Mean transverse energy, surface chemical and physical characterization of CERN-made Cs-Te photocathodes

Cesium telluride photocathodes are known to offer high quantum efficiencies under UV illumination combined with good lifetimes compared to other semiconductor photocathodes, making them very popular electron sources for particle accelerator applications. The development of photocathode preparation, characterization, and related expertise at a single accelerator laboratory can be challenging, expensive, and time consuming. Recognizing this, we explored the use of a custom-designed ultrahigh vacuum suitcase for transportation of CERN-made (Switzerland) cesium telluride photocathodes to Daresbury Laboratory (UK) for characterization. We report the synthesis and characterization of a batch of four cesium telluride photocathodes corresponding to our second attempt of transport, following design and process improvements through lessons learned from our first attempt. The photocathode characterization involved, where possible, measurements of the surface elemental composition using x-ray photoelectron spectroscopy (XPS), surface roughness with an in-vacuum scanning tunneling microscope (STM), and quantum efficiency (QE) measurements. Transverse energy distribution curves were obtained over a wide range of illumination wavelengths using the transverse energy spread spectrometer (TESS) at room- and cryogenic temperatures, and the values for mean transverse energy (MTE) were extracted. The photocathodes exhibited distinct thicknesses ranging from -50 to -120 nm and significant MTE beyond the photoemission threshold which is attributed to the presence of CsxO and Cs phases, as confirmed by XPS analysis. The photocathode that exhibited no carbon or oxygen contamination was measured to have the highest QE of 2.9% at a wavelength of 265 nm at the end of the performance characterization process. The results presented herein offer an insight into the achievements possible through international collaborations by successfully utilizing long-distance transportation of photocathodes by land under vacuum conditions.