  We calibrate a two-phase xenon detector at 0.27 keV in the charge channel and
at 2.8 keV in both the light and charge channels using a $^{37}$Ar source that
is directly released into the detector. We map the light and charge yields as a
function of electric drift field. For the 2.8 keV peak, we calculate the
Thomas-Imel box parameter for recombination and determine its dependence on
drift field. For the same peak, we achieve an energy resolution,
$E_{\sigma}/E_{mean}$, between 9.8% and 10.8% for 0.1 kV/cm to 2 kV/cm electric
drift fields.
