
Physicists searching for dark matter using lead from the bottom of the sea - sohkamyung
https://www.theatlantic.com/science/archive/2019/10/search-dark-matter-depends-ancient-shipwrecks/600718/
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kingbirdy
> its unstable lead-210 isotope would have largely decayed away over the
> centuries into stable lead-206

I know very little about radiation or metallurgy. Why are the few centuries
the lead was underwater relevant compared to the thousands or millions of
years it existed in the ground prior to being mined? Compared to the timescale
that the lead exists on, 100 years either way seems like it should be
irrelevant.

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ISL
What is special is that it was chemically refined as lead in ancient times.

Lead-210 is in the natural Uranium decay chain:
[https://en.wikipedia.org/wiki/Decay_chain#Uranium_series](https://en.wikipedia.org/wiki/Decay_chain#Uranium_series)

The uranium that feeds the chain has a half-life of ~4.5 billion years, so a
handful of good 'ol dirt generally has the entire chain's worth of isotopes
present. Refining the lead out of ore concentrates almost-pure lead (with all
of its isotopes). Lead-210 has a 22-year half-life, so the lead-210 in Roman
lead has almost entirely decayed away at the present day.

Being underwater is _also_ special. Cosmic rays strike nuclei on Earth's
surface all the time, sometimes transmuting them into other, potentially-
radioactive isotopes. They aren't of safety concern, but they are readily
observed in low-background physics experiments. "Cosmogenic activation" is a
serious problem -- the best facilities these days re-refine copper, from which
the experiments are built, deep underground in order to remove cosmogenics.
Similarly, detectors are transported by boat in order to prevent the
activation that happens via air travel.

Ancient Roman lead avoids _both_ problems. It is old, removing the 210Pb, and
has low cosmogenic activation.

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garmaine
Also, nuclear bombs. Iron from old (pre-1945) sunken ships is valued for its
lack of trace radioactive isotopes in making sensitive detection equipment.

~~~
kpU8efre7r
[https://en.wikipedia.org/wiki/Low-
background_steel?wprov=sfl...](https://en.wikipedia.org/wiki/Low-
background_steel?wprov=sfla1)

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yummypaint
The majorana experiment spent years electroforming copper on site (bottom of a
mineshaft) to remove traces of uranium and other contaminants. If it were
brought to the surface it would get activated by cosmic rays so all the
machining also happens underground. I think they finished growing it this
year.

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nkrisc
This makes me wonder if anyone's stockpiling lead in underground vaults to let
it "mature" for scientific use in the coming centuries. Or are there better
way potential ways to make low (or non) radioactive shielding that might be
viable in the coming decades or centuries?

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abeppu
Lay person question: in the same way that we use centrifuges to purify nuclear
fuel, could we not similarly use them to refine the stable isotopes from lead
(or other metals)? If not, is there an interesting reason?

~~~
sohkamyung
You could in theory: but you would have to turn the metals into a gas first as
the process for purifying uranium depends on gas diffusion [1]

[1]
[https://en.wikipedia.org/wiki/Gas_centrifuge](https://en.wikipedia.org/wiki/Gas_centrifuge)

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ta1234567890
Could the black matter effects be explained by thinking of light as having
mass? (my understanding is that light/electromagnetic radiation is considered
as not having mass)

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qtplatypus
Light does have relativistic mass but it is insufficient to account for dark
matter.

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evanb
Light does not have relativistic mass, but it does carry energy. The full
version of Einstein's famous equation is E^2 = m^2c^4 + p^2c^2; a photon has
m=0, so E=pc.

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senectus1
that was a cool read.

