The measurement of late-pulses and after-pulses in the large area Hamamatsu R7081 photomultiplier with improved quantum-efficiency photocathode

This paper reports on laboratory measurements of late-pulses and after-pulses in a large-area Hamamatsu R7081 photomultiplier tube with an improved quantum-efficiency photocathode, finding that while the late-pulse contribution is small, it remains non-negligible for accurate neutrino detection reconstruction.

The Gist

Imagine you are trying to listen to a single, quiet whisper in a massive, echoing cathedral. That's essentially what scientists are doing when they build giant underwater telescopes to catch messages from deep space (specifically, high-energy neutrinos).

To "hear" these whispers, they use huge light sensors called Photomultiplier Tubes (PMTs). When a neutrino bumps into water, it creates a flash of blue light (Cherenkov light). The PMT catches this flash and turns it into an electrical signal.

However, there's a problem. Just like a bad echo in a cathedral, the PMT doesn't just record the original flash. It sometimes creates ghost signals or false echoes that arrive a split second later. If the scientists don't understand these ghosts, they might think a second neutrino arrived when it was actually just a glitch in the machine.

This paper is a report on how the scientists at the INFN (Italian National Institute of Nuclear Physics) studied these "ghosts" in a specific, high-quality sensor called the Hamamatsu R7081.

Here is a breakdown of what they found, using simple analogies:

1. The Setup: A Controlled Test Lab

The scientists didn't do this underwater. They put the giant sensor in a black, light-proof box in their lab. They used a super-fast laser (a "light gun") to fire tiny, single flashes of light at the sensor, mimicking the real cosmic events. They then used a high-speed camera (a digitizer) to record exactly what the sensor "saw" for 16 microseconds after every flash.

2. The Four Types of "Ghosts"

The paper explains that the sensor creates four different kinds of false signals, depending on when they arrive after the real flash:

• Type 1 (The Immediate Echo): These happen almost instantly (within 80 nanoseconds).
  • Analogy: Imagine a runner (an electron) hitting a wall (the dynode) and bouncing back, or a spark jumping off the wall and hitting the runner. It's a quick, messy reaction right after the main event.
• Type 2 (The Gas Delay): These happen between 80 nanoseconds and 16 microseconds.
  • Analogy: Imagine the runner hits a patch of fog (gas molecules) inside the tube. The fog gets excited and sends a signal back later. Different types of fog (ions like Helium or Oxygen) take different amounts of time to clear, creating distinct delays.
• Late Pulses (The Detour): These are the main focus of the study.
  • Analogy: Imagine the runner starts running, hits a wall, bounces all the way back to the starting line, runs a full loop, and then finishes the race. Because they took a detour, they arrive late. The scientists found these happen about 5% of the time.
• Pre-pulses (The Early Bird): These arrive before the main signal.
  • Analogy: A runner who starts running before the starting gun because they saw a flash of light through the starting gate. (The paper noted they didn't see many of these in their data).

3. What They Discovered

The scientists measured these "ghosts" very carefully:

• The Late Pulses: They found that about 5% of the time, the signal takes a "detour" and arrives late. While this is a small number, it's not zero. In the underwater telescopes, these late signals look exactly like light bouncing off particles in the water. If the computer doesn't know these "detours" exist, it might calculate the wrong path for the neutrino.
• The After-Pulses (The Echoes):
  • Type 1 echoes happened very quickly (25–40 nanoseconds later).
  • Type 2 echoes happened later, specifically in two big clusters: one around 1–2 microseconds and another around 7–8 microseconds.
  • The Surprise: They found that about 8.1% of the signals were Type 2 echoes. This is a higher percentage than they expected for this specific "high-efficiency" sensor.
  • The Mystery: They also spotted a tiny, faint signal about 0.5 to 0.8 microseconds after the main flash. It's so small it's hard to explain, but it looks like a tiny spark happening inside the sensor's internal machinery.

4. Why This Matters

The paper concludes that while this specific sensor is very good, it still has these "ghosts."

• The Problem: If you are trying to map the path of a neutrino underwater, a "late pulse" looks just like a photon scattering in the water. A "large after-pulse" looks like a very bright flash from a nearby particle.
• The Solution: By measuring exactly when these ghosts happen and how big they are, the scientists can teach their computer simulations (Monte Carlo models) to recognize them. This helps the computer ignore the noise and focus on the real message from the stars.

In short: The scientists took a giant, sensitive light sensor, fired lasers at it, and mapped out all the times it "lied" to them. They found that while the lies are rare, they are frequent enough that if you don't account for them, your map of the universe will be slightly wrong.

Technical Summary

Technical Summary: Measurement of Late-Pulses and After-Pulses in the Large Area Hamamatsu R7081 Photomultiplier

Problem Statement
In the context of large underwater telescopes designed to detect high-energy astrophysical neutrinos, the reconstruction of muon tracks relies on the precise measurement of Cherenkov light arrival times and photo-electron counts by Photomultiplier Tube (PMT) arrays. Accurate reconstruction requires distinguishing between genuine signal pulses and "spurious" pulses generated by internal physical processes within the PMT. These spurious pulses, categorized as pre-pulses, late-pulses, and after-pulses, can mimic the scattering of light in water or be misinterpreted as additional photons from a muon track, thereby degrading reconstruction fidelity. While the general nature of these effects is known, specific characterization is required for new PMT models, particularly those with improved quantum efficiency (QE) photocathodes, to ensure correct simulation and data analysis.

Methodology
The study utilized a Hamamatsu R7081MOD 10-inch PMT featuring a high QE photocathode and a modified active ISEG base. The experimental setup consisted of a light-tight box housing the PMT and a Hamamatsu Picosecond Light Pulser (PLP10-044C) emitting 440 nm laser pulses with a width of a few picoseconds.

• Single Photoelectron (SPE) Mode: The laser intensity was attenuated to approximately $10^{-2}$ photons/pulse at the photocathode, ensuring an average of one signal every 500 laser pulses to operate in the SPE regime.
• Data Acquisition: The PMT anode signal was split between a discriminator (set to a 0.3 pe threshold) and a 1 GHz CAEN V1731 digitizer. The discriminator output was synchronized with the laser SYNC signal to trigger the digitizer, capturing a 16 µs time window following each laser pulse.
• Analysis: Digitized waveforms underwent pedestal subtraction. Peak detection algorithms identified pulses exceeding a 10 mV threshold (~0.2–0.3 pe). Charge was calculated via Simpson integration, and timing was measured relative to the laser pulse. The main pulse was defined as occurring within the first 200 ns; events outside this window were analyzed as late or after-pulses.

Key Contributions and Results
The paper provides a detailed characterization of the timing and amplitude distributions of spurious pulses in the R7081MOD PMT:

1. Late-Pulses:

   • Timing: A distribution of late-pulses was observed with a contribution of approximately 30% when considering times >2σ from the main peak, or roughly 5% when measured from the distribution minimum at 30 ns.
   • Origin: A structure observed at ~60 ns is hypothesized to correspond to the "round trip" of an electron reflecting from the first dynode to the photocathode and back, assuming a uniform electric field of ~60 V/cm.
   • Amplitude: No significant correlation was found between the arrival time and amplitude of late-pulses; their average charge matched that of the main pulses.

2. After-Pulses (Type 1 and Type 2):

   • Type 1 (25–40 ns): Attributed to luminous reactions on dynodes or electron escape, these pulses appeared with an average charge of 1 pe.
   • Type 2 (200 ns – 16 µs): Caused by the ionization of residual gas (H+, He+, heavy ions) by photoelectrons. The study identified distinct clusters at 1–2 µs and 7–8 µs, corresponding to the drift times of light and heavy ions, respectively.
   • Amplitude Anomalies: While many Type 2 pulses had large amplitudes, a specific region of low-amplitude pulses (~1/3 pe) was observed between 500–1000 ns. The origin of these small signals remains unclear, though their timing suggests a dynode-region event rather than ion drift.
   • Quantification: The total contribution of after-pulses was measured at 1.2% for Type 1 and 8.1% for Type 2. Notably, the percentage of Type 2 after-pulses was found to be larger in this high QE tube compared to previous measurements on other large-area PMTs.

Significance and Claims
The authors assert that the measured response functions for late- and after-pulses in the high QE R7081MOD are consistent with general PMT behaviors but exhibit a higher rate of Type 2 after-pulses. The primary significance of this work lies in the provision of precise timing and amplitude data necessary for Monte Carlo simulations. By incorporating these specific measured effects into simulation models, the agreement between simulated and experimental data in underwater neutrino detectors is expected to improve. This refinement is critical for correctly reconstructing muon tracks and distinguishing between genuine Cherenkov light scattering and internal PMT artifacts.