Measurement of saturated electron extraction efficiency for dual-phase xenon dark matter experiments

We report a new measurement of the electron extraction efficiency (EEE) from liquid xenon into gaseous xenon in a dual-phase xenon time projection chamber (TPC). By using the L- and K-shell capture lines from 37Ar decays and by developing excellent high voltage capabilities, we measured the relative EEE values at the highest extraction electric field strength reported to date. For the first time, a clear saturation of the EEE is observed for extraction electric fields above 7.5 kV/cm in the liquid; and the EEE stays stable at 3% level up to 10.4 kV/cm. We discuss how the observed saturation may help calibrate the absolute EEE values, and present the implications of this result to current and future xenon-based dark matter searches.