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Why the Aluminum Foil?

(Photo - Tin Foil) Perhaps you've noticed that physicists seem to love aluminum foil. Give them a high-precision, expensive vacuum chamber and what do they do with it? Wrap the whole thing like leftovers.

The real story, of course, is more complicated than an arbitrary love for shiny things. Foil is used for many things in the lab, and it turns out that when it comes to vacuum chambers, aluminum foil is crucial to developing an ultra-high vacuum.

Most vacuum chambers are constructed from stainless steel, which has a tendency to collect substances on its surface when exposed to air. Whenever a steel vacuum chamber is opened or develops a leak, atmospheric air can enter and create a thin film of things like water vapor or hydrocarbons. Once the leak is repaired or the chamber is closed, this residue disrupts the ability to achieve a high vacuum by gradually releasing a stream of molecules as the air is pumped out, preventing the chamber from being emptied enough to conduct research.

To get rid of this film, researchers perform a "bake out" of the equipment. This is where the aluminum foil comes in. In a typical bake out, the equipment is blanketed in foil, wrapped with electrical heat tape, and then covered in foil again. Heat tape is used to heat the metal chamber just enough to loosen any residues that could cause trouble. The aluminum foil helps spread the heat evenly.

After heating for a period, the residues that have been liberated are sucked out with the rest of the air. The newly clean surfaces allow researchers to achieve very high vacuum pressures—on the order of 10-10 Torr. That represents 10,000,000,000,000 times fewer air molecules bouncing around than atmospheric pressure.

óBrad Plummer
   SLAC Today, June 7, 2006

Image: Beamline 5-1's foil-swaddled monochromator. (Click on image for larger version.)