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

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.

💡 Research Summary

This paper presents a systematic Geant4‑based study of how the vertical placement of the top photosensor array influences XY position reconstruction using the S2 electroluminescence signal in a compact dual‑phase argon time projection chamber (TPC). The authors employ the G4DS simulation framework to model a cylindrical liquid argon target (80 mm diameter, 76.4 mm height) topped by a 7 mm gas electroluminescence region. Seven Hamamatsu R8520‑506 photomultiplier tubes (PMTs) are arranged in a central‑plus‑six configuration, each with a 20.5 mm × 20.5 mm photocathode. The distance h between the PMT photocathode plane and a reference plane 3 mm above the gas‑liquid interface is varied from 0 mm to 50 mm in 5 mm steps. For each configuration, 100 k electron‑recoil events are generated at two energies: 41.5 keV (the combined 83mKr calibration line) and 1.0 keV (to probe the low‑S2 regime). The simulation includes ionization, electron drift, diffusion, and a constant electroluminescence yield across the gas gap, which is discretized into seven 1 mm slices. Optical properties such as TPB wavelength shifting, ESR wall reflectivity (0.98), and a PMT quantum efficiency of 26 % are incorporated.

Reconstruction is performed with a geometrical solid‑angle (GSA) algorithm. For a hypothesized XY position, the solid angle Ω_i subtended by each square PMT is calculated analytically using the arctangent formulation for a rectangular aperture. The contribution from each of the seven gas slices is summed, yielding an expected fractional light distribution P_i(x,y). The observed photo‑electron counts n_i are normalized to ˆp_i, and a χ² metric comparing ˆp_i to P_i(x,y) is minimized to obtain (x_rec, y_rec). Reconstruction bias is quantified by the mean radial deviation ⟨Δr⟩, and resolution by its standard deviation σ_Δr.

The results reveal a clear non‑monotonic dependence of both bias and resolution on h. When the PMTs are placed very close to the gas pocket (h ≤ 5 mm), the S2 light is highly concentrated in the nearest channel, producing a flat normalized PE pattern that is insensitive to lateral shifts; consequently, both bias and resolution deteriorate. Conversely, at large separations (h ≥ 30 mm) the total collected light drops, and the channel responses become increasingly similar, again reducing positional information and worsening performance. Between these extremes, an optimum emerges where the trade‑off between light‑sharing sensitivity and photon‑statistics is balanced. For the 41.5 keV events the optimal height is ≈ 10 mm, while for the 1.0 keV events the optimum shifts to ≈ 5 mm, reflecting the stronger impact of photon statistics at low energy. Radial dependence studies show generally uniform reconstruction across the active region, with modest bias increases near the detector edge where reflections from the ESR‑coated walls and reduced S2 size become significant.

The authors conclude that careful geometry optimisation—specifically selecting an appropriate PMT‑to‑gas‑gap distance—can substantially improve XY reconstruction, achieving millimetre‑scale precision even for sub‑keV S2 signals. They outline a forthcoming experimental program: a small dual‑phase argon prototype with adjustable PMT height will be built to validate the simulation, employing 83mKr, 37Ar, and other low‑energy calibration sources. Additionally, a novel calibration technique using gold‑coated cathode spots to generate time‑separated S3 electroluminescence will provide absolute XY reference points, enabling direct measurement of bias and resolution in real data. This work thus provides both quantitative guidance for future low‑threshold argon dark‑matter detectors (e.g., DarkSide‑LowMass) and a methodological framework for evaluating geometry‑driven performance trade‑offs.