  We present a measurement of scintillation efficiency for a few tens of keV
nuclear recoils (NR) with a liquid argon time projection chamber under electric
fields ranging from $0$ to $3\ {\rm kV/cm}$. The calibration data are taken
with $^{252}{\rm Cf}$ radioactive source. Observed scintillation and
electroluminescence spectra are simultaneously fit with spectra derived from
Geant4-based Monte Carlo simulation and an NR model. The scintillation
efficiency extracted from the fit is reported as a function of recoil energy
and electric field. This result can be used for designing the detector and for
the interpretation of experimental data in searching for scintillation and
ionization signals induced by WIMP dark matter.
