Temperature effects on lead against radiation I would like to know if bringing lead to near absolute zero temperatures would have any affects on how resistive it is against gamma radiation. It takes 40 centimeters of lead to reduce gamma radiation effects by a factor of a billion (medium energy levels). Since atoms come closer to each other at colder temperatures, I would imagine this would increase the amount of atoms in a given surface area, thus increasing the chances a gamma ray will interact with an atom. Does anyone know if this could possibly reduce the thickness of lead needed to block gamma rays? I have been looking everywhere and have not been able to find this answer. Any help would be much appreciated. I am still very new to physics and have so many questions that I have no one to talk with about, now that I have gotten involved with physics on this level. Thanks, everyone's help is appreciated.
 A: It is the mass of material more than the thickness that determines the stopping power (which incidentally is a function of energy - so you can't simply state "40 cm reduces gamma flux one billion times" without specifying the energy).
Lead has a positive coefficient of thermal expansion - so the same amount of lead will become slightly thinner at colder temperatures. If you take into account that the lead sheet shrinks in all three dimensions, then the number of atoms per unit area goes up. This increases the probability of an interaction.
So yes - the same sheet of lead, cooled down, will be a slightly better shielding material. 
At room temperature the coefficient of thermal expansion is approximately $3\cdot 10^{-5}/\mathrm{K}$ so if you cool it by 300 degrees it will shrink by about 1% in all directions. At that point it will be 3% denser - a sheet of the same thickness will have 3% better attenuation. The same sheet (which got thinner) will have about 2% greater attenuation.
By contrast, changing to denser materials (eg tungsten, gold or uranium) would give a much bigger jump in shielding effectiveness per unit thickness.
