Hacker News Reader

Comment by ams6110 | Hacker News Reader

What causes "new" lead (whatever that means) to be more radioactive?

I mean, there's nothing really creating lead naturally in the environment, so isn't all lead pretty much the same age?

Part of naturally-abundant lead is Pb-210 (Wikipedia says 'trace', but from my low-background colleagues many talks, Pb-210 is a persistent problem). Pb-210 is part of the uranium-thorium decay chain, and it has the longest halflife (22 yr) after thorium.

If someone refines lead, it pulls out almost all of the impurities, except, of course, lead. As Pb-210 is chemically almost indistinguishable from the rest of the lead isotopes.

If, however, someone many years ago refined the lead, much of the Pb-210 will have decayed, lowering the backgrounds. This effect is important enough that even 50-100 year old lead of reasonable quality is worth expending considerable resources to salvage. One group in our lab spent a couple of people-years removing mercury from contaminated 50-100 year old lead .

The really precious stuff is ancient Roman lead, detailed in the article, that was submerged beneath the ocean for millenia. Not only has the 210 contamination decayed almost completely, but it has been shielded from cosmic-ray re-activation.

You seem to imply the Pb-210 only starts decaying after the refining occurs. I'm as curious about this as the grandparent - why?

Edit: Figured it out in the reddit thread linked elsewhere ( https://www.reddit.com/r/todayilearned/comments/pg14g/til_th... )

The key appears to be isolating the lead from co-occurring uranium which can decay into the Pb-210. It's not that the refined lead is losing Pb-210 any faster, it's just that there's no longer any uranium generating more of it. Fascinating!

I see how you can read it that way -- yes, your inferred 'key' is correct.

It makes me wonder if we shouldn't be smelting a large cache of high-purity lead today and stashing it away deep underground, for the science experiments a few hundred years hence.

Considering the relentless increase of lead production in the past few hundred years [1], with every passing year, old lead is probably becoming more and more readily available, to the point that in another 100 years it might be cheaper to just scrape it off rusted car wheels/etc. than deliberately store it for such a long time. As long as we don't get too greedy with recycling.

[1] http://historum.com/general-history/21841-world-lead-product...

Like, say, in the water pipes of a northern midwest city?

I assume it is for the same reason as with steel[1] - the refinement process involves atmospheric air, and after we started detonating nuclear bombs, we raised the background radiation level of the atmosphere, and thus of the metals we produce.

[1] https://en.wikipedia.org/wiki/Low-background_steel

There's a longer article explaining about Low-Background Steel on Hackaday:

http://hackaday.com/2017/03/27/low-background-steel-so-hot-r...

Atmospheric nuclear detonations have contaminated all un-so-shielded metals: https://en.m.wikipedia.org/wiki/Low-background_steel

Reply on news.ycombinator.com