Embedded underwater front-end electronics for the 3-inch photomultipliers in the JUNO experiment

This paper presents the design, validation, and performance of the underwater front-end electronics for the 25,600 3-inch photomultipliers in the JUNO experiment, detailing a system that achieves low noise, minimal crosstalk, and high bandwidth to support the detector's physics goals.

The Gist

Imagine the JUNO experiment as a giant, ultra-sensitive underwater camera buried deep beneath the Earth's surface in China. Its job is to catch tiny flashes of light (scintillation) produced when ghostly particles called neutrinos interact with a massive tank of liquid.

To see these faint flashes, the camera needs eyes. It has two types of eyes:

1. The Big Eyes: 17,612 huge 20-inch cameras (Photomultipliers or PMTs) that do the heavy lifting.
2. The Tiny Eyes: 25,600 smaller 3-inch cameras squeezed into the gaps between the big ones.

This paper is all about the brain and nervous system built specifically for those 25,600 "Tiny Eyes." Since these cameras are deep underwater (about 693 meters down), they can't just be plugged into a wall outlet. They need a special, waterproof, high-tech nervous system to talk to the surface.

Here is how the system works, explained simply:

1. The "Underwater City" (The Boxes)

Imagine you have 200 waterproof metal canisters (called Underwater Boxes or UWBs) sitting on the ocean floor. Each canister is a tiny city that manages a neighborhood of 128 tiny cameras.

• The Challenge: These cameras need electricity (high voltage) to work, but they also need to send delicate signals back to the surface. Usually, you'd need two thick cables for this.
• The Solution: The engineers used a "magic trick" where the high-voltage electricity and the delicate signal travel together in a single cable. It's like sending a letter and a power bill in the same envelope. Inside the box, a special board separates the power from the message so the message doesn't get fried.

2. The "Traffic Controllers" (The Electronics Boards)

Inside each of these 200 metal boxes, there are three main types of circuit boards working together like a team:

• The Power Distributor (HVS Board): Think of this as the electrician. It takes the high voltage coming from the surface and splits it up to power the 128 cameras. It also acts as a filter, making sure the high voltage doesn't crash into the delicate signal wires.
• The Digital Translator (ABC Board): This is the translator. When a tiny camera sees a flash of light, it sends a tiny electrical pulse. This board has 8 special chips (called CATIROC) that act like super-fast scribes. They instantly count how many photons (light particles) hit the camera and record exactly when they arrived. They turn these analog pulses into digital numbers (0s and 1s).
• The Manager (GCU Board): This is the boss. It controls the electrician and the translators. It takes all the digital notes from the translators, packages them up, and sends them up to the surface computers. It also keeps an eye on the temperature and makes sure everything is running smoothly.

3. Keeping Cool and Quiet

Because these electronics are packed tight inside a metal box underwater, they generate heat.

• The Cooling: Imagine a sandwich. The hot chips are the filling, and thick copper plates are the bread. The heat flows from the chips, through the copper, and out into the surrounding water, keeping the electronics cool enough to last for decades.
• The Silence: The system is so sensitive that it can hear a single photon (a single particle of light). To do this, the electronics must be incredibly quiet. The paper claims the system is so quiet that its own "static noise" is only about 4% of a single photon's signal. It's like trying to hear a whisper in a library, but the library itself is completely silent.

4. What Can It Handle?

The paper tests if this system can handle a "traffic jam" of light.

• Normal Days: It easily counts single photons with high precision.
• Supernova Day: If a star explodes nearby (a supernova), the detector would be flooded with light. The system was tested to see if it would get overwhelmed. The results show it can handle the rush, keeping about 90% to 100% of the data even during a massive burst, ensuring scientists don't miss the event.

5. The "Cleanliness" Factor

Since JUNO is looking for extremely rare events, even tiny bits of natural radiation from the electronics themselves could create "fake" signals.

• The team screened every single screw, wire, and chip to ensure they are made of ultra-pure materials. They calculated that the electronics themselves will only create a tiny, manageable amount of "background noise," well within the safety limits for the experiment.

Summary

In short, this paper describes the successful design and testing of a robust, waterproof, and ultra-sensitive nervous system for 25,600 small cameras deep underwater. It proves that this system can:

• Power the cameras and read their signals through a single cable.
• Count single particles of light with almost zero error.
• Stay cool and quiet for 20 years.
• Handle massive bursts of data without crashing.

The system is now installed and ready to help JUNO solve the mystery of neutrino mass and watch for exploding stars.

Technical Summary

1. Problem Statement

The Jiangmen Underground Neutrino Observatory (JUNO) is a massive 20-kiloton liquid scintillator neutrino detector designed to determine the neutrino mass ordering. While the detector relies on 17,612 large 20-inch photomultiplier tubes (PMTs) for primary energy resolution, it also deploys 25,600 small 3-inch PMTs (SPMTs) in the gaps between the large tubes.

The SPMT system faces unique engineering challenges:

• Underwater Environment: The electronics must operate submerged in ultrapure water at a depth of 693 meters, requiring extreme reliability, waterproofing, and low radiopurity to avoid background noise.
• High Channel Density: The system requires reading out 25,600 channels with high granularity.
• Signal Integrity: The SPMTs operate primarily in Single Photoelectron (SPE) counting mode. The electronics must detect extremely low-amplitude signals (approx. 2 mV) with minimal noise and crosstalk, while avoiding the non-linearities that affect the larger PMTs at high light levels.
• Cabling Constraints: To minimize failure points and cost, the system uses a single coaxial cable per PMT to carry both High Voltage (HV) and signal, necessitating complex decoupling underwater.
• High-Rate Events: The system must handle transient high-rate events, such as Core-Collapse Supernovae (CCSN), without data loss.

2. Methodology and System Architecture

The authors designed a modular, underwater front-end electronics system housed in 200 independent Underwater Boxes (UWBs). Each UWB manages 128 SPMT channels. The system architecture consists of three main electronic boards and a robust mechanical housing:

A. Mechanical and Environmental Design

• Underwater Box (UWB): A cylindrical stainless steel (SS304L) vessel (50 cm long, 27 cm diameter) with a removable lid sealed by three O-rings (two axial, one radial) for redundancy.
• Thermal Management: A sandwich structure using copper heat sinks, thermal silicone rubber, and natural convection in the water cools the electronics, maintaining chip temperatures below 60°C despite a 15W power draw per unit.
• Radiopurity: All materials (PCBs, connectors, structural components) underwent rigorous low-background gamma spectroscopy screening to ensure the total single-hit rate from radioactivity remains below the 0.2 Hz requirement.

B. Electronic Components

1. High Voltage Splitter (HVS) Board:
   • Contains eight custom High Voltage Units (HVUs) (DC-DC converters) capable of generating 800–1,500 V.
   • Implements a 1:1 redundancy scheme where four HVUs are active and four are backups.
   • Decouples the PMT signal from the HV line using 3.9 nF capacitors and splits the HV to 16 channels per unit via 20 MΩ resistors.
2. ABC (ASIC Battery Card) Front-End Board:
   • The core processing unit featuring eight 16-channel CATIROC ASICs (custom chips developed at OMEGA, France) and a Kintex-7 FPGA.
   • Digitizes the signal, measuring both charge and arrival time.
   • Operates with a 160 MHz clock for the ASICs and 80 MHz for data transfer.
3. Global Control Unit (GCU) Board:
   • Acts as the central hub, housing a Kintex-7 FPGA (for data aggregation) and a Spartan-6 FPGA (for system initialization and firmware updates).
   • Interfaces with surface systems via Gigabit Ethernet (DAQ) and synchronous links (White Rabbit protocol for timing).
   • Manages power distribution (36V input from surface) and HV control for the HVS boards.

C. Firmware and Data Flow

• Synchronization: Uses the White Rabbit protocol for sub-nanosecond time synchronization across all 200 UWBs.
• Data Pipeline: CATIROC ASICs $\rightarrow$ ABC FPGA (FIFO buffering) $\rightarrow$ GCU FPGA $\rightarrow$ Surface DAQ via TCP/IP.
• Configuration: Uses IPBus protocol for remote configuration of the 328 slow-control parameters per CATIROC chip.

3. Key Contributions

• Compact Underwater Integration: Successfully integrated 128 channels of high-voltage generation, signal decoupling, and high-speed digitization into a single 200-liter underwater box.
• Low-Noise ASIC Readout: Adapted the CATIROC ASIC (originally designed for other applications) to achieve ultra-low noise performance suitable for single-photon detection in a high-voltage environment.
• Redundant HV Architecture: Developed a novel HV splitting and backup system that ensures continuous operation even if individual HV units fail.
• Radiopurity Validation: Demonstrated that the total radiogenic background from the readout electronics contributes only ~21% of the total SPMT system background, meeting the stringent <0.2 Hz requirement.
• Supernova Resilience Modeling: Validated the system's ability to handle the extreme hit rates of a nearby supernova (up to 1 kpc) without data saturation.

4. Results and Performance Metrics

Comprehensive validation tests were conducted on full-scale prototypes (128 channels) mimicking the final JUNO configuration:

• Noise Performance:
  • Achieved an electronic noise level of 0.04 Photoelectrons (PE) (2.3 ADC units).
  • This is negligible compared to the PMT's intrinsic resolution (33.2%), contributing less than 0.7% to the total SPE resolution.
• Crosstalk:
  • Measured crosstalk between channels is < 0.4%, well within specifications.
• Linearity and Resolution:
  • Charge linearity is better than 0.05% in the 0–10 PE range.
  • Time resolution is 0.23 ns, significantly better than the PMT's Transit Time Spread (1.6 ns).
• Bandwidth and Dead Time:
  • The system achieves a sustained data throughput of 57 MB/s.
  • The primary bottleneck is the hardware link between the ABC and GCU boards.
  • Supernova Simulation: For a 20-solar-mass supernova at 1 kpc, the system maintains >90% hit acceptance. The onboard 2 GB DDR3 memory per GCU is sufficient to buffer the entire burst without loss.
• Reliability:
  • Accelerated aging tests (95°C for 500+ hours) confirmed a Failure In Time (FIT) rate of <1900.
  • Pressure testing (8 bar at 77°C for 1,800 hours) confirmed the integrity of the O-ring seals.

5. Significance

This paper presents the successful design, validation, and deployment of a critical subsystem for the JUNO experiment. The SPMT front-end electronics enable:

1. Independent Calibration: Providing a linear, non-saturating readout to cross-calibrate the large PMT system and correct for non-linearities in the energy scale.
2. Enhanced Physics Reach: Improving muon track reconstruction and enabling the detection of rare events like supernova neutrinos and proton decay.
3. Technological Benchmark: Establishing a new standard for high-density, ultra-low-noise, and ultra-low-radiopurity underwater electronics, which is essential for future large-scale neutrino and dark matter experiments.

The system is now fully integrated and installed in the JUNO detector, ready to support the experiment's physics program starting in 2025.