Geometry-driven impact of photosensor placement on S2-based XY reconstruction in a dual-phase argon TPC

TL;DR

This work addresses the challenge of reconstructing the horizontal position from S2 light patterns in dual-phase argon TPCs and shows that detector geometry, particularly the distance between the top photodetector plane and the gas pocket, has a non-trivial influence on XY accuracy. Using a Geant4-based framework (G4DS) with a seven-PMT top array, the authors simulate S2 emission across a gas pocket and apply a geometrical solid-angle forward model to infer XY by minimizing the mismatch between observed and expected light shares. They find a non-monotonic dependence of reconstruction bias and resolution on the PMT height, with energy-dependent optimal heights (approximately 10 mm for 41.5 keV and 5 mm for 1 keV), highlighting a trade-off between light sharing and photon statistics. These results guide geometry optimization for future low-threshold argon detectors and set the stage for prototype validation using Kr-83m and additional calibration approaches such as time-separated S3 references for XY benchmarking.

Abstract

Accurate reconstruction of the horizontal vertex $(x,y)$ from the S2 electroluminescence pattern is essential for fiducialization and background rejection in dual-phase argon time projection chambers. In this work, we perform a Geant4-based simulation study using the G4DS framework to investigate how detector geometry, in particular the distance between the top photodetector plane and the gas pocket, impacts S2-based XY reconstruction. A compact dual-phase argon TPC instrumented with seven Hamamatsu R8520-506 PMTs is simulated with electron recoils at 41.5 keV (corresponding to the ${}^{83m}\mathrm{Kr}$ calibration energy), as well as 1.0 keV to probe the low-S2 regime. The PMT array height is scanned from 0 mm to 50 mm, and XY positions are reconstructed using a geometrical solid-angle (GSA) method with the S2 emission modeled by 1 mm-thick slices across the 7 mm gas pocket. The results show a clear non-monotonic dependence of reconstruction bias and resolution on PMT height, driven by the trade-off between S2 light sharing and photon statistics. These findings provide guidance for geometry optimization in future low-threshold dual-phase argon detectors and will be validated with upcoming prototype measurements.

Figures (3)

• Figure 1: Schematic of the seven PMT array. The square solid line refer to the photocathode area of each PMT; the solid circle refer to the area of the gas pocket; and the dashed circle refer to the area of the liquid argon. The right side zoom-in PMT shows how to define the coordinates of each corner which is used in the algorithm mentioned in section \ref{['subsec:3.1']}.
• Figure 2: Reconstruction bias as a function of the PMT height $h$ for two ER energies. Markers show the mean radial deviation $\langle\Delta r\rangle$ and the error bars indicate one standard deviation of $\Delta r$.
• Figure 3: Reconstruction performance and S2 size versus the true radial position $R$. For each ER energy, $h$ is set to the corresponding near-optimal value inferred from figure \ref{['fig:2']}. Figure \ref{['fig:3a']} and \ref{['fig:3b']} show $R_{\mathrm{rec}}$ versus $R_{\mathrm{true}}$ and the binned mean bias (with error bars) as a function of $R_{\mathrm{true}}$. Figure \ref{['fig:3c']} and \ref{['fig:3d']} show the total detected S2 photoelectrons as a function of $R_{\mathrm{true}}$.