Timing performance of large prototype based on $\upmu$RWELL- PICOSEC detector technology with $10 \times 10\ \mathrm{cm}^{2}$ active area

Beam tests of a large-area ($10 \times 10\ \mathrm{cm}^2$) $\upmu$RWELL-PICOSEC prototype with a CsI photocathode using a 150 GeV/$c$ muon beam demonstrated timing resolutions of approximately 48 ps and 52 ps, validating the technology's potential for high-precision time-of-flight applications.

The Gist

Imagine you are trying to catch a speeding bullet with a camera. If your camera is too slow, the bullet will just look like a blurry streak. But if your camera is incredibly fast, you can freeze the bullet in mid-air and see exactly where it is.

In the world of particle physics, scientists are trying to do the same thing, but with subatomic particles (like muons) moving at nearly the speed of light. The paper you shared is about building a super-fast camera for these particles, specifically a large one that can cover a big area.

Here is the story of their experiment, explained simply:

1. The Goal: Catching Particles in the Act

Scientists at CERN (the place where the Large Hadron Collider lives) are building bigger and more powerful machines. The problem? When they smash particles together, they happen so fast and so frequently that the "pictures" get messy. It's like trying to take a photo of a crowded dance floor where everyone is moving in slow motion, but the camera shutter is stuck open too long.

To fix this, they need detectors that can tell exactly when a particle arrives, down to the picosecond (one-trillionth of a second). This is called "timing."

2. The Invention: The "µRWELL-PICOSEC" Detector

The team built a new type of detector called µRWELL-PICOSEC. Think of it as a high-tech sandwich:

• The Top Bun (The Radiator): When a fast particle hits this layer, it flashes a tiny burst of light (like a camera flash).
• The Filling (The Photocathode): This layer catches that flash of light and instantly turns it into an electron (a tiny electric spark).
• The Bottom Bun (The Amplifier): This is a special grid (the µRWELL) that takes that tiny spark and multiplies it into a huge, loud signal that electronics can hear.

The magic of this design is that the light-to-electron conversion happens in a very tiny space. Because the distance is so short, the signal arrives almost instantly, allowing for super-precise timing.

3. The Challenge: Making it Big

Previous versions of this detector were tiny (like a postage stamp). They worked great, but real experiments need detectors the size of a pizza box (10 cm x 10 cm) to catch enough particles.

The team built a large prototype with 100 little "pixels" (called pads) on it. Imagine a 10x10 grid of tiny sensors, all working together.

4. The Test: The 150 GeV Muon Beam

To test their new "pizza box" detector, they took it to CERN and fired a beam of muons (heavy cousins of electrons) at it. These muons were traveling at 99.9% the speed of light.

They used two ways to read the data:

1. The "Microscope" Method (Oscilloscope): They looked at just one or two pixels at a time with a very expensive, high-speed oscilloscope. This is like looking at a single pixel on a TV screen with a magnifying glass. It gives perfect detail but is slow and hard to scale up.
2. The "Wide-Angle" Method (SAMPIC): They used a special digital chip (SAMPIC) that could read all 100 pixels at once. This is like using a wide-angle lens to see the whole TV screen at once.

5. The Results: How Fast Was It?

The results were impressive, but not perfect yet.

• The Best Time: On two specific pixels, they measured a timing precision of about 48 to 52 picoseconds. To put that in perspective: If a second were the age of the universe, 50 picoseconds would be the blink of an eye.
• The Problem: They expected it to be even faster (around 20 picoseconds, like their tiny prototypes). Why was the big one slower?
  • The "Frosting" Issue: The layer that turns light into electrons (the CsI photocathode) wasn't perfectly smooth or high quality on the big version. It was like trying to frost a cake unevenly; some spots catch the light better than others.
  • The "Table" Issue: The surface of the detector wasn't perfectly flat. If the "sandwich" layers aren't perfectly parallel, the electrons get lost or delayed.

6. The Conclusion: A Work in Progress

The team proved that they can build a large-area detector that works. It's like proving you can build a giant, high-speed camera.

Even though the current version is a bit "blurry" compared to the tiny prototypes (due to the uneven frosting and wobbly table), the scientists are confident they can fix it. They believe that with better materials and a flatter surface, they can get that timing down to under 20 picoseconds across the whole detector.

Why does this matter?
If they succeed, future particle colliders can sort through massive amounts of data instantly, separating the "interesting" collisions from the background noise. It's the difference between finding a needle in a haystack and finding a specific grain of sand in a desert.

In short: They built a giant, super-fast particle stopwatch. It's currently good enough to be useful, but they are still polishing it to make it the world's best timekeeper.

Technical Summary

1. Problem Statement

The High Luminosity Large Hadron Collider (HL-LHC) presents a significant challenge regarding "pileup" (overlapping collision events), which creates background noise that degrades particle identification. To mitigate this, there is an urgent need for gaseous detectors capable of precise timing (sub-nanosecond to tens of picoseconds) at high interaction rates. While previous small-scale prototypes of the µRWELL-PICOSEC technology demonstrated excellent timing resolution (23–37 ps), it remained unproven whether this technology could be successfully scaled to large active areas (10 × 10 cm²) while maintaining performance and uniformity. Additionally, the scalability of readout electronics for hundreds of channels in such large detectors was a critical engineering hurdle.

2. Methodology

The study focused on the construction, assembly, and beam testing of a large-area prototype.

• Detector Architecture:

  • Concept: The µRWELL-PICOSEC replaces the standard 3 mm ionization gap of a µRWELL detector with a compact photon conversion scheme. It consists of a Cerenkov radiator (where high-energy particles emit photons) placed above a photocathode (Cesium Iodide, CsI).
  • Operation: Incident muons generate Cerenkov photons, which release photoelectrons from the CsI cathode. These electrons drift through a narrow gap (100–200 µm) and undergo avalanche multiplication within the µRWELL amplification structure.
  • Prototype Specs: A 10 × 10 cm² active area with a 100-pad readout (1 × 1 cm² per pad). The µRWELL PCB used 50 µm thick Kapton with 80/100 µm holes at a 120 µm pitch.
  • Gas Mixture: Ne:C₂H₆:CF4 (80:10:10) at ambient pressure.

• Experimental Setup:

  • Beam: 150 GeV/c muon beam at the CERN SPS H4 beamline.
  • Reference System: A compact telescope comprising three Triple-GEM detectors (70 µm spatial resolution) for tracking and a MCP-PMT (Hamamatsu R3809U-50) serving as the timing reference.
  • Readout Electronics:
    1. Single-Channel (Oscilloscope): Used for high-precision waveform analysis of specific pads (Pad #45 and #28). Signals were amplified (650 MHz bandwidth) and digitized by a LECROY WR8104 oscilloscope (1.0 GHz bandwidth, 10 GS/s).
    2. Multi-Channel (SAMPIC): To address scalability, a SAMPIC digitizer (up to 128 channels, 8.5 GS/s) was tested to read out all 100 pads simultaneously.

• Analysis Methodology:

  • Timing stamps were extracted by fitting sigmoid functions to the leading edges of the electron peaks.
  • The 20% Constant Fraction (CF) method was used to determine the signal arrival time (SAT).
  • Time resolution was calculated as the standard deviation of the SAT distribution (Detector Time – MCP-PMT Reference Time). Note: The MCP-PMT contribution was not subtracted from the final results.

3. Key Contributions

• Scalability Demonstration: Successfully constructed and tested the first 10 × 10 cm² µRWELL-PICOSEC prototype, proving the mechanical and electronic feasibility of scaling this technology from single-channel to large-area modules.
• Dual-Readout Validation: Validated both high-precision oscilloscope readout for detailed characterization and the SAMPIC multi-channel system for scalable, high-speed data acquisition of 100 channels.
• Performance Benchmarking: Provided the first timing resolution data for a large-area µRWELL-PICOSEC device, establishing a baseline for future optimization.

4. Results

• Timing Resolution:
  • Pad #45: Achieved an intrinsic timing resolution of 51.8 ± 4.1 ps (Total RMS: 57.1 ps) at a cathode HV of 500 V and anode bias of 220 V.
  • Pad #28: Achieved an intrinsic timing resolution of 47.9 ± 1.0 ps (Total RMS: 50.4 ps) at a cathode HV of 465 V and anode bias of 250 V.
  • The results indicate that resolution generally improves with higher cathode voltage due to better electron transport, though saturation effects were observed at very high voltages.
• Uniformity Issues:
  • A 2D scan of the detector revealed significant non-uniformity in signal amplitude across the active area.
  • The authors attribute this primarily to poor quality of the CsI photocathode and flatness issues with the µRWELL PCB surface, which likely caused variations in the drift gap.
• Comparison to Small Prototypes: The large prototype's performance (~48–52 ps) is roughly twice as poor as previous small-scale prototypes (23–37 ps). The authors explicitly state this is due to the manufacturing imperfections (photocathode/PCB flatness) rather than a fundamental limitation of the technology.

5. Significance

• Feasibility for Future Experiments: The study confirms that the µRWELL-PICOSEC concept is scalable to large areas required for future high-energy physics experiments (like the HL-LHC) and potentially medical imaging (Time-of-Flight PET).
• Path to <20 ps: Despite the current performance gap, the authors project that by addressing the identified manufacturing defects (improving CsI deposition and PCB flatness), the technology can achieve <20 ps timing resolution across the full active area.
• Readout Scalability: The successful integration of the SAMPIC digitizer demonstrates a viable pathway for reading out large arrays of fast-timing gaseous detectors without the prohibitive cost and complexity of using hundreds of oscilloscopes.

In conclusion, while the specific prototype tested suffered from manufacturing limitations that degraded its timing performance, the paper successfully validates the architectural scalability of the µRWELL-PICOSEC technology, paving the way for next-generation ultra-fast gaseous detectors.