PURPOSE: To permit simultaneous measurement of the evaporation amount of alkali atoms deposited on the surface of an electron emitting crystal, which is used as an electron source utilizing the photoemission phenomenon, and a work function value of the crystal surface after evaporation, using a tungsten thin plate.

CONSTITUTION: An evaporation amount of Cs deposited on a GaAs plate 2 is measured using a tungsten (W) thin plate 3. Since all incident Cs atoms are ionized to Cs+ ions on the surface of the heated W plate 3, an evaporation amount of Cs can be determined by measuring the current of Cs+ ions. Then, after cleaning the surface of the same W plate 3, a work function of the GaAs plate 2 is calibrated using a kelvin probe 5. Thereafter, the work function of the GaAs plate 2 is actually measured using the kelvin probe 5. Light 9 is irradiated to the GaAs plate 2 to emit photoelectrons, thereby to measure the angular dependence of energy distribution using a spherical grid 8 and a Faraday cup 6. Finally, a spherical anode 7 is placed over the grid 8 to measure the spatial distribution of photoelectrons.