Yoneda peak (also Yoneda–Vineyard peak) is an enhancement in scattering intensity from scattered radiation propagating as an evanescent wave underneath a surface or another material interface. It appears under an exit angle αf that is close to the critical angle for total external reflection αc. For x-rays and neutrons, critical angles are small. The Yoneda peak is typically observed in off-specular reflectometry and in grazing-incidence small-angle scattering (GISAS), where it is also used for calibration. The phenomenon was described by Yasuharu Yoneda in 1963 and later given a rigorous theoretical foundation by George H. Vineyard.
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