Low-energy radon backgrounds from electrode grids in dual-phase xenon TPCs

The dual-phase xenon time projection chamber (TPC) is a powerful technology to detect rare interactions such as scatters of dark matter particles on nuclei. In particular, the built-in gain of ionization signals in a dual-phase TPC makes it sensitive to events in the few-electron regime, as expected from low-mass dark matter interactions. The pursuit of this low-energy sensitivity through ionization-only signal detection has so far been hindered by excessive electron backgrounds observed across experiments. Much of this background is attributed to the plate-out of Rn 222 decay chain isotopes on the high-voltage electrode grid surfaces that span the full cross section of the TPC. This work presents a first-principles model constructed for this background, the predictions of which are consistent with data from the LZ and LUX experiments. We then discuss strategies to mitigate this background in future dual-phase TPCs and the possibility of applying this grid background model to ionization-only dark matter searches.