Electrical conductivity of glass and sapphire cells exposed to dry cesium vapor

We demonstrate that the walls of glass cells become electrically conductive when exposed to a dry cesium vapor, glasses of different compositions leading to similar effects. We find dramatically different results for monocrystalline sapphire cells, which exhibit typical resistances of a few G Omega, nearly five orders of magnitude larger than glass cells in similar conditions. In spectroscopy experiments requiring the application of an electric field, low cell resistances imply leakage currents generating stray magnetic fields. The latter, at the origin of harmful effects in precise tests of the fundamental laws of physics performed in cesium cells, will thus be suppressed in sapphire cells. Moreover, with such cells it becomes possible to place the electrodes outside. A tentative interpretation of the surface electrical conductivity of glass cells is propounded by establishing a connection with the surface coverage by cesium atoms physically adsorbed on the glass surface. This results from the observed dependences of the cell conductance versus Ca vapor density in the mtorr range and versus the wall temperature up to 200 degrees C, which indicates an activation energy of 0.66 +/- 0.05 eV. In the sapphire cell, where there is no hint of surface effects, the conductivity looks instead attributable to collisional processes occuring inside the vapor for Ca number densities greater than or similar to 10(14) at/ cm(3).