Collisional properties of sympathetically cooled K-39

We report experimental evidence of the sympathetic cooling of K-39 with Rb-87 down to 1 mu K, obtained in a tight-confining magnetic trap. This allowed us to perform a direct measurement of the elastic cross section of K-39 below 50 mu K. The result obtained for the triplet scattering length, a(T)=-51(7) Bohr radii, agrees with previous results derived from photoassociation spectra and from Feshbach spectroscopy of K-40.