Adsorption isotherms of H₂ and mixtures of H₂, CH₄, CO, and CO₂ on copper plated stainless steel at 4.2 K

Adsorption isotherms in the pressure range 10⁻¹¹–10⁻⁶ Torr have been measured for H₂ and mixtures of H₂ and CH₄, CO, and CO₂ on copper plated stainless steel at 4.2 K. The measurements have been focused on the behavior 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. Coadsorption measurements of H₂ with CH₄, CO, and CO₂ as well as adsorption of H₂ on condensates of CH₄, CO, and CO₂ have been made. A cryotrapping effect of H₂ is seen when coadsorbing the gas mixtures, which is especially strong for the mixture of H₂ and CO₂. The measurements show that CO₂ condensed at 4.2 K may have a porous structure that H₂ can penetrate, while CO and CH₄ have rather dense structures when adsorbed at 4.2 K. The measurements have been done within the framework of the Large Hadron Collider project [The Large Hadron Collider Accelerator Project, CERN/AC/95‐05 (LHC), 20 October 1995] 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 quasicircular accelerator. Synchrotron radiation produced by the circulating proton beam in the superconducting magnets will induce desorption of neutral gas molecules, mainly H₂, CH₄, CO, and CO₂, 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.