1. Apr 21, 2015

### Sean Conley

I've read that the Tsar Bomb used a lead casing for the second and third stages of the bomb instead of a uranium casing. A uranium casing would have resulted in fast fusion but would have created too much fallout. Typical Hydrogen bombs use uranium casings. A uranium casing would have indeed resulted in fast fusion but would have created too much fallout therefore the soviets chose to use lead instead.

My question is, what happens to lead after it has absorbed so much radiation? Does it change and become another element?

2. Apr 22, 2015

### ChrisVer

I am not sure if that's what applied on nukes....but, lead is a good absorbing matterial for gamma rays.
The gammas are either absorbed (photoelectric effect) or scatter off (compton effect) the electrons. So the gamma rays mainly interact with the lead's electrons and not nucleus,so they don't create other elements.
Lead has a relatively large attenuation (or absorption) parameter $\mu$ which suppresses/brings down the intensity of radiation $I_0$ exponentially with distance:

$I(r) = I_0 e^{-\mu r}$

Last edited: Apr 22, 2015
3. Apr 22, 2015

### Staff: Mentor

The lead nuclei will absorb some of the neutrons that get emitted, and change their isotope. Some neutrons convert stable isotopes to other stable isotopes, but some produce radioactive isotopes that decay afterwards (mainly to bismuth). Still much better than uranium.

4. Apr 22, 2015

### ChrisVer

Last edited: Apr 22, 2015
5. Apr 22, 2015

### Staff: Mentor

A good choice for what? For simulating additional mass of the bomb and absorbing gamma rays, it is not bad (everything more dense is much more expensive).

6. Apr 25, 2015

### Sean Conley

7. Apr 25, 2015

### Staff: Mentor

Outside of nuclear weapons and reactors, you never get enough irradiation to change a significant fraction of your material.

8. Apr 27, 2015

### e.bar.goum

Further, the places you find lead shielding in most nuclear labs is for creating "low background environments" for sensitive gamma-counting experiments, where the amount of radiation is very small. So the possibility of activating the lead enough to be significant is even lower.

Now, there are situations where you do worry about activating your material - in vacuum chamber design. You're never going to "change a significant fraction of your material", but you don't need to do much to make something radioactive, and in nuclear experiments, you tend to get a fair amount of neutrons (not a lot, but enough to worry about).

So to deal with this, in general, vacuum chambers for nuclear physics experiments are made from aluminium rather than stainless steel, as aluminium has a smaller probability of neutron capture than steel.

9. Apr 28, 2015

### Staff: Mentor

In addition to the lower cross-section, aluminium has nice (non-)activation chains. Natural aluminium is 27Al, if it catches a neutron the 28Al quickly (minutes) decays to 28Si which is stable even if it captures one or two additional neutrons.

Natural iron has about 5% 54Fe, if it catches a neutron to become 55Fe it has a half-life of 2.7 years. Too long to wait, too short to ignore. A small fraction of 59Fe with a half-life of 44 days does not help either. And steel has tons of other elements in it, some of them produce other nasty isotopes when activated.