Adsorption isotherms of H$_{2}$ and mixtures of H$_{2}$, CH$_{4}$, CO and CO$_{2}$ on copper plated stainless steel at 4.2 K

Adsorption isotherms in the pressure range 10$^{-11}$ to 10$^{-6} Torr have been measured for H$_2$ and mixtures of H$_2$ and CH$_4$, CO and CO$_2$ on copper plated stainless steel at 4.2 K. The measurements have been focused on the behaviour of the isotherms at low surface coverage, up to a few monolayers of adsorbed gas on the cold surface. The isotherms were measured in a static situation with a small amount of gas injected for each point on the isotherm. Co-adsorption measurements of H$_2$ with CH$_4$, CO and CO$_2$ as well as adsorption of H$_2$ on condensates of CH$_4$, CO and CO$_2$ have been made. A cryotrapping effect of H$_2$ is seen when co-adsorbing the gas mixtures, especially strong for the mixture of H$_2$ and CO$_2$. The measurements show that CO$_2$ condensed at 4.2 K may have a porous structure that H$_2$ can penetrate, while CO and CH$_4$ have rather dense structures when adsorbed at 4.2K. The measurements have been done within the framework of the Large Hadron Collider (LHC) project at CERN, Geneva, Switzerland, where superconducting magnets will be used to produce a magnetic field of about 9 T around the arcs of the 27 km long quasi-circular accelerator. Synchrotron radiation produced by the circulating proton beam in the superconducting magnets will induce desorption of neutral gas molecules, mainly H$_2$, CH$_4$, CO and CO$_2$, from the inner surface of the vacuum chamber. The desorbed gas molecules will be physisorbed on the cold surfaces in the vacuum chamber and they may induce vacuum instabilities in the accelerator. Hence it is of importance to have knowledge of the mixed adsorption isotherms of these gases at low temperatures.