Development and characterization of hybrid MCP-PMT with embedded Timepix4 ASIC used as pixelated anode

We present a novel single-photon detector based on a vacuum tube incorporating a photocathode, a microchannel plate (MCP), and a Timepix4 CMOS ASIC functioning as a pixelated anode. Designed to handle photon rates up to 1 billion per second across a 7 cm$^2$ active area, the detector achieves outstanding spatial and temporal resolutions of 5-10 $μ$m and below 50 ps r.m.s., respectively. The Timepix4 ASIC comprises approximately 230,000 pixels, each integrating analog and digital front-end electronics. This enables data-driven acquisition and supports data transmission rates up to 160 Gb/s. External FPGA-based electronics manage both configuration and readout. In order to test the timing performance of the Timepix4 ASIC we performed preliminary characterization of an assembly bonded to a 100 $μ$m thick n-on-p silicon sensor using a pulsed infrared laser, which demonstrated a per-pixel timing resolution of 110 ps, with cluster-based averaging methods improving to below 50 ps. Six prototype detectors incorporating different MCP stack configurations and end-spoiling depths were produced by Hamamatsu Photonics. We report on their characterization, including dark count rates, gain, and spatial and timing resolutions, assessed both in laboratory conditions and during a test-beam campaign at CERN’s SPS facility.

💡 Research Summary

The paper presents the development and comprehensive characterization of a novel hybrid single‑photon detector that integrates a vacuum‑tube photomultiplier architecture with a state‑of‑the‑art Timepix4 CMOS ASIC serving as a pixelated anode. The detector combines a high‑quantum‑efficiency photocathode, a micro‑channel‑plate (MCP) amplification stage (Chevron or Z‑stack, 6 µm channel diameter, 7.5 µm pitch), and a bare Timepix4 chip (448 × 512 pixels, 55 µm pitch, 195 ps TDC bin). The ASIC is bump‑bonded directly to the ceramic carrier that also seals the vacuum tube, providing a compact, high‑rate (up to 10⁹ photons s⁻¹ over a 7 cm² active area) solution with sub‑10 µm spatial and sub‑50 ps timing capabilities.

Key technical contributions include:

• Hybrid architecture – The electron cloud emerging from the MCP is deposited directly onto the ASIC’s pixel pads, eliminating intermediate readout stages and preserving the intrinsic fast response of the MCP.
• Timepix4 performance – Detailed calibration of per‑pixel threshold equalization, VCO frequency correction, and time‑walk compensation yields a single‑pixel timing resolution of σ≈107 ps (after subtracting a 72 ps reference contribution). When multiple pixels belonging to the same charge cluster are averaged, the resolution improves dramatically to σ≈33 ps, confirming the benefit of multi‑pixel sampling.
• Prototype production – Six MCP‑PMT variants were fabricated by Hamamatsu, differing in the number of MCP stages (2 or 3) and end‑spoiling depth (1D, 2D, 3D). A custom ceramic carrier board, designed by INFN and manufactured by Kyocera, provides mechanical sealing, high‑speed pin‑grid interconnects, and thermal coupling for the ASIC’s 5 W dissipation.
• Thermal and environmental control – Active liquid cooling of the ASIC and MCP, together with a dry‑air purge maintaining ~5 % relative humidity, prevents condensation while keeping the detector at ~0 °C, thereby reducing dark noise.
• Dark count and gain – Dark count rates remain below 30 Hz cm⁻² for MCP bias > 2.2 kV. Gain scales linearly with MCP voltage, reaching ~240 ke⁻ per photo‑electron at 2.4 kV, as measured in the central, most uniform region of the pixel matrix.
• Timing with the full hybrid – Operating the MCP at 240 ke⁻/ph.e. and attenuating a picosecond infrared laser to a single‑photon probability of 0.1, typical clusters span 3–4 pixels. The resulting single‑pixel timing is ≈95 ps, while clusters of ≥4 pixels achieve ≈65 ps, limited primarily by the ASIC’s TDC granularity and the reference timing jitter.

The authors demonstrate that the hybrid detector meets its design goals: large active area, high photon‑rate capability, and combined spatial‑temporal precision surpassing most existing MCP‑PMTs. Remaining challenges are the non‑uniform gain across the pixel matrix and the intrinsic ASIC timing floor; ongoing work focuses on ASIC front‑end optimization and further MCP stack refinements to push timing below 30 ps and dark count rates even lower.

Overall, this work establishes a new platform for applications requiring simultaneous high‑resolution imaging and precise time stamping, such as Ring Imaging Cherenkov detectors in high‑luminosity colliders, time‑of‑flight PET, and quantum optics experiments. The integration of a mature vacuum‑tube amplification stage with a modern, data‑driven CMOS readout opens a pathway toward scalable, low‑noise, ultra‑fast photon detection systems.