Methods of preparation and results of sealed gas photomultipliers for visible light

We describe the preparation of a sealed, atmospheric pressure gaseous photomultiplier (GPMT) for the visible spectral range and present the properties of the first prototypes. They consist of a 50 mm diameter semitransparent bialkali photocathode coupled to a 30×30 mm Kapton-made multi-GEM electron multiplier. High gain of 2×10⁴ in two-GEM mode and a quantum efficiency of 13% at 405 nm have been reached at atmospheric pressure of Ar/CH₄ (95:5). The detector structure and experimental setup are described; results are presented on the GPMT gain, ion-feedback and its suppression, stability, and other critical parameters in various gas mixtures. We examine also hot sealing techniques with In/Sn and In/Bi solders.