Long-lived, non-lethal radioisotope for fiction
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 &quot;cold war&quot;. Each time one of these superpowers had developed a &quot;more powerful&quot; 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 &quot;in the interest of the peace in the world&quot;. 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 ?