Edge effect on helium outgassing from ULE glass

Determination of the diffusion constant for specific cases can be achieved in different ways. One of them is by using the outgassing of some gas in high vacuum from a solid body. In this paper, the diffusion constant of helium from ULE (ultra-low expansion) glass is determined by using experimental data for outgassing rate of helium from three ULE glass plates with different dimensions. ULE glasses are one of the candidates which may be used in the project for quantum pascal. Usually, the diffusion constant is found by mathematical models which neglect the thickness of the plate. In this paper, a mathematical model for the outgassing of the helium by ULE glass is constructed, taking into account also the thickness of the plates. Optimum values for diffusion coefficient D are determined, such that the mathematical model for outgassing will fit in the best way possible the experimental data. Optimum values for D are calculated based on the fitting by the method of least squares. Values obtained in this manner are compared with the values taken by the previous models which considers plates as infinitely thin surfaces. Thus, the difference between these values, namely the edge effect on the outgassing of helium by ULE glass, is determined.