Studies of photon induced desorption of surface gas within an aluminium vacuum chamber using an X-ray source

The role of photon induced desorption of gases arising from hard X-rays has been investigated within a vacuum chamber of aluminium alloy (Extrudal) as proposed for the new European LEP accelerator. The radiation was derived from a commercial 100 kV X-ray unit, and was introduced into the test chamber of length around 1 m by means of an end window formed of aluminium foil 25 μm thick. In this manner a photon flux density of up to some 109 photons cm2 s-1 irradiated a total surface area of around 4 x 103cm2. The mean energy of the radiation was continuously variable in the range 9-30 keV. Measurements of desorption efficiency appropriate to each species of desorbed gas have been carried out with chamber pressures at around 1.5 x 10-8 and 5 x 10-11 torr attained, respectively, with evacuation at room temperature and following bakeout at 1500C. Studies have further been extended to assess the contribution of the pretreatment of the chamber by glow discharge cleaning in argon gas. In all cases the desorbed gas species under irradiation are predominantly H2 and CO2 which contribute in excess of 80% of the total desorption. The relative preponderance of these species is influenced by the treatments of bakeout and glow discharge cleaning, although the total desorption efficiency at around 10-1 mol photon-1 is not strongly affected by these actions. The desorption efficiencies of all species and at all stages of preparation of the chamber are found to decrease with increasing mean energy of the radiation, a factor of reduction of one third being observed over the range of energies available in the test. (author)