Quantum efficiency as a function of temperature in metal photocathodes

Abstract

Photocathodes, in which light is used to extract electrons from materials by the photoelectric effect, are the principal electron sources for many linear accelerators and Free Electron Lasers (FELs). There is an increasing interest in the use of superconducting radiofrequency electron guns, which work at cryogenic temperatures, and therefore require photocathodes that work at cryogenic temperatures as well. The primary metric used to quantify photocathode performance is the cathodes Quantum Efficiency (QE), which is the ratio between the number of incoming laser photons and outgoing electrons. The objective of this thesis is to measure the QE of metal photocathodes as a function of temperature. To accomplish this, a photocathode test stand capable of varying the temperature of metal samples from 80 K to 400 K was developed, and copper and niobium samples were tested using it. The QE of copper was found to vary by a factor of more than four over this temperature range, while the QE of niobium showed only slight temperature dependence.