A number of very serious problems will occur if atmospheric air is not excluded from the window housing interior and housing/liner annulus. Atmospheric air is primarily composed of nitrogen, oxygen, argon, water vapor, and several other gases in minute quantities. Of these constituents, oxygen and water vapor are very deleterious to the heavily leaded shielding glass. Oxygen will combine with the lead to form a grayish lead oxide on the glass surface. Water vapor will cause "fogging" of the interior glass surfaces as the water vapor condenses on the glass surfaces. Water, in a radiation field, also forms hydrogen peroxide which will etch the glass surfaces. Atmospheric air also carries dust which deposits as a film on the internal glass surfaces.
It is therefore mandatory that atmospheric air be excluded from all interior shielding window and annulus glass surfaces if a pristine viewing condition is to be maintained over the useful lifetime of the facility. Two methods, with several variations, are used to do this: gas purge or oil fill.
Dry shielding windows filled with inert gas generally have a modest nitrogen gas purge (argon gas is more suitable because it is 2 ½ times more hydrophilic than nitrogen, but too expensive to use if not recovered) so the gas is constantly circulating and replenished in the window housing, preventing any condensation of the water entrained in the gas from fogging the glass surfaces. Even "dry" nitrogen bottle gas contains up to 6 ppm water content unless a dehumidifier has been installed in the gas supply system to reduce the vapor content to less than or equal to 2 ppm. Therefore, cost of a proper facility inert gas supply system can be substantial.
Shielding windows can also be filled with oil with an inert cover gas under a slight pressure on the top of the oil reservoir / expansion chamber to exclude air from the oil surface. Either nitrogen or argon gas may be utilized, with no purge being necessary on the chamber. Argon gas is most suitable for this application as it is heavier than air and cannot be displaced by air if the supply pressure fails for any reason. Nitrogen, being lighter than air, can be displaced over the oil in the reservoir expansion tank if the supply pressure fails. Because oil has a modest expansion coefficient, small bladders (which have a useable life of at least a year) can be used to maintain a cover gas on the oil reservoir.
Oil windows are far superior to gas windows in respect to seal leakage prevention. In a dry window, a small gas leakage may go undetected and allow an interchange of nitrogen with atmospheric air. However, in an oil window an oil molecule, being much larger than a nitrogen or air atom, cannot pass through a small nitrogen or air atom leak path, and thus maintains the seal integrity of the window.
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Data and information contained in the tutorial was written by Dale A. Tobias, Premier Technology, Inc. and Hienz E. Hoffman and William G. Wash, Schott Glass Technologies, Inc. and may be reproduced only with written consent.
For information, contact Lyle Freeman
Vice President of Business Development
(208) 782-9129 lfreeman@ptius.net
