Internal calibration of the PandaX-II detector with radon gaseous sources

We have developed a low-energy electron recoil (ER) calibration method with ²²⁰Rn for the PandaX-II detector. ²²⁰Rn, 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 ²²²Rn were on the order of 1% of that of ²²⁰Rn and thorium particulate contamination was negligible, especially in 2018. We also measured the background contribution from ²¹⁴Pb for the first time in PandaX-II with the help from a ²²²Rn injection. Calibration strategy with ²²⁰Rn and ²²²Rn will be implemented in the upcoming PandaX-4T experiment and can be useful for other xenon-based detectors as well.