 In 1931 Rabi returned to particle beam experiments . In collaboration with Gregory Breit , he developed the Breit @-@ Rabi equation , and predicted that the Stern – Gerlach experiment could be modified to confirm the properties of the atomic nucleus . The next step was to do so . With the help of Victor W. Cohen , Rabi built a molecular beam apparatus at Columbia . Their idea was to employ a weak magnetic field instead of a strong one , with which they hoped to detect the nuclear spin of sodium . When the experiment was conducted , four beamlets were found , from which they deduced a nuclear spin of 3 ⁄ 2 . 
