Design and performance of the field cage for the XENONnT experiment

TL;DR

This paper presents the design, simulation, and calibration of the field cage in the XENONnT experiment, improving electric field uniformity and charge signal correction for better event reconstruction in a dark matter detector.

Contribution

It introduces a novel field cage design with an independently biased top ring and validates it through simulations and calibration data, enhancing electric field control in the detector.

Findings

Optimized field cage design improves electric field homogeneity.

Calibration data shows charge accumulation is stable over time.

Field correction resolves drift electron lifetime measurement inconsistencies.

Abstract

The precision in reconstructing events detected in a dual-phase time projection chamber depends on an homogeneous and well understood electric field within the liquid target. In the XENONnT TPC the field homogeneity is achieved through a double-array field cage, consisting of two nested arrays of field shaping rings connected by an easily accessible resistor chain. Rather than being connected to the gate electrode, the topmost field shaping ring is independently biased, adding a degree of freedom to tune the electric field during operation. Two-dimensional finite element simulations were used to optimize the field cage, as well as its operation. Simulation results were compared to ${}^{83m}\mathrm{Kr}$ calibration data. This comparison indicates an accumulation of charge on the panels of the TPC which is constant over time, as no evolution of the reconstructed position distribution of events is observed. The simulated electric field was then used to correct the charge signal for the field dependence of the charge yield. This correction resolves the inconsistent measurement of the drift electron lifetime when using different calibrations sources and different field cage tuning voltages.