Measurements of the rate of evaporation of caesium from the tri-alkali photocathode in an alkali-free environment

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Abstract
A Langmuir ion gauge has been used to measure the rate of evaporation of caesium from a tri-alkali photocathode transferred from its activation cell into high vacuum. The variation with temperature has been studied over the range 22-40 °C, and a value obtained for the latent heat of evaporation. The loss of caesium was found to be relatively slow at room temperature, about 5 × 104 atoms cm−2 s−1, and it is concluded that the photocathode is stable in an alkali-free environment. The experimental method employed is thought to be inherently more reliable than that used by other workers, since practically all free caesium, which could condense on the gauge and influence the results, is excluded from the test cell.