  We report the first measurements of the absolute ionization yield of nuclear
recoils in liquid xenon, as a function of energy and electric-field.
Independent experiments were carried out with two dual-phase time projection
chamber prototypes, developed for the XENON Dark Matter project. We find that
the charge yield increases with decreasing recoil energy, and exhibits only a
weak field dependence. These results are a first demonstration of the
capability of dual phase xenon detectors to discriminate between electron and
nuclear recoils, a key requirement for a sensitive dark matter search at recoil
energies down to 20 keV.
