Mechanism for reduction of the afterpulsing rate of PMTs

This study demonstrates that the reduction of afterpulsing rates in Cherenkov Telescope Array PMTs, which had previously increased during storage, is driven by a mechanism requiring both light illumination and high-voltage operation that facilitates the ionization and subsequent trapping of residual gas ions at later dynodes.

The Gist

The Problem: The "Ghost" Signals

Imagine you have a super-sensitive microphone (a Photomultiplier Tube, or PMT) designed to hear the faintest whisper of a firefly in a dark forest. This microphone is used by giant telescopes to catch the tiny flashes of light created when cosmic rays hit our atmosphere.

However, these microphones have a annoying habit: sometimes, after hearing a real sound, they make a fake "echo" or "ghost noise" called an afterpulse. These ghosts mess up the data, making it look like there are more fireflies than there actually are.

The scientists noticed something strange:

• The Sleeping Microphones: If they left these microphones sitting in a box (unused) for a while, the "ghost noises" got worse.
• The Working Microphones: But, if the microphones were actually turned on and listening to the night sky, the "ghost noises" slowly disappeared.

The big question was: Why does using the microphone make the ghosts go away?

The Experiment: The "Burn-In" Test

To solve the mystery, the team set up a laboratory experiment with 21 of these microphones. They treated them like different groups of students in a classroom to see what made the ghosts vanish:

1. Group A: Turned on, high voltage, and shining a bright light on them (The "Hard Workers").
2. Group B: Turned on, high voltage, but no light (The "Silent Workers").
3. Group C: Light on them, but no power (The "Daydreamers").
4. Group D: No light, no power (The "Sleepers").

They watched these groups for three weeks, checking the "ghost noise" levels every day.

The Discovery: It Takes Two to Tango

The results were clear: The ghosts only disappeared if the microphone was both turned on with high voltage AND blasted with light.

• If you just had the light but no power? No change.
• If you just had the power but no light? No change.

The Analogy: Think of the inside of the microphone as a room filled with invisible, sticky dust (residual gas). This dust causes the "ghost" echoes.

• Light is like sending people into the room.
• High Voltage is like giving those people a running start (acceleration).

Only when you send people running fast into the room do they kick up enough dust to clean the walls. If they just walk in (light only) or if the room is empty (no light), the dust stays stuck.

The Twist: Where the Cleaning Happens

The scientists thought the cleaning happened right at the entrance (the photocathode), where the light hits first. But they found a surprise.

They realized the cleaning power didn't depend on how many people entered the room, but on how fast they were moving when they hit the back wall.

• The "ghosts" are caused by dust near the front door.
• But the "cleaning crew" (ions) is actually generated by the electrons bouncing around at the very back of the tube (the later dynodes).

The Metaphor: Imagine a long hallway.

• The "dust" (residual gas) is stuck near the front door.
• The "cleaning crew" is actually generated by people running down the hallway and slamming into the back wall with high energy.
• When they slam into the back wall, they create a shockwave that sucks the dust out of the air and traps it on the back wall.
• Once the dust is gone from the air, it can't stick to the front door anymore, so the "ghost echoes" stop.

The Conclusion

The paper concludes that to fix these sensitive detectors, you don't just need to turn them on; you need to run them at full speed (high voltage) while they are looking at light. This creates a "self-cleaning" effect where the electrons act like a vacuum cleaner, sucking up the invisible gas that causes the noise, specifically by slamming into the back of the tube.

In short: To stop the ghosts, you have to let the machine work hard. The more it works (and the harder it works), the cleaner it gets.

Technical Summary

1. Problem Statement

Photomultiplier tubes (PMTs) are critical components in Imaging Atmospheric Cherenkov Telescopes (IACTs), such as the Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO). A significant operational challenge is afterpulsing, a phenomenon where residual gas molecules inside the vacuum tube are ionized by accelerated electrons, creating ions that drift back to the photocathode and generate spurious signals.

Previous observations revealed a paradoxical behavior regarding the afterpulsing rate of PMTs used in CTAO:

• Spare PMTs in storage: The afterpulsing rate increases over time (likely due to helium permeation).
• Operational PMTs (in the first LST): The afterpulsing rate decreases over time.

While it was hypothesized that the decrease was caused by the removal of residual gas via ionization (where accelerated photoelectrons ionize gas molecules, and the resulting ions are trapped by dynodes), the specific conditions required for this reduction and the precise location of the ionization within the PMT remained unclear. Specifically, it was unknown if both light illumination and high voltage (HV) were strictly necessary, and whether the ionization was driven by primary photoelectrons or secondary electrons at later dynodes.

2. Methodology

To investigate the mechanisms behind the afterpulsing rate evolution, the authors conducted a controlled, three-week laboratory experiment using 21 spare R12992-100 PMTs.

Experimental Setup:

• Conditions: The PMTs were divided into five groups subjected to different combinations of light illumination and high voltage (HV):
  • Illuminated: 1100 V, 750 V, and 0 V.
  • Masked (Dark): 1100 V and 0 V.
  • Light Source: An LED providing a stable, diffused light corresponding to a photoelectron rate of ~2.5 GHz (an order of magnitude higher than typical night-sky background).
• Monitoring:
  • Daily Measurements: The afterpulsing rate was measured daily.
  • Stimulus: PMTs were fired with sub-nanosecond laser pulses (~50 photoelectrons) at a measurement voltage of 1100 V.
  • Data Acquisition: 1 µs waveforms were recorded at 1 GHz sampling to distinguish main pulses from afterpulses.
  • Gain Calibration: PMT gain was measured daily to convert charge to photoelectron numbers and to calculate cathode and anode currents.
• Variables Tracked: The study correlated the evolution of the afterpulsing rate against:
  • Integrated cathode charge (representing primary photoelectron flow).
  • Integrated anode charge (representing total electron multiplication).
  • Applied HV levels.

3. Key Contributions and Results

A. Necessity of Combined Conditions

The study confirmed that the reduction of the afterpulsing rate requires both light illumination and high-voltage operation.

• PMTs operated with light but no HV (0 V) showed no reduction.
• PMTs operated with HV but no light (masked) showed no reduction.
• Only the group with both illumination and HV (1100 V) exhibited a significant decline in the afterpulsing rate.
• Conclusion: The reduction is driven by the ionization of residual gas by accelerated electrons, which only occurs when a current is flowing under an electric field.

B. Location of Ionization (Front vs. Rear Region)

The authors distinguished between ionization occurring near the photocathode (front region) and ionization occurring at later dynodes (rear region).

• Cathode Charge Analysis: When plotting the afterpulsing rate reduction against the integrated cathode charge, the reduction was significantly larger at 1100 V than at 750 V, despite the potential difference between the photocathode and the first dynode being fixed at 350 V for both.
  • Implication: If primary photoelectrons (front region) were the main cause, the reduction should have been similar for both voltages. The difference suggests the primary driver is not the front region.
• Anode Charge Analysis: When plotting against the integrated anode charge, the reduction rates for 1100 V and 750 V were comparable.
  • Implication: The ionization responsible for gas removal occurs primarily in the rear region (between later dynodes and the anode), where the electron energy is determined by the total applied HV.

C. Mechanism of Reduction

• Ionization Cross-Section: The difference in reduction efficiency between 1100 V and 750 V correlates with the ionization cross-section of helium (the dominant residual gas). At 1100 V, electrons reach ~100 eV, while at 750 V they reach ~60 eV. The higher energy at 1100 V leads to a higher ionization probability.
• Gas Depletion and Diffusion: The study concludes that secondary electrons generated at the later dynodes ionize the residual gas. The resulting ions are trapped by the dynode surfaces. This creates a local depletion of gas in the rear region, driving diffusion of gas from the front region (where afterpulses originate) to the rear. This homogenization and overall reduction of gas density lower the afterpulsing rate.

D. Voltage Independence of Measurement

Interestingly, the measured afterpulsing rate itself showed little dependence on the measurement voltage (1400 V vs. 1100 V). This is because the observed afterpulses originate mainly from the front region (where the electric field is fixed), whereas the gas removal mechanism is driven by the rear region (where the field varies with HV).

4. Significance

• Operational Strategy: The findings provide a clear operational guideline for CTAO and other IACTs. To mitigate the increase in afterpulsing rates (or to reverse the increase in stored spares), PMTs must be operated with both light and high voltage. Simply applying voltage without light, or vice versa, is ineffective.
• Physical Understanding: The paper resolves the ambiguity regarding the location of the ionization process. It shifts the understanding from a photocathode-centric view to a later-dynode-centric view, explaining that the "burn-in" or conditioning effect is driven by secondary electron multiplication in the rear stages of the tube.
• Detector Longevity: Understanding this mechanism allows for better management of PMT lifecycles, potentially extending the useful life of detectors by actively managing residual gas levels through controlled operation.

In summary, the paper establishes that the reduction of PMT afterpulsing is a diffusion-driven process initiated by ionization in the rear dynode stages, requiring simultaneous illumination and high-voltage operation to effectively pump out residual gas.