Investigation of deposition parameters for VUV reflective coatings on the HADES RICH mirrors

The HADES RICH detector at GSI is equipped with a spherical mirror segments of 433 mm focal length. The complete mirror consisting of six sectors has a diameter of 144 cm. The Al reflection layer was protected by a MgF2 film of the optimal thickness for best reflectivity in the wavelength range of 140 nm <λ<220 nm. Since the optical quality of the film might be spoiled by sticking of residual gases during the vacuum process, highest attainable deposition rates were applied and the switching time Ts from the Al to the MgF2 evaporation was reduced as much as possible. It was not evident if this conception leads to the best results. In order to understand better and to improve the deposition process, needed for the further 12 SIGRADUR® G panels, Ts was enlarged in several steps up to 5 min. Lower deposition rates were tried for the MgF2 evaporation. The substrate temperature was increased up to 570 K. The influence of all these parameters on the VUV reflectivity was investigated.