[HyperSane]

Reminds me of the way nuclear explosions contaminated all steel with radioactive fallout. For applications that require the lowest possible radiation levels they have to use steel created before the first nuclear bomb was detonated.

[kuhewa]

We use where bomb radiocarbon appears along the rings of a fish's earstones to validate ring-counting to age long-lived species.

[HyperSane]

We should detonate nukes at precise intervals like every 1, 5, or 10 years. Maybe with distinct radio-nuclide signatures.

[kuhewa]

Haha it honestly would make a lot of work much easier. Instead we inject a little oxytetracycline into 1000s of fish which stains calcified structures and release them again in the hopes someone recaptures one or so x years later in the hopes they report it and there are y rings between the dark ring and the margin of the otolith/vertebrae.

Should be some type of nuke that would release weird isotopes but with minimal toxicity?

[MereInterest]

That’s true, but it’s more the timescale that helps. There’s a decent amount of radioactive background produced by cosmic rays hitting the upper atmosphere, much of it as gaseous elements that are easily incorporated into steel while smelting. It isn’t harmful at that level, but you do need to wait a few decades for those to decay away.

World War Two is rather convenient in that respect, as there are large quantities of steel that were left to sit around for several decades after those ships sank.

It’s been a while since I’ve done low-background gamma-ray spectroscopy, but I believe there were some setups that went even further, using lead that had been smelted by the Romans. That way, any contamination present at the time of smelting would have a few thousand years to decay away.

[HyperSane]

Wikipedia says that for the lowest radiation levels high-purity copper is used.

[MereInterest]

That would make sense, since the longest lifetime of an unstable copper isotope is ~13 hours, and so chemical separation plus a week of waiting would give you a low background. By comparison, iron has Fe-60, a naturally occurring gamma-ray emitter with a 2.6M year half-life, and 10 million years is too long to wait. For iron shielding, you'd need isotopic separation to remove the Fe-60, which is wicked expensive.