Recent GasPM advances: photon-feedback mitigation and LaB$_{6}$ photocathode studies

This paper reports recent advancements in GasPM technology aimed at mitigating photon-feedback-induced time-resolution degradation through improved beam tests with a high-speed digitizer and exploring the robustness of LaB$_6$ photocathodes for future detector upgrades.

The Gist

Imagine you are trying to listen to a single, quiet whisper in a room where a massive, chaotic party is happening. The "whisper" is a tiny signal from a particle collision in a physics experiment, and the "party" is a storm of background noise created by the machine itself.

This paper is about a team of scientists trying to build a super-sensitive microphone (called a GasPM) that can hear that whisper clearly, even while the party is raging. They are working on an upgrade for a giant particle collider in Japan called Belle II.

Here is the story of their journey, broken down into simple concepts:

1. The Problem: The "Echo" in the Room

The Belle II collider smashes particles together to study the universe's origins. However, the machine creates a lot of "background noise"—unwanted photons (light particles) that hit the detectors at the wrong time. These noise particles confuse the scientists, making it hard to see the real data.

The scientists need a detector that can tell the difference between a signal from the collision (the whisper) and the background noise (the party chatter) based on timing. They need to know exactly when a particle arrived, down to a trillionth of a second.

2. The Solution: The GasPM Microphone

The team built a device called a GasPM. Think of it as a high-tech sandwich:

• The Bread: A special window and a "photocathode" (a surface that turns light into electricity).
• The Filling: A thin layer of gas.
• The Mechanism: When a particle hits the gas, it creates a tiny spark (an avalanche of electrons). This spark is amplified to create a readable signal.

It's like a snowball rolling down a hill: one tiny snowflake (a photon) hits the top, and by the time it reaches the bottom, it has gathered enough snow to become a massive, easy-to-see avalanche.

3. The Hiccup: The "Ghost Echo" (Photon Feedback)

In 2022, the team made a great microphone that could time things perfectly (25 picoseconds!). But in 2023, when they tried a different setup, the timing got messy (70 picoseconds).

Why? They discovered a problem called "Photon Feedback."

Imagine you are shouting in a canyon. You shout (the primary signal), but the sound bounces off the walls and comes back to you as an echo (the secondary signal). If the echo comes back too fast, it mixes with your original shout, making it sound garbled and confusing.

In the GasPM, when the gas spark happens, it emits its own tiny flash of light. This light hits the sensor again, creating a second spark right on top of the first one. It's like the microphone hearing its own echo and getting confused about when the original sound actually happened.

4. The Fix: The Super-Fast Camera

To fix this, the team went back to the drawing board with a new plan:

• Tighter Sandwich: They made the gas layer thinner to speed things up.
• Better Glass: They used a thicker window to catch more light.
• The Secret Weapon: They installed a 10 GSPS digitizer.

Think of the old digitizer as a camera taking 5 photos per second. It missed the fast echo. The new one is a super-slow-motion camera taking 10 billion photos per second.

With this super-speed camera, they could look at the shape of the electrical signal and see the "bump" caused by the echo. They wrote a computer algorithm (a set of rules) to act like a smart editor: "If I see this specific wobble in the signal, I know it's an echo. I will ignore it and only record the original shout."

5. The New Material: The "Indestructible" Sensor

The team also tested a new material for the sensor's surface called LaB6.

• The Old Material (CsI): Like a delicate flower. It works well but gets ruined easily if exposed to air or if charged particles (ions) drift back and hit it.
• The New Material (LaB6): Like a tough cactus. It can survive the harsh environment of the particle collider and the "backwards drift" of ions much better.

However, the cactus isn't very good at "hearing" light yet (it has low sensitivity). The team is currently testing it with cosmic rays (natural particles from space) to see if they can make it sensitive enough to be useful.

The Bottom Line

The scientists are building a super-timed, noise-canceling microphone for the Belle II collider.

1. They identified that the machine was "hearing its own echo" (photon feedback).
2. They built a super-fast camera to spot and delete those echoes.
3. They are testing a tougher sensor material that won't break down in the harsh environment.

If successful, this will allow the Belle II experiment to ignore the background noise and hear the "whispers" of the universe much more clearly, leading to new discoveries about how our universe works.

Technical Summary

Here is a detailed technical summary of the paper "Recent GasPM advances: photon-feedback mitigation and LaB6 photocathode studies."

1. Problem Statement

The Belle II experiment at KEK utilizes a CsI crystal electromagnetic calorimeter for precision physics measurements. However, the detector is highly sensitive to beam-induced background photons (energies 1–2 MeV) originating from processes like radiative Bhabha scattering and beam-gas interactions. These background photons are often "out-of-time" relative to the collision event and degrade calorimeter performance.

To mitigate this, the collaboration developed the Gaseous Photomultiplier (GasPM), a photosensor combining a photocathode with a Resistive-Plate Chamber (RPC) to achieve $\mathcal{O}(10)$ ps time resolution. While a 2022 test achieved a promising 25 ps single-photon time resolution, a 2023 beam test using a CsI photocathode and MgF2 window resulted in a significant degradation to 70–73 ps. The primary causes identified were:

• Photon Feedback: UV photons emitted during gas excitation/de-excitation trigger secondary avalanches, creating delayed signals that overlap with the primary signal.
• Ion Feedback: Positive ions drifting backward damage the photocathode (specifically CsI), reducing efficiency over time.
• Signal Overlap: The 2023 configuration (200 $\mu$m gap, lower electric field) resulted in slower electron drift, exacerbating pulse overlap.

2. Methodology

The authors conducted a new beam test at the KEK PF-AR facility using a 5 GeV electron beam to address the time-resolution degradation. The methodology involved:

• Hardware Optimization:
  • Geometry: Reduced the gas gap to 150 $\mu$m and increased the electric field to 187 kV/cm to improve electron drift velocity.
  • Window: Increased the MgF2 window thickness to 5 mm to enhance Cherenkov photon yield.
  • Materials: Switched the resistive plate to soda glass and utilized a LaB6 photocathode in a separate cosmic-ray study to test resistance to ion feedback and air exposure.
  • Readout: Upgraded the digitizer to a 10 GSPS Nalu DSA-C10-8+ (from 5 GSPS) to capture finer waveform details.
• Algorithm Development:
  • Since direct correlation between charge and photon feedback is diluted in beam tests compared to laser tests, the team focused on the rising edge of the signal.
  • They fitted the waveform (pedestal to peak) with an 8th-order polynomial.
  • Selection Criterion: Events were tagged as "photon feedback" if the second derivative of the fit exhibited at least two zero-crossings within the rising edge (excluding fixed margins near the pedestal and peak). This indicates multiple curvature changes inconsistent with a single avalanche.
• Validation:
  • Compared the rise-time distributions (50% to 100%) of selected events.
  • Conducted a cosmic-ray test with the LaB6 photocathode to measure hit rates and isolate ionization-only signals.

3. Key Contributions

• Photon Feedback Mitigation Strategy: Successfully designed and executed a high-sampling-rate beam test specifically to isolate and characterize photon feedback.
• Novel Signal Processing Algorithm: Developed a robust algorithm based on the second derivative of the signal rising edge to distinguish between single-avalanche events and those contaminated by photon feedback.
• Material Investigation: Initiated a study on LaB6 photocathodes as a potential alternative to CsI, specifically targeting resistance to ion feedback and air exposure, despite initial concerns about quantum efficiency (QE).
• Hardware Upgrade: Demonstrated the necessity of high-frequency digitization (10 GSPS) for resolving sub-nanosecond signal overlaps in gaseous detectors.

4. Results

• Event Classification: The new algorithm successfully classified (53.2 ± 2.3)% of beam-test events as being affected by photon feedback.
• Timing Validation: The distribution of the 50%-to-100% rise time showed a clear qualitative separation between "single avalanche" events (cyan) and "photon feedback" events (orange), validating the selection criterion.
• LaB6 Performance:
  • In cosmic-ray tests, the GasPM hit rate was (7.19 ± 0.49)%, consistent with the RPC-only rate of (7.66 ± 0.18)%.
  • This suggests the observed signals were primarily due to ionization rather than photoelectric effect, indicating the quantum efficiency (QE) of the LaB6 photocathode is currently too low for the target Cherenkov UV wavelengths.
• Time Resolution: While the paper focuses on the method to improve resolution, the updated determination of time resolution (after applying the algorithm) is noted as "ongoing," implying the 70 ps degradation is expected to be mitigated by removing the feedback-contaminated events.

5. Significance

This work represents a critical step toward the operational deployment of GasPMs for the Belle II detector upgrade.

• Background Suppression: By achieving $\mathcal{O}(10)$ ps time resolution and effectively suppressing photon feedback, the GasPM can distinguish collision photons from out-of-time beam backgrounds, significantly improving the signal-to-noise ratio for the electromagnetic calorimeter.
• Scalability: The GasPM offers a cost-effective, large-area alternative to Micro-Channel Plate PMTs (MCP-PMTs) for Cherenkov detectors and timing applications.
• Future Direction: The study highlights the trade-off between photocathode durability (LaB6) and sensitivity (CsI). Future work will focus on optimizing the LaB6 QE and refining the feedback suppression algorithm to finalize the time-resolution performance for the Belle II upgrade.