Helium Diffraction

In 1930, Immanuel Estermann and Otto Stern obtained a diffraction pattern from a beam of room temperature helium atoms scattered off a lithium fluoride crystal. Estermann and Stern were able to demonstrate that Equation (4.5) applies to helium atoms—and, by implication, to protons and neutrons—as well as to electrons. The de Broglie wavelength of a proton is

$$\lambda_p = 2.9\times 10^{-11}\,[E({\rm eV})]^{-1/2}\,{\rm m}.$$ (4.7)