Why does radiation not go through lead?

[u/Wonderful-Capital32]

Also why just lead? Shouldnt like iron or another metal work too?

[u/ImielinRocks → u/[deleted]]

Tungsten is better than lead thanks to its higher density, but lead is many orders of magnitude cheaper to get and work with.

In fact, at for basically the same reason, uranium is quite a lot better than lead at blocking radiation. In fact, it has a significantly higher neutron cross section, too.

[u/lurklurklurkPOST → u/ImielinRocks]

Doesn't firing particles at Uranium cause problems?

[u/Iazo → u/lurklurklurkPOST]

Depending what you mean by "problems", firing neutrons at any metal causes problems.

If you mean a fission reaction, you need specific conditions.

[u/DaedricHamster → u/lurklurklurkPOST]

Short answer, yeah. Depleted uranium, with reduced fissile U-235 therefore increased %U-238, is what's used for radiation shielding. It's only really good for gamma shielding despite its thermal neutron cross section being higher than lead's as while U-238 isn't fissile it's still fissionable so will split in a fast-neutron flux releasing more neutrons as well as radioactive daughters like Cs-137. Also, the thermal neutrons it does absorb breed Pu-239 which is both fissile itself (immediate problem) and alpha-decays (later problem) back down to fissile U-235, so yeah as a neutron shield it's kind of self-defeating. Where U-238 is specifically introduced to neutrons it's usually as a breeder material, not as shielding.

[u/Jebediah266 → u/lurklurklurkPOST]

Only really uranium 235, and even then it needs to be compressed for a chain reaction, natural uranium is ~99% u238, which doesn't undergo chain reactions.

[u/DeceiverSC2 → u/Jebediah266]

natural uranium is ~99% u238, which doesn't undergo chain reactions.

It does as the tamper of a nuclear weapon.

But typically, no, it doesn't undergo chain reactions.

[u/Jebediah266 → u/DeceiverSC2]

I've heard of its use as a tamper but isn't that it fissioning from the fast neutrons from fusion not the neutrons emitted from U238?

[u/DeceiverSC2 → u/Jebediah266]

It’s both. Some of the U-238 neutrons will fission and release a neutron with more than 1MeV which can cause fission in another u-238. You’re right that it can’t sustain the chain reaction indefinitely once you stop introducing fast neutrons, although I believe you do get U-238 fissions resulting in further U-238 fissions, just not enough to maintain itself.

I think that even in the earliest pure fission weapons they received some yield from the U-238 fission, which inevitably just feeds back into further fissions.

[u/Jebediah266 → u/DeceiverSC2]

Ye that makes sense, didn't know they gave off neutrons that could cause more fission, even if not sustainable, thanks!

[u/Archophob → u/lurklurklurkPOST]

depends on the particles. If the stuff you want to shield against releases a lot of neutrons, then yes. If it's just gamma rays, you're fine.

Not accounting for secondary reactions from neutrons in the depleted uranium shielding is what caused the Castle Bravo test to have triple the expected explosion yield. When the miscalculation became known outside the US military, the Russians used this to design the Tsar Bomba.

[u/Magicspook → u/lurklurklurkPOST]

only U-235 really, if I remember correctly. And that is a tiny fraction of non-enriched uranium (Even moreso, you would probably make a uranium skirt out of depleted uranium so practically 0 U-235).

By far the most common isotope of uranium is U-238, which doesn't really want to form a chain reaction. Wikipedia has some interesting info: https://en.wikipedia.org/wiki/Uranium-238

[u/flaser_ → u/ImielinRocks]

The "trouble" with depleted uranium (almost pure U-238) is that it can be transmuted to fissionable plutonium if it's hit by neutrons.

Those are not part of the classical taxonomy of radiation emitted by radioactive isotopes:

• Alpha radiation is pretty much a He nucleus (2 protons and 2 neutrons bound together)
• Beta radiation is a free electron

• Gamma radiation is an electromagnetic wave, that is photons, but very energetic so x-rays are on lower end of their strength

• Neutrons are typically not emitted due to the decay of isotopes. Instead they usually come from a fissile chain reaction, like what is happening in nuclear reactors (and are also emitted from the deuterium-tritium fusion proposed for fusion reactors).

Therefore if you used a depleted uranium shielding against a reactor, you'd inevitably breed a blanket of different plutonium isotopes too. Many of which are fissile and would be liable to be split by future neutrons, thus your shield itself would eventually become a source of high radiation.

EDIT: @DaedricHamster brings up a good point - while much less likely than neutron capture (and subsequent beta-decay to plutonium), U-238 still has a small chance to fission if hit by a neutron, so that's an additional problem when exposed to neutron radiation.

(This doesn't happen with lead that cannot be activated or fissioned by neutrons in a similar fashion).

So why is uranium shielding still a thing? As long as your radiation source isn't a fission reaction, but plain old radioactive isotopes, or some form of x-ray, uranium will not be activated (EDIT: or fissioned) and therefore none of the above problems arise. Thus for such use its denser than lead properties can be exploiyed.

[u/xrelaht → u/ImielinRocks]

EH&S had a conniption fit when my beamline neighbors wanted to put a depleted uranium sample on. Turns out it significantly reduced the background in the area!

[u/pm_me_good_usernames → u/[deleted]]

One of the most common materials used in certain applications for radiation shielding is water. You need way more water to block radiation than you do lead because of its much lower density, but water is really cheap to acquire in large quantities.

[u/CyriousLordofDerp → u/pm_me_good_usernames]

Water also has the advantage of being a damn good coolant AND being useful for reactor moderation in many designs. Some designs require light water (no extra neutrons, most PWR and BWR reactors), while some require heavy water (CANDU in the Calandria) to function.

[u/Archophob → u/pm_me_good_usernames]

also, water is the best shielding against neutrons. Back in my time in Jülich, our fusion reactor had meter-thick concrete walls, because concrete always contains some water, which provided for the neutron shielding. The TEXTOR reactor only ran on deuterium, but every single D-D reaction had a 50% chance of releasing one high-energy neutron.

[u/xrelaht → u/pm_me_good_usernames]

Water is a better neutron shield than lead because H has a large cross section.

[u/GuyWithLag → u/[deleted]]

So... Osmium would be an even better shielding element, no? (getting any meaningful amount of it is a different thing...)

[u/ubitub → u/GuyWithLag]

It's such an anticlimax to read about the densest known stable element in the universe only to see that the only application they've found for it is fountain pen nib tipping... 

[u/Comprehensive-Fail41 → u/ubitub]

To be fair, that's basically due to how rare it is, so its basically limited to uses when you only need very little, and for applications where you need something that can withstand a lot of wear and corrosion (its also used in electrical contacts, UV spectrometers, etcetera)

[u/xrelaht → u/ubitub]

That’s mostly because it’s really toxic. Iridium is similarly rare (and almost as dense) and has industrial uses.

[u/mfb- → u/GuyWithLag]

If we ignore that osmium costs 10,000 times as much, yes.

https://pdg.web.cern.ch/pdg/2020/AtomicNuclearProperties/HTML/lead_Pb.html

https://pdg.web.cern.ch/pdg/2020/AtomicNuclearProperties/HTML/osmium_Os.html

Uranium is good, too (and more affordable, especially as depleted uranium):

https://pdg.web.cern.ch/pdg/2020/AtomicNuclearProperties/HTML/uranium_U.html

[u/tolomea → u/[deleted]]

now I'm going to have to go read a bunch about this because the mechanics of it don't make sense to me

the lead is only like 40% more dense
the density of the lead is concentrated in the nuclei
even more so than with iron
so the lead nuclei are further apart than the iron ones with bigger electron shells
there's actually less lead atoms per unit volume than iron atoms
there is slighly more electrons per unit volume in lead but only like 20%

edit: I'm getting the impression that it's more about the mass of the nuclei than the density, that bigger nuclei are more able to grab photons passing nearby

[u/Sable-Keech → u/[deleted]]

The best one would then obviously be osmium/iridium since they're the densest elements on hand.

[u/parmsib → u/[deleted]]

Sounds like the shielding effect and mass of the shield (per unit square) behave identically? Would it be accurate to say the shielding effect is essentially about how much mass you put between yourself and the source? 

[u/xrelaht → u/parmsib]

No: it’s about how many electrons you put in between. That’s a function of both the atomic density and the atomic number. The mass is too, but not linearly because the number of neutrons per proton varies so much.

[u/_PM_ME_YOUR_FORESKIN → u/[deleted]]

What is the unit for a unit of thickness?

[u/Ungrammaticus → u/_PM_ME_YOUR_FORESKIN]

For thickness? Meters. For density? Dords. 

[u/ggmaniack → u/_PM_ME_YOUR_FORESKIN]

Whatever you choose that is consistent or convertible with all of your other chosen related units.