measuring the scattered atoms after the pulse. The He beam velocity ranges from 644–2037 m/s. The other involves measuring the momentum of the scattered He atoms by a LiF grating monochromator. It is important to note that the He nozzle beam source used in many He scattering experiments poses some risk of error, as it adds components to the velocity distributions that can mimic phonon peaks; particularly in time-of-flight measurements, these peaks can look very much like inelastic phonon peaks. Thus, these false peaks have come to be known by the names "deceptons" or "phonions". Comparison of techniques EELS and helium scattering techniques each have their own particular merits that warrant the use of either depending on the sample type, resolution desired, etc. Helium scattering has a higher resolution than EELS, with a resolution of 0.5–1 meV compared to 7 meV. However, He scattering is available only for energy differences, Ei−Es, of less than about 30 meV, while EELS can be used for up to 500 meV. During He scattering, the He atom does not actually penetrate into the material, being scattered only once at the surface; in EELS, the electron can go as deep as a few monolayers, scattering more than once during the course of the interaction. Thus, the resulting data is easier to understand and analyze for He atom scattering than for EELS, since there are no multiple collisions to account for. He beams have a capabilities of delivering a beam of higher flux than electrons in EELS, but the detection of electrons is easier than the detection of He atoms. He scattering is also more sensitive to very low frequency vibrations, on the order of 1 meV. This is the reason for its high resolution in comparison to EELS. References Quasiparticles Bosons