Optimization of the light detection system of the ICARUS detector

This paper investigates the progressive gain degradation observed in the ICARUS detector's cryogenic photomultiplier tubes, characterizes the irreversible performance loss at low temperatures through experimental testing and modeling, and implements mitigation strategies to ensure reliable operation.

The Gist

Imagine the ICARUS detector as a giant, ultra-sensitive underwater camera designed to take pictures of ghostly particles called neutrinos. To make these pictures, the camera uses a special liquid called liquid argon. When a neutrino bumps into the argon, it creates two things: a tiny electrical signal and a flash of invisible light.

To catch that flash of light, the camera is equipped with 360 "super-eyes" called Photomultiplier Tubes (PMTs). Think of these PMTs as highly sensitive microphones that can hear the faintest whisper of light. Their job is to amplify that whisper into a loud shout so the computer can record it.

The Problem: The Super-Eyes Got Tired

When the ICARUS detector started working at Fermilab (a massive particle physics lab), the scientists noticed a strange problem. The "super-eyes" were getting tired. Specifically, they were losing their ability to amplify the light signals.

Imagine you have a microphone that is supposed to turn a whisper into a shout. Over time, it started turning the whisper into just a murmur. If this keeps happening, the computer might miss the neutrino events entirely, or confuse them with background noise.

Scientists suspected the problem wasn't that the "ears" (the part that first hears the light) were broken, but that the "amplifiers" inside the tube were wearing out. They noticed this happened faster when the tubes were operating in the freezing cold of liquid argon.

The Investigation: A Controlled Test

To figure out exactly what was going on, the team built a special "weather chamber" in their lab in Catania, Italy. They put a single PMT inside and slowly cooled it down to -70°C (which is cold, but not quite as cold as liquid argon).

They shined a steady laser light on the tube and watched what happened. Here is what they discovered:

• At Room Temperature: The tube was fine. It could handle the work without getting tired.
• At Low Temperatures: When they cooled it down, the tube started to lose its amplifying power.
• The Twist: Some of the loss was temporary (like a muscle cramp that goes away when you warm up), but some of it was permanent. Once the tube got cold and worked hard, it was permanently damaged, even after warming back up.

The "Why": A Broken Chain Reaction

The scientists built a simple model to explain this. Imagine the PMT as a relay race with 10 runners (called dynodes). Each runner catches a baton (an electron) and passes it to the next, but they also multiply the number of batons. By the end of the race, one baton has become millions.

The team realized that the damage wasn't happening to the first few runners. It was happening to the last few runners in the chain. Because the race is a relay, the last runners have to handle a massive crowd of batons (high electrical current).

When it's freezing cold, the materials inside these last runners expand and contract at different rates. It's like a metal bridge in winter: if the different parts of the bridge shrink at different speeds, tiny cracks can form. In the PMT, these microscopic cracks or layers peeling apart meant the runners couldn't pass the batons as efficiently anymore. The more batons they had to handle (the higher the current), the more damage they suffered.

The Fix: Slowing Down the Race

The scientists didn't just watch the problem; they fixed it. They implemented three main strategies to save the super-eyes:

1. Building a Shield: They added a thick layer of concrete over the detector. This acted like a heavy blanket, blocking cosmic rays (natural background radiation) from hitting the tubes. Fewer hits meant the tubes didn't have to work as hard.
2. Turning Down the Volume: They lowered the "gain" (the amplification power) of the tubes. Instead of trying to shout as loud as possible, they spoke at a comfortable volume. This reduced the stress on the last runners in the relay race, slowing down the damage significantly.
3. Better Wires: They replaced the old signal cables with new, high-performance ones. These new cables were so good at carrying the signal that the scientists could lower the amplification even further without losing any picture quality.

The Result

Thanks to these changes, the "super-eyes" are now stable. The rate at which they were getting tired dropped from losing about 2% of their power every month to less than 0.3%.

The paper concludes that the ICARUS detector is now healthy and robust. It can continue to take clear, long-term "photos" of neutrinos for the rest of the program's life, ensuring that scientists can achieve their goals of understanding these mysterious particles.

Technical Summary

Technical Summary: Optimization of the Light Detection System of the ICARUS Detector

Problem Statement
The ICARUS detector, a 600-ton Liquid Argon Time Projection Chamber (LArTPC) central to the Short Baseline Neutrino (SBN) Program at Fermi National Accelerator Laboratory (FNAL), relies on a Light Detection System (LDS) comprising 360 Hamamatsu R5912-MOD 8-inch photomultiplier tubes (PMTs). These PMTs are critical for triggering, event timing, spatial resolution, and cosmic ray mitigation. Since operations began in 2021 at liquid argon temperatures (~87 K), the system exhibited a progressive and significant reduction in PMT gain, initially measured at a loss rate of approximately 1.93% per month.

Initial diagnostics ruled out photocathode degradation, as the rate of background optical pulses remained constant, indicating that the quantum efficiency for converting photons to photoelectrons was stable. Instead, the gain loss was attributed to a deterioration within the dynode amplification chain. Uncontrolled gain reduction poses a severe risk to detector performance: it compromises the ability to distinguish real neutrino events from electronic noise, lowers trigger efficiency, and degrades time resolution ($t_0$), which is essential for 3D track reconstruction and cosmic ray rejection.

Methodology
To investigate the underlying mechanism of this gain loss, the authors developed a controlled experimental setup at INFN Catania. A single PMT (without TPB coating) was placed in an environmental chamber capable of reaching temperatures down to -70°C. The PMT was illuminated by a continuous laser (520 nm) and operated in current mode to simulate the high-charge conditions observed at FNAL, stressing the device with a maximum anodic current of ~28 µA.

The study monitored the anode current over time while varying temperature and illumination conditions. The researchers distinguished between reversible effects (such as changes in quantum efficiency or effective gain due to thermal shifts) and irreversible degradation. To interpret the observed trends, the authors developed a simplified physical model of the PMT dynode chain. This model hypothesized that degradation depends on the current flowing through specific stages, specifically targeting the secondary electron emission parameters ($A$ or $\alpha$) of the dynodes.

Key Results
The experimental campaign yielded three primary observations regarding PMT behavior at low temperatures:

1. Room Temperature Stability: PMTs operated for extended periods at room temperature under high current showed no gain loss, confirming that the degradation mechanism is temperature-dependent.
2. Irreversible Low-Temperature Degradation: During cooling cycles to -60°C and -70°C, a permanent loss of gain was observed. While some current reduction upon cooling was reversible (attributable to quantum efficiency or gain shifts), a portion of the degradation persisted even after returning to room temperature, indicating permanent physical damage.
3. Current-Dependent Mechanism: Measurements under different illumination and voltage conditions, but with identical initial anode currents, exhibited highly correlated degradation trends. This suggests the degradation is driven primarily by the anode current rather than specific light intensity or voltage settings.

The proposed physical model (Model A) successfully reproduced the observed degradation trend by assuming a loss factor proportional to the current affecting the parameter $A$ (related to the secondary emission coefficient) of the dynodes. The authors posit that at low temperatures, mismatches in the thermal expansion coefficients of multilayer dynode materials induce mechanical stress, leading to microcracks or delamination. This surface degradation reduces the secondary emission efficiency, a phenomenon exacerbated by the impingement of accelerated electrons.

Mitigation Strategies and Outcomes
Based on these findings, a series of mitigation strategies were implemented in the ICARUS detector to preserve PMT performance:

• Overburden Installation (June 2022): A 2.85 m concrete overburden was installed to reduce the cosmic ray flux, thereby decreasing the background light hitting the PMTs and slowing the degradation rate.
• Operating Gain Reduction: The operating gain was lowered from $0.7 \times 10^7$ to $0.46 \times 10^7$ during RUN1 and RUN2. This reduced the stress on the dynodes, decreasing the monthly loss rate to 0.64%.
• Signal Cable Replacement: Prior to RUN3, new high-performance signal cables were installed to improve signal shape and amplitude. This allowed for a further reduction in operating gain to $0.39 \times 10^7$ while maintaining signal quality, ultimately lowering the gain loss rate to 0.31% per month.

Significance
The study confirms that the gain loss observed in the ICARUS detector is driven by a current-dependent degradation mechanism affecting the last dynodes of the PMTs, likely caused by thermal stress-induced surface damage at cryogenic temperatures. Although the environmental tests were conducted at temperatures higher than liquid argon, the qualitative behavior matched the operational data from FNAL.

The implementation of the identified mitigation strategies has proven effective in stabilizing the PMT response. These optimizations ensure that the Light Detection System provides reliable triggering and accurate timing information, allowing the ICARUS detector to sustain stable operation throughout the expected lifetime of the SBN Program and fully achieve its physics goals regarding neutrino oscillations and cross-section measurements.