A simple method of velocity selection of arbitrary resolving power for atomic and molecular beams is described. The method consists in spreading the beam into a velocity spectrum, through the action of external fields, and then selecting a portion thereof by means of a movable selector slit. Since all the atoms of the beam of the given velocity interval are utilized, this type of selection has the maximum efficiency. In addition a method of focussing the selected beam to increase intensity and resolution is applied. A ballistic method of using the surface ionization detector was evolved extending its use to beams of exceedingly low intensity. A selected beam of slow Na atoms, obtained and measured in this fashion, was analyzed in a weak and inhomogeneous magnetic field. Four distinct peaks of equal intensity were obtained which represent the ($2i+1$) nuclear magnetic levels. The spin of the Na nucleus is accordingly equal to $\frac{3}{2}$, in units of $\frac{h}{2\pi }$.

• Received 24 August 1934

DOI:https://doi.org/10.1103/PhysRev.46.707