Studies on photon-feedback and LaB$_6$ photocathode for the GasPM development

This paper investigates and addresses time-resolution degradation in GasPM detectors caused by photon-feedback signals through improved beam tests with high-speed digitizers and cosmic-ray studies utilizing a robust LaB$_6$ photocathode, aiming to enhance performance for future Belle II upgrades.

The Gist

Imagine you are trying to listen to a single, quiet whisper in a crowded, noisy room. That is the challenge scientists face with the Belle II experiment, a giant machine in Japan that smashes particles together to study the building blocks of the universe.

The machine has a very sensitive "ear" (a detector) that listens for specific signals from these collisions. However, the machine is so powerful that it creates a lot of "background noise"—unwanted flashes of light that happen at the wrong time. These flashes confuse the detector, making it hard to hear the important whispers.

To fix this, the scientists are building a new, super-fast "noise-canceling headphone" called the GasPM. Here is how they are trying to make it work, explained simply:

1. The Goal: Catching Light in a Flash

The GasPM is designed to detect light particles (photons) with incredible speed—so fast that it can tell the difference between a signal that happened at the exact right moment and one that happened a tiny fraction of a second later. If it can do this, it can filter out the background noise and save the quality of the experiment.

2. How It Works: The Avalanche Effect

Think of the GasPM like a snowball rolling down a hill.

• The Trigger: A photon hits a special surface (the photocathode) and knocks a tiny electron loose.
• The Snowball: This electron enters a narrow gap filled with gas. A strong electric field acts like a steep hill, accelerating the electron. As it zooms, it crashes into gas molecules, knocking off more electrons.
• The Avalanche: This creates a chain reaction, a massive "avalanche" of electrons that creates a strong electrical signal the scientists can read.

3. The Problem: The "Echo"

In their first tests, the scientists got a good signal, but it was muddy. They realized there was a problem called "photon feedback."

Imagine you shout in a canyon. You hear your voice, but then you also hear an echo bouncing off the walls a split second later.

• In the GasPM, when the electron avalanche happens, the excited gas molecules glow with ultraviolet light (the "echo").
• This light hits the photocathode again and creates a second, smaller avalanche.
• Because this second avalanche happens just a tiny bit later, it overlaps with the first one. It's like your shout and the echo merging into a messy, indistinct noise. This "echo" made the timing measurements blurry, turning a sharp 25-picosecond resolution into a fuzzy 70-picosecond one.

4. The Solution: High-Speed Cameras

To fix the "echo" problem, the scientists upgraded their equipment.

• The Upgrade: They replaced their old recording device with a super-fast digital camera (a 10 GSPS digitizer). This camera takes pictures of the electrical signal 10 billion times a second.
• The Trick: Because the camera is so fast, it can see the shape of the signal in extreme detail. The scientists found that the "echo" (photon feedback) changes the shape of the signal's rising edge in a specific way.
• The Filter: They wrote a computer algorithm that acts like a smart filter. It looks at the shape of the signal and says, "This looks like a clean, single shout," or "This looks like a shout with an echo." By ignoring the "echo" signals, they can isolate the true signal and improve the timing.

5. Testing a New Material: The "Tough Cookie"

The scientists also tried a new material for the light-catching surface called LaB6 (Lanthanum Hexaboride).

• Why try it? The old material (CsI) is like a delicate flower; if a stray ion (a charged particle) hits it, it gets damaged and stops working well over time. LaB6 is like a "tough cookie"—it can withstand being hit by ions and exposed to air much better.
• The Result: Unfortunately, while LaB6 is tough, it wasn't very good at catching the specific type of light they needed (it had low "Quantum Efficiency"). It was like having a very durable microphone that just didn't pick up the sound well enough. So, for now, this material isn't ready for the next big test.

Summary

The scientists are building a super-fast detector to clean up the "noise" in a particle physics experiment. They discovered that the detector was getting confused by its own internal "echoes." By using a super-fast digital recorder to spot and filter out these echoes, they are learning how to make the detector sharp and precise again. They also tested a tougher material to protect the detector, but found it wasn't sensitive enough yet. The work is ongoing to perfect this tool for the future of the Belle II experiment.

Technical Summary

Technical Summary: Studies on Photon-Feedback and LaB6 Photocathode for GasPM Development

Problem Statement
The Belle II experiment requires effective mitigation of beam-induced background photons to preserve the performance of its electromagnetic calorimeter. These background photons, originating from single-beam interactions with residual gas, typically arrive out-of-time relative to collisions and possess energies (1–2 MeV) sufficient to mimic signal deposits. The proposed solution is the Gaseous Photomultiplier (GasPM), a resistive-plate chamber (RPC) coupled with a photocathode, designed to achieve O(10) ps time resolution for precise timing discrimination.

However, previous iterations of the GasPM faced significant performance degradation. While a 2022 laser test using a LaB6 photocathode achieved a single-photon time resolution of 25 ps, a 2023 test utilizing a CsI photocathode and a MgF2 window for Cherenkov applications resulted in a degraded resolution of 70 ps. The primary causes identified were:

1. Photon Feedback: UV photons emitted during the de-excitation of gas molecules (C2H2F4/SF6 mixture) strike the photocathode, generating secondary avalanches that overlap with primary signals.
2. Ion Feedback: Back-drifting avalanche ions damage the photocathode over time, reducing efficiency.
3. Signal Overlap: In the 2023 configuration, signals overlapped within a ~600 ns window, complicating timing determination.

Methodology
To address these limitations, the authors conducted a new beam test using a 5 GeV electron beam at the KEK PF-AR test beamline. The experimental approach involved several key modifications:

• Hardware Configuration: The GasPM prototype was modified with a 150 µm gas gap (increasing the electric field to 187 kV/cm) and a 5 mm thick MgF2 window to enhance photon yield. A soda-glass resistive plate was employed.
• High-Speed Digitization: A 10 GSPS Nalu DSA-C10-8+ digitizer was utilized to capture waveforms with high temporal fidelity, enabling the separation of primary signals from delayed photon-feedback pulses.
• Event Classification Algorithm: Instead of relying on pulse height or charge (which were correlated with multiple processes), the team focused on the signal rising edge. Waveforms were fitted with an 8th-degree polynomial. Events were classified as "photon feedback" if the second derivative of the fit, evaluated 0.2 ns from the boundaries, exhibited two or more zero-crossings. This criterion identifies the multiple curvatures characteristic of overlapping pulses.
• Photocathode Evaluation: The LaB6 photocathode was tested under cosmic rays to evaluate its resistance to air exposure and ion feedback. The setup included a GasPM and a standalone RPC to subtract ionization contributions and isolate Cherenkov signals. LED tests were also performed to assess pure photon detection rates.

Key Results

• Photon Feedback Identification: The rising-edge analysis successfully identified approximately 53.2% of events as undergoing photon feedback. The method relies on the statistical distinction of waveform curvature between single avalanches and overlapping signals.
• Resolution and Separation: The study confirmed that thickening the gas gap increases the time separation between primary and photon-feedback signals, facilitating their distinction. However, the authors noted a trade-off, as larger gap thicknesses negatively impact gain.
• LaB6 Photocathode Performance: Despite its known robustness against ion feedback and air exposure, the LaB6 photocathode demonstrated poor Quantum Efficiency (QE) at the target wavelengths.
  • Cosmic ray data showed hit rates indistinguishable between the GasPM and the RPC (approx. 7.2% vs. 7.7%), indicating that ionization-only signals dominated and the photocathode contribution was negligible.
  • LED tests confirmed the photocathode was responsive to light but yielded a low response rate.
  • Consequently, the authors concluded that the LaB6 photocathode, in its current configuration, is not a viable option for the next beam test.

Significance and Claims
The paper presents a modest but critical step in the development of the GasPM for the Belle II upgrade. The primary contribution is the demonstration of a signal-processing algorithm capable of statistically separating genuine primary signals from photon-feedback noise using high-sampling-rate digitizers. This approach offers a pathway to recover high time resolution in the presence of photon feedback without necessarily requiring immediate hardware changes to the gas mixture or photocathode materials.

While the study successfully characterized the photon-feedback mechanism and validated a suppression strategy, it also delivered a negative result regarding the LaB6 photocathode, ruling it out as a solution for the immediate next phase due to insufficient QE. The work underscores that while the GasPM architecture holds promise for O(10) ps resolution, overcoming the specific limitations of photon feedback and photocathode durability remains an active challenge. Validation of the time resolution using the new algorithm is ongoing.