https://doi.org/10.1351/goldbook.09438
X-ray energy at which there is a significant increase of absorption and for which the increased absorption occurs at a greater energy than a core-level binding energy.
Note: For many elements, there is a significant increase of absorption when the X-ray energy is equal to the electron-binding energy for a sub-shell. A delayed onset occurs for some elements and sub-shells when the corresponding increase of absorption occurs, instead, at an energy larger than the sub-shell binding energy.