the types of radiation detected; you should know the half life of what your target has on it, and by time elapsed since it went on you could calculate the energy you expect it to still be emitting. I discussed medically used radioisotopes because there exist real examples but you are not limited in that way - use plutonium as your tag if you want.
Downside: you have smeared radioactive stuff on a person, and not a little bit: if you want them to be detectable at a distance you need something that is kicking out a lot of radiation. The radiation going away from their body is what you detect. Also the gamma rays that go through their bodies and do not stop. But a fair amount of this radiation will go into their bodies and hit something. The people who took iodine-131 did it to destroy their thyroid glands (or thyroid cancers). People who are treated with hot phosphorus are treated to destroy overactive bone marrow or blood producing cells. Radiation is not good for you. Your target may suffer side effects of radiation both acute and delayed.
Other downside: others are watching for radioactive people now. Your plutonium smeared target will be pulled over by Homeland Security if he tries to go thru the Holland Tunnel or attend a Christmas Tree lighting, and he will become aware he has been tagged with radioactive stuff. Although they will probably take care of that for him once he is in their custody.