Hybrid MCP-PMT characterisation on a testbeam with Cherenkov setup

This paper reports the successful testbeam characterization of a novel hybrid MCP-PMT with an encapsulated CMOS ASIC at CERN, demonstrating its capability for single-photon Cherenkov detection with a gain of $10^4$ and a timing resolution of approximately 280 ps.

The Gist

Imagine you are trying to take a photograph of a single, tiny spark of light created when a fast-moving particle zips through a special glass. This is exactly what a team of scientists did at CERN (the world's largest particle physics lab) to test a brand-new "camera" for light.

Here is a breakdown of their experiment, explained simply:

The Goal: Catching a Ghostly Spark

The scientists wanted to test a new type of detector called a Hybrid MCP-PMT. Think of this device as a super-sensitive camera that can see individual photons (particles of light).

• The Challenge: These light particles are incredibly faint and fast. To see them, you need a camera that can amplify the signal (like turning up the volume on a whisper) and record exactly when the sound happened, down to a trillionth of a second.
• The Innovation: This new camera combines a vacuum tube (which multiplies electrons) with a tiny computer chip (called Timepix4) that acts as the digital sensor. It's like putting a high-tech digital brain inside a classic vacuum tube.

The Setup: A Particle Race Track

To test this camera, they set up a mini-race track at CERN:

1. The Racers: They fired a beam of high-speed particles (mostly protons and pions) down a tunnel.
2. The Spark Factory: When these particles hit a special block of glass (a radiator), they created a cone of blue light called Cherenkov radiation. Imagine a sonic boom, but made of light instead of sound.
3. The Lens System: A complex series of mirrors and lenses acted like a giant periscope. They caught that cone of light and focused it into a perfect ring, projecting it onto the new camera (the "Device Under Test").
4. The GPS: Before the light hit the camera, two other detectors tracked the path of the particles to ensure they were going exactly where the scientists expected.

The Experiment: What Happened?

The team ran the experiment for a week, collecting data from thousands of particle collisions. Here is what they found:

• It Worked: The camera successfully captured the rings of light. The size and shape of the rings matched their computer simulations perfectly. It was like drawing a circle on a piece of paper and having the camera draw the exact same circle back.
• The Speed: The camera was incredibly fast. It could tell the difference between two events happening just 280 picoseconds apart. To put that in perspective, a picosecond is to a second what a second is to about 31,000 years. The camera is fast enough to see the difference between a blink and the time it takes light to travel across a human hair.
• The Volume: The camera was operating at a "low volume" setting (low gain). Usually, these detectors need to be turned up very loud to work, but this new design worked well even when the signal was quiet. This is good because it means the camera is stable and less likely to get "noisy" or confused.
• The Count: They counted about 15 light particles per ring. This matched their predictions, proving the camera is efficient at catching these faint sparks.

The Hiccups

It wasn't a perfect run.

• The Reference Clock: They planned to use a separate, ultra-fast clock to time the events, but that clock had some issues and couldn't be used for the final math.
• The Workaround: Instead of relying on the external clock, the scientists used a clever trick. They split the data from each light ring into two groups and compared them against each other. This canceled out many errors and still allowed them to calculate the speed accurately.
• The Jitter: The main reason the timing wasn't even faster (it was 280 ps instead of, say, 50 ps) was that the electronic "front-end" of the camera got a little jittery when handling the small electrical signals. It's like trying to hear a whisper in a windy room; the wind (electronic noise) adds a little fuzz to the sound.

The Conclusion

The team successfully proved that this new hybrid camera works. It can:

1. See single particles of light.
2. Create clear images of light rings.
3. Time events with extreme precision (about 280 picoseconds).

They didn't test it for medical use or future space missions in this specific paper; they simply built a prototype, tested it on a particle beam, and confirmed that the technology works as designed. It's a successful "proof of concept" for a very fast, very sensitive light detector.

Technical Summary

Technical Summary: Hybrid MCP-PMT Characterisation on a Testbeam with Cherenkov Setup

Problem and Motivation
The paper addresses the characterization of a novel photodetector developed within the 4DPHOTON project. This device integrates a vacuum tube design—featuring a transmission photocathode and a microchannel plate (MCP) for electron multiplication—with a pixelated Timepix4 ASIC used as an anode. The primary objective was to evaluate the performance of the first prototypes of this hybrid MCP-PMT in a Ring-Imaging Cherenkov (RICH) configuration, specifically focusing on single-photon detection capabilities, spatial reconstruction of Cherenkov rings, and timing resolution.

Experimental Methodology
The characterization was conducted at the CERN SPS North Area (H8 beamline) using a 150 GeV/c secondary hadron beam (comprising ~80% protons and ~20% pions). The experimental setup consisted of three main subsystems:

1. Tracking System: Two Timepix4 detectors with bump-bonded silicon sensors placed upstream to reconstruct particle trajectories with high precision.
2. RICH System: A light-tight box containing a solid Cherenkov radiator (a metallized plano-convex LA4545 lens) and a complex optical train (including LA4078, three LA4795, and one LA4384 fused silica lenses) designed to project Cherenkov rings onto the Device Under Test (DUT).
3. Timing Reference: A pair of plexiglass bars coupled to standard single-anode MCP-PMTs, intended to serve as a fast timing reference, though issues with this system prevented its use in the final analysis.

The DUT, a Hamamatsu Photonics prototype with a dual-MCP stack and 1D end-spoiling, was operated at a temperature of −10°C. The MCP stack was biased at 2100 V, with specific potential differences applied between the photocathode, MCPs, and the Timepix4 anode. Data acquisition was synchronized via a 40 MHz common clock and triggered by the SPS extraction signal.

Simulation
A comprehensive Geant4 simulation was developed to model the entire setup, including beam interactions, photon production in the radiator, optical transport, and detector response. The simulation predicted a Cherenkov ring radius of approximately 8.66 mm and an average of 70 detected photons per event (accounting for photocathode quantum efficiency), with an intrinsic photon timing spread of ~20 ps.

Key Results

• Spatial Performance: The detector successfully imaged Cherenkov rings. The reconstructed ring radius from experimental data was measured at 8.62 ± 0.01 mm, showing excellent agreement with the simulated value of 8.66 ± 0.01 mm. The mean number of detected photons per ring was 15 ± 1, which is compatible with the simulation prediction of 13 ± 1.
• Gain and Charge Distribution: The detector operated stably at a gain of 9.9 ± 0.1 k𝑒⁻ (approximately $10^4$). The charge cloud generated by the MCP stack was found to be shared across an average of 3.5 pixels, resulting in a per-pixel charge of approximately 2.7 k𝑒⁻.
• Timing Resolution: Due to issues with the external timing reference and the limited resolution of the tracking sensors (~1 ns), the timing resolution was derived internally by splitting Cherenkov ring candidates into two statistically independent subsets and analyzing the time difference distribution. The measured resolution was 400 ± 10 ps for the difference distribution. Correcting for the statistical subtraction, the intrinsic timing resolution was determined to be 283 ± 7 ps.

Significance and Claims
The paper claims that the 4DPHOTON hybrid detector successfully demonstrated reliable operation over a one-week testbeam period. The device achieved single-photon Cherenkov detection with a timing resolution of approximately 280 ps while operating at a relatively low gain of $10^4$. The authors note that the timing performance is currently limited by the analog front-end jitter associated with low-gain signals and the charge spread across multiple pixels. While spatial resolution was not explicitly measured due to optical system limitations, the strong agreement between the experimental ring radius and photon counts with Geant4 simulations validates the detector's geometric and optical performance. The work establishes the feasibility of using Timepix4-based anodes in hybrid MCP-PMTs for high-precision timing applications.