Atomic absorption evaporation flow rate measurements of alkali metal dispensers

The preparation of photosensitive surfaces in phototubes is generally perfoemed with alkali metal dispensers (AMDs). Since a very well controlled evaporation flow rate of the alkali vapours released from AMDs are of great importance, a method based on the atomic absorption technique, has been developed for its accurate measurement. This method can overcome some limitations of other methods such as those based on the work function change of cathodes in the presence of alkali vapours. The test AMD is mounted in a specifically designed glass cell which is evacuated and baked overnight at 350°C to have a residual pressure in the range of 10^?6 Pa. The cell is provided with a Ba getter to ensure a good vacuum, essential for the application of this technique, and with an Sb generator to capture the alkali metal vapour. After tip-off, the cell is mounted on the atomic absorption spectrophotometer in the burner position. An absorption signal is generated by the alkali atoms released by the AMD, when they intercept a monochromatic light beam resonant with the alkali metal. With an appropriate calibration the absorption signal can be converted into evaporation flow rate. This technique has been applied to the study of caesium generators and practical results are given. The sensitivity of the method is a flow of about 1 μg min^?1.