Sulphonic fluorinated ionomers crosslinked by radical crosslinking of: A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 geq and comprising: from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene ; from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups SO2F; from 0. A phosphor screen is formed on the inner surface of the front substrate. Another approach uses dummy word lines.