Study of Gas Permeability across Polymers at Low Temperature

The study of permeability of gases through polymeric materials is a topic of interest in many fields of science and technology: for example, polymeric membranes are largely used both as barriers against atmospheric gases and to separate different gases in medical and industrial fields; polymers with high barrier properties are used for packaging of carbonated soft drinks, where they prevent the permeation of carbon dioxide, oxygen and water; they can also be used to contain greases and oils, which have to be preserved from contact with oxygen. This work focuses on the study of the mechanisms of diffusion and permeation of gases through polymeric membranes at low temperatures, in order to understand if they can be used for Ultra High Vacuum (UHV) applications and for some cryogenic CERN experiments. This study has been performed in the Cryogenics Group, under commission of the group leading the COMPASS experiment (Common Muon and Proton Apparatus for Structure and Spectroscopy®), which has a target made of liquid hydrogen enclosed inside a Kapton tube with two end-caps in Mylar. By the design concept, the experiment requires the diffusion of the hydrogen and helium through these polymers to be studied at low temperature, to properly separate the target gas from the UHV environment of the particle beam. Measurements have been performed in a permeating low temperature set-up, present in the CERN Cryogenics Laboratory; the sample has been tested and two gases, hydrogen and helium, have been used as permeants. To improve the performance of the sample at low temperature an innovative solution has been tested, consisting of diffusing a heavy noble gas, i.e. krypton, through the sample, cooling it down and purging the system before injecting the light gases of interest; in this way, the helium or hydrogen should have a reduced amount of path-ways through the sample, reducing its permeability. The aim of this work is to find the best polymer sample by composition and structure, among the tested candidates, to be employed for the new target of COMPASS, and to establish the temperature dependance of the permeation and diffusion of hydrogen and helium through its constituent material.