Why use helium atoms?
Using helium atoms to probe a surface holds several advantages compared with X-rays, neutrons or electrons. Hence the lightweight helium atoms in the energy range of 10-200 meV do not penetrate into the bulk material giving rise to the only surface characterization method that is strictly surface sensitive. This means in addition, that using low energetic helium atoms to probe a surface is truly non-destructive to the sample.
Since the helium atoms are neutral they are insensitive to surface charges thus allowing the investigation of insulating materials. Furthermore, as a noble gas, the helium atoms are chemically inert thus offering a completely inert investigation method (chemically, electrically, magnetically and mechenically).
Hence a helium beam is capable of studying the surface structure and dynamics of a wide range of materials, including those with reactive or metastable surfaces.
The drawing on the right side may give you an impression of the whole apparatus.