Evaporative cooling of sodium atoms

We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the {ital F}=1, {ital m}{sub {ital F}}={minus}1 hyperfine state was determined to be 6{times}10{sup {minus}12} cm{sup 2}, which implies a positive value of the scattering length.