Photo Cathode Production and Detector Sealing

An ultra high vacuum facility with two vacuum chambers (cathode chamber and detector chamber) enables the production of photo cathodes and their transfer onto a detector body. Using an indium sealing the cathode is soldered to the detector body and thus sealed vaccum tight. The vaccum achieved in this facility is in the range of 10^-10 mbar.

We investigate the production of Cs₂Te and GaN(Cs) photo cathodes.

For producing Cs₂Te cathodes first a tellurium layer is deposited onto the cathode substrate. Subsequently this layer is activated with cesium vapour while observing the photo current. For testing purposes we initially used fused silica discs of 25 mm diameter as cathode window and transfered these onto a ceramics body. The cathode window is soldered with indium to this ceramics body. This ceramics body with brazed kovar electrodes was produced by Empa (Eidgenössische Materialprüfungs-Anstalt) in Dübendorf (Switzerland) in the Laboratory for Joining Technologies & Corrosion.

After completion of the cathode it is measured with an UV monochromator. This monochromator is illuminated by an deuterium lamp, which has a spectral continuum up to the ultraviolet range. By comparison to a NIST calibrated silicon photodiode the absolute quantum efficiency of the cathode is determined. The lower limit at 190 nm of the measurable wavelength range is determined by the fused silica windows and also by the absorption of oxygen in the air. By using a pure nitrogen atmosphere this limit may be reduced to 180 nm.