Quantum efficiency mapping of the GaAs photocathode

An accurate quantum efficiency (QE) map is very important in characterizing the performances of semiconductor photocathodes. This paper presents the study of a QE mapping system built on a DC high-voltage GaAs photocathode injector. A focusing lens is used to move the laser spot to different locations on the photocathode and to map the quantum efficiency. The control system and the data acquisition are established by LabVIEW. With a single sampling time of 2.3 s and a resolution of 0.32 mm, several quantum efficiency maps of the GaAs photocathode have been measured during 330 h. Both the distribution and the decay of the quantum efficiency have been observed from these maps as non-uniform. The quantum efficiency in the high quantum efficiency region decays slower than that in other regions. This QE mapping system is critically useful in improving the preparation technique of the photocathode.