Edge effect on helium outgassing from ultralow expansion glass

Abstract

Determination of the diffusion constant for specific cases can be achieved in different ways. One of them is by outgassing some gas in high vacuum from a solid body. In this paper, the diffusion constant of helium from ultralow expansion (ULE) glass is determined by using experimental data for the outgassing rate of helium from three ULE glass plates with different dimensions. ULE glasses are one of the candidates that may be used in the project for quantum pascal. Usually, the diffusion constant is found by using mathematical models that neglect the thickness of the plate. In this paper, a mathematical model for the outgassing of helium by ULE glass is constructed, taking into account also the thickness of the plates. Optimum values of the diffusion coefficient D are determined, such that the mathematical model for outgassing will fit in the best way possible the experimental data. Optimum values of 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 previous models that consider 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.