The MUSE Target Chamber Post Veto

This paper describes the design and performance of the Target Chamber Post Veto (TCPV) detector, which was installed inside the MUSE experiment's vacuum chamber to eliminate background triggers caused by beam particles striking structural support posts.

The Gist

The Big Picture: Solving a Cosmic Riddle

Imagine scientists are trying to measure the size of a tiny, invisible marble (a proton) to solve a mystery known as the "Proton Radius Puzzle." For years, two different ways of measuring this marble gave different answers, leaving physicists confused.

To solve this, the MUSE experiment was built. It shoots a mixed stream of particles (electrons and muons) at a target made of liquid hydrogen. By watching how these particles bounce off the hydrogen, scientists hope to get the correct measurement of the proton's size.

The Problem: The "Bouncer" in the Room

To keep the liquid hydrogen cold and stable, it must be kept inside a vacuum chamber (a box with no air). However, the walls of this box have to be very thin to let the particles pass through without getting bumped around.

Because the pressure outside the box is much higher than the pressure inside, the thin walls want to cave in. To stop this, the engineers built support posts (like pillars) inside the chamber to hold the walls up.

Here is the trouble:
The beam of particles isn't a perfect laser; it's a bit fuzzy, with some particles wandering off to the edges (the "tails" of the beam). These wandering particles hit the support posts instead of the hydrogen target.

• The Analogy: Imagine trying to take a photo of a butterfly in a garden, but there are large tree trunks right in front of your camera. Every time a bird flies into a tree trunk, it makes a loud crash that drowns out the sound of the butterfly.
• The Result: These "crashes" (particles hitting the posts) create a massive amount of noise. They clog up the data system, causing it to pause and miss the real, important data (the butterfly). In fact, at certain angles, these "post crashes" made up 94% of the events the computer was trying to record!

The Solution: The "Veto" Detector

The team built a special detector called the Target Chamber Post Veto (TCPV). Its job is simple: If a particle hits a post, ignore it.

Think of the TCPV as a bouncer standing right next to the support posts.

1. The Setup: They placed thin, plastic "paddles" (scintillators) right next to the posts inside the vacuum chamber.
2. The Trigger: When a particle hits a post, it hits the paddle. The paddle glows with a tiny flash of light.
3. The Action: The bouncer sees the flash and immediately yells, "Stop! Ignore this!" before the computer even finishes processing the data. This saves the computer from wasting time on useless noise.

How It Works (The Two-Track System)

Because the chamber contains liquid hydrogen (which is flammable if it leaks and mixes with air), putting electronics inside is risky. If a spark happens, it could cause an explosion. To be safe, they designed the detector with two parallel systems:

1. The "Direct" System (The In-Chamber Team):

   • They glued tiny light sensors (SiPMs) directly onto the paddles inside the vacuum.
   • Pros: It's super fast and very sensitive. It catches almost every particle hitting the post.
   • Cons: It requires high voltage inside a hydrogen-filled room, which is a safety risk. They had to prove mathematically that the pressure is so low that a spark couldn't possibly ignite the hydrogen.

2. The "Fiber" System (The Remote Team):

   • They used special light-guiding fibers (Wavelength-Shifting fibers) to carry the light from the paddles out of the vacuum chamber to sensors sitting safely outside.
   • Pros: No high voltage inside the dangerous zone.
   • Cons: The light gets a bit dimmer and slower as it travels through the fiber. It's less efficient at catching the "bad" particles.

The Results: A Cleaner Experiment

The paper reports on how well this bouncer system worked:

• Noise Reduction: When they turned on the "Direct" system (the in-chamber sensors), it successfully vetoed (blocked) up to 63% of the background noise at lower energies. The fiber system was about half as effective.
• Safety: The team did a deep dive into the physics of sparks and hydrogen. They calculated that even if a leak happened, the pressure inside the chamber is so low that a spark couldn't ignite the gas. They also added a safety "interlock" that cuts all power if the pressure rises even slightly.
• Conclusion: The TCPV detector is a success. It acts like a noise-canceling headphone for the experiment, filtering out the "tree trunk crashes" so the scientists can finally hear the "butterfly" and solve the proton radius puzzle.

Summary

The MUSE experiment needed to stop its data from being drowned out by particles hitting support beams. They built a smart, dual-system detector inside the vacuum chamber that acts as a bouncer, instantly rejecting those bad hits. This allows them to collect clean, high-quality data to finally figure out the true size of the proton.

Technical Summary

1. Problem Statement

The Muon Scattering Experiment (MUSE) at the Paul Scherrer Institute (PSI) aims to resolve the "proton radius puzzle" by simultaneously measuring elastic electron-proton and muon-proton scattering. The experiment utilizes a liquid hydrogen (LH2) target housed within a vacuum chamber to maintain stable target density.

• Structural Necessity: To withstand the pressure differential between the external atmosphere and the internal vacuum, the target chamber requires structural support. This is achieved via an upstream "strong back" and two downstream support posts located at small scattering angles.
• The Background Issue: While these posts are outside the primary acceptance of the large-solid-angle spectrometer, particles in the tails of the beam distribution frequently strike them.
• Consequences: Scattering off these posts generates a massive background (accounting for approximately 94% of events at certain angles). This overwhelms the Data Acquisition (DAQ) system, significantly increasing dead time and limiting the readout of genuine scattering events from the liquid hydrogen target.

2. Methodology: The TCPV Detector

To mitigate this background, the authors designed and installed the Target Chamber Post Veto (TCPV) detector inside the vacuum chamber, positioned adjacent to the upstream face of the support posts.

Design and Construction

• Geometry: The TCPV consists of two plastic scintillator paddles (BC404, 4 mm thick, 20.5 mm wide, 200 mm long) mounted on a frame inside the chamber. They are positioned to cover the posts without obstructing scattered particles or touching the vacuum windows (which deform inward by ~2 mm under vacuum).
• Dual Readout Systems: To address safety concerns regarding high voltage near flammable hydrogen, two parallel readout systems were implemented:
  1. In-Chamber Readout: Silicon Photomultipliers (SiPMs) are glued directly to the ends of the scintillator paddles. These provide mechanical support and direct signal collection.
  2. Wavelength-Shifting (WLS) Fiber Readout: WLS fibers are embedded in grooves within the scintillators to transport light to SiPMs located outside the vacuum chamber.
• Safety Measures:
  • The TCPV is not directly attached to the posts (which distort under vacuum) but to a separate internal frame.
  • All electronics are sealed with epoxy to prevent sparking.
  • A rigorous safety analysis (Appendix A) confirmed that the operating voltages (30V for SiPMs, 150V for connectors) cannot cause a spark-induced hydrogen explosion within the operational pressure range ($10^{-4}$ mbar), as the breakdown voltage is too high for the low-pressure environment, and the interlock system shuts down power if pressure rises above $10^{-2}$ mbar.

Electronics and Signal Processing

• Signal Chain: SiPM signals are amplified, discriminated by Constant Fraction Discriminators (CFDs), and sent to Time-to-Digital Converters (TDCs) and Charge-to-Digital Converters (QDCs).
• Trigger Logic: The TCPV signals are integrated into the master trigger as a veto. If the TCPV fires, the event is rejected at the trigger level, preventing DAQ saturation.
• Timing: The system requires nanosecond-level resolution due to the 50.6 MHz beam structure. The in-chamber SiPMs meet this easily, while the WLS fiber readout has a slightly slower resolution (4 ns) due to the WLS time constant (12 ns).

3. Key Contributions

• Novel Detector Integration: Successful design and installation of a veto detector inside a liquid hydrogen target vacuum chamber, a challenging environment due to flammability and vacuum constraints.
• Dual-Readout Architecture: Implementation of a redundant readout system (direct SiPM vs. WLS fiber) that allowed for safe commissioning. The in-chamber SiPMs were initially used only for calibration with an empty target, then fully deployed once safety was verified.
• Safety Analysis: A comprehensive theoretical and experimental safety analysis (Appendix A) proving that operating high-voltage SiPMs inside a hydrogen-filled vacuum chamber is safe, utilizing Paschen's Law and combustion heat calculations to demonstrate that ignition is impossible under the experiment's operating conditions.

4. Results and Performance

The paper reports on the performance of the TCPV during the 2022 and 2023 beam times.

• Trigger Rate Reduction:
  • The TCPV significantly reduced the trigger rate.
  • Efficiency: The in-chamber SiPM readout was approximately twice as efficient as the WLS fiber readout.
    • At +115 MeV/c, the in-chamber veto removed 54.1% of triggers, compared to 23.5% for the WLS readout.
    • At -115 MeV/c, the in-chamber veto removed 62.8%, compared to 24.6% for WLS.
  • Reason for Difference: The in-chamber readout has a superior signal-to-noise ratio due to a higher number of photoelectrons. The WLS readout suffers from light loss and a lower photon count, making it harder to distinguish signals from noise without increasing the false-veto rate.
• Background Rejection:
  • Analysis of reconstructed particle tracks (using GEM detectors) showed that the TCPV effectively removed events originating from the support posts ($x \approx \pm 40$ mm).
  • Without the veto, the posts generated event rates comparable to the target itself. With the in-chamber SiPM veto, these background events were nearly eliminated.
• Time Resolution:
  • In-chamber SiPMs achieved the required nanosecond resolution.
  • WLS fiber readout achieved ~4 ns resolution, which, while sufficient, is limited by the WLS time constant and lower photon statistics.

5. Significance

• Enabling MUSE Physics: The TCPV is critical for the success of the MUSE experiment. By suppressing the ~94% background from support posts, it drastically reduces DAQ dead time, allowing for the collection of sufficient statistics to extract precise form factors and the proton charge radius.
• Lepton Universality Test: The ability to cleanly separate scattering events from the liquid hydrogen target allows for a direct test of lepton universality (comparing electron vs. muon scattering) and the measurement of two-photon exchange effects, which are central to resolving the proton radius puzzle.
• Safety Precedent: The paper establishes a validated methodology for operating high-voltage silicon detectors inside cryogenic, flammable gas environments, providing a blueprint for future experiments with similar constraints.

In conclusion, the TCPV detector successfully mitigated a major source of background noise, enabling the MUSE experiment to operate with the necessary statistical precision to address fundamental questions in nuclear physics.