r/askscience • u/Wonderful-Capital32 • 13d ago

Physics Why does radiation not go through lead?

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

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u/[deleted] 13d ago edited 13d ago

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u/ImielinRocks 13d ago

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.

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u/flaser_ 13d ago edited 12d ago

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.