Flexible PMT Base Design for Next-generation LXe Detectors

Designed a compact flexible PMT base integrating voltage division, signal readout, and mechanical adaptability for dense PMT arrays in future liquid xenon experiments.

This project develops and validates a flex-rigid PMT base (FRP) for LXe detectors, integrating voltage division and long-distance signal readout into a single structure to replace traditional cables and hub boards.

Motivation

  • Future LXe detectors require increasingly dense photosensor layouts with strict space and radiopurity constraints.
  • Conventional rigid PCB bases can introduce mechanical interference, routing complexity, and assembly limitations in compact PMT arrays.
  • A flexible base design provides a route toward improved mechanical compatibility while preserving stable voltage division and signal readout.

System Architecture

  • 3×3 PMT module (9 HV channels, 36 signal channels).
  • Flex–rigid structure:
    • Rigid base board: ~0.2 m × 0.2 m
    • Flexible readout tail: ~0.9 m (total length ~1.1 m)
  • Direct connection to flange via Micro-D connectors (no external cable bundle).
  • Cut-out regions introduced to reduce material and background.

Design Evolution

  • V1–V2: FR4 rigid boards + cable/hub → wiring complexity, failure risk.
  • V3: PI rigid base → lower background, validated fabrication.
  • V4: Flex–rigid PCB → integrated base + signal routing, cable-free design.
  • V5 (ongoing): optimized layout and stack-up based on test feedback.

Mechanical & Thermal Validation

  • Repeated bending test: >100 cycles with stable electrical continuity.
  • Thermal cycling: down to ~−37°C and back to room temperature.
  • Observed issues:
    • Partial delamination after thermal cycles
    • Local PI swelling during soldering → Indicates need for improved stack-up and material control.

Electrical Performance

  • High-voltage stability:
    • Tested at 1000–1200 V for 3 hours
    • One discharge observed at 1200 V
  • Noise:
    • ~10 mVpp baseline noise (no shielding)
    • Reduced to ~5 mVpp with shielding (~50% improvement)
  • Signal attenuation:
    • Strong frequency dependence; improves at low temperature
  • Crosstalk:
    • ~10–15% between adjacent traces
    • Strongly dependent on trace spacing, not region location
  • Waveform quality:
    • Noticeable distortion observed → identified as a critical issue for next iteration

Key Engineering Insights

  • Signal integrity is dominated by routing geometry (spacing, layer separation).
  • Shielding layers are essential for noise suppression.
  • HV and signal lines must be strictly separated across layers.
  • Flex–rigid integration introduces new failure modes (delamination, mechanical stress coupling).

Optimization Strategy (Next Iteration)

  • Introduce multi-layer stack-up:
    • Signal / Ground / HV / Ground / Signal
    • Top/bottom shielding layers
  • Increase trace spacing to suppress crosstalk.
  • Improve impedance matching and routing continuity.
  • Evaluate high-frequency materials (e.g., LCP) for reduced attenuation.
  • Strengthen mechanical robustness (thickness, bonding process). Outcome
  • Demonstrated feasibility of a cable-free PMT readout architecture.
  • Established a unified framework linking:
    • mechanical design
    • electrical performance
    • detector integration constraints
  • Identified concrete upgrade directions toward deployable systems in future detectors (e.g., PandaX-20T / xT).