Scintillation and ionization ratio of liquid argon for electronic and nuclear recoils at drift-fields up to 3Â kV/cm

A two-phase argon detector has high discrimination power between electron recoil and nuclear recoil events based on the pulse shape discrimination and the ionization/scintillation ratio (S2/S1). This character is very suitable for the dark matter search to establish the low background experiment. However, the basic properties of S2/S1 of argon are not well known, as compared with xenon. We report the evaluation of S2/S1 properties with a two-phase detector at drift-fields of 0.2-3.0 kV/cm. Finally, the discrimination power against electron recoil background of S2/S1 is discussed.