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The Center for Bright Beams, A National Science Foundation Science and Technology Center

Mean Transverse Energy at the Thermal Limit

Mean Transvers Highlight image
MTE vs photon energy. Blue and black curves is the data measured in this work and shows near-thermal MTE at threshold. The red points are previous measurements from literature.

The Mean Transverse Energy (MTE) of Cs3Sb has been shown to approach the thermal limit at room temperature when illuminated with light at the emission threshold. The emission threshold was observed to be lower than in previous studies.

MTE is the ultimate limit on the achievable brightness of an electron source. If this performance can be replicated in a full injector with a higher QE, it will enable hard X-ray operation of LCLS-II HE.

•  Quantum Efficiency and MTE have been measured as a function of incident photon energy for several preparations of Cesium Antimonide
•  MTE reached the thermal limit at threshold
•  The MTE is lower than would be expected from ballistic emission, suggesting some thermalization is occurring
•  QE at threshold was also found to be lower than expected, making it crucial to better understand the photoemission process from alkali-antimonides and the role of defect states within the gap.

In summary, this discovery takes a big step towards finding the secret recipe for making brighter and more efficient X-ray sources. It brings us closer to realizing the full potential of advanced X-ray technology, with applications that could range from improving medical imaging to unlocking the secrets of matter at the smallest scales.


A. Kachwala, P. Saha, P. Bhattacharyya, E. Montgomery, O. Chubenko, and S. Karkare, “Demonstration of thermal limit mean transverse energy from cesium antimonide photocathodes,” Applied Physics Letters, vol. 123, no. 4, p. 044106, Jul. 2023, doi: 10.1063/5.0159924. Available: