Question

In an atomic beam experiment a collimated beam of neutral Na atoms traverses a vacuum chamber for a distance of 10 cm. How good a vacuum is required for the beam to remain well-collimated during the transit?

Answer 1

To keep well collimated the atoms should suffer no scattering from air molecules.The condition for well collimated is

where

where n is the density of air molecule and is cross section of collision between Na atoms and air molecule.

Thus

roughly

L=10cm

so

So air molecule less than is required for well colimated beam