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.
1 Answer
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.
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$\begingroup$ You are absolutely right. I thought not making that clear would confuse people. I will edit my post so people know this is low to medium energy levels. Do you honestly think it would only be slightly though? Has this been tested? I mean by slightly do you mean 39.9 inches of lead for a medium energy level gamma ray? $\endgroup$ May 15, 2015 at 2:33
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$\begingroup$ By "slightly" I mean that 40 cm of lead (why did you switch to inches?) when cooled to near absolute zero (and assuming the coefficient of thermal expansion is constant) would shrink to 39.6 cm thickness. 1% of 40 is 0.4 not 0.1! $\endgroup$– FlorisMay 15, 2015 at 2:39
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1$\begingroup$ The inches was a typo error. Was talking to someone near me. At the time I made the comment, I did not see the edited piece you added to your comment. That makes much more sense to me now. Thank you. I have a better understanding now. $\endgroup$ May 15, 2015 at 2:46