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 $F=1\mathrm{}$, $m_F=-1\mathrm{}$ hyperfine state was determined to be $6\times {10}^{-12\mathrm{}}$ c${\mathrm{m}}^2$, which implies a positive value of the scattering length.