  The ionization yield in the two-phase liquid xenon dark-matter detector has
been studied in keV nuclear-recoil energy region. The newly-obtained nuclear
quenching as well as the recently-measured average energy required to produce
an electron-ion pair are used to calculate the total electric charges produced.
To estimate the fraction of the electron charges collected, the Thomas-Imel
model is generalized to describing the field dependence for nuclear recoils in
liquid xenon. With free parameters fitted to experiment measured 56.5 keV
nuclear recoils, the energy dependence of ionization yield for nuclear recoils
is predicted, which increases with the decreasing of the recoiling energy and
reaches the maximum value at 2~3 keV. This prediction agrees well with existing
data and may help to lower the energy detection threshold for nuclear recoils
to ~1 keV.
