We have developed a low-energy electron recoil (ER) calibration method with 220 Rn for the PandaX-II detector. 220 Rn, emanated from natural thorium compounds, was fed into the detector through the xenon purification system. From 2017 to 2019, we performed three dedicated calibration campaigns with different radon sources. We studied the detector response to 𝛼, 𝛽, and 𝛾 particles with focus on low energy ER events. During the runs in 2017 and 2018, the amount of radioactivity of 222 Rn were on the order of 1% of that of 220 Rn and thorium particulate contamination was negligible, especially in 2018. We also measured the background contribution from 214 Pb for the first time in PandaX-II with the help from a 222 Rn injection. Calibration strategy with 220 Rn and 222 Rn will be implemented in the upcoming PandaX-4T experiment and can be useful for other xenon-based detectors as well.

Dual-phase noble-gas time projection chambers (TPCs) have improved the sensitivities for dark matter direct search in past decades. The capability of TPCs to reconstruct 3-D vertexes of keV scale recoilings is one of the most advantageous features. In this work, we develop two horizontal position reconstruction algorithms for the PandaX-II dark matter search experiment using the dual-phase liquid xenon TPC. Both algorithms are optimized by the 83mKr calibration events and use photon distribution of ionization signals among photomultiplier tubes to infer the positions. According to the events coming from the gate electrode, the uncertainties in the horizontal positions are 3.4 mm (3.9 mm) in the analytical (simulation-based) algorithm for an ionization signal with several thousand photon electrons in the center of the TPC.

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