A new carbon based VUV mirror of high radiation length for the HADES RICH

We report on the development and production of a spherical (f=0.436 m) VUV mirror for the HADES RICH detector. Glassy carbon substrates of X0=28 cm radiation length were fabricated with a thickness of d=2 mm and an overall slope error SSE80⩽0.85 mrad. After coating with thin Al/MgF2 layers, the measured reflectivities in the wavelength region 120 nm<λ<240 nm yielded R=82% at λ=150 nm.