Design of the DUNE horizontal drift far detector charge readout electronics and performance in ProtoDUNE-HD

This paper describes the design of the custom cryogenic charge readout electronics for the DUNE horizontal drift far detector and presents their performance results based on data from the ProtoDUNE-HD prototype operated at CERN.

The Gist

The Giant Underwater Ear: How DUNE "Hears" Ghost Particles

Imagine you are trying to listen to a single, tiny whisper in the middle of a roaring, heavy metal concert. That is essentially the challenge facing the DUNE (Deep Underground Neutrino Experiment).

DUNE is building massive tanks filled with liquid argon, buried deep underground to shield them from cosmic noise. They are looking for neutrinos—subatomic particles so small and "ghostly" that they can fly through a lead wall a light-year thick without touching a single atom. When a neutrino does finally hit an argon atom in the tank, it leaves a tiny, faint trail of electricity.

This paper describes the "high-tech hearing aids" being built to catch those whispers.

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1. The "Cold" Electronics: Working in a Deep Freeze

Most electronics (like your phone or laptop) hate the cold. But DUNE’s detectors are filled with liquid argon, which is incredibly cold—about -196°C (-320°F).

Instead of running long, noisy wires from the cold tank to a warm room (which would be like trying to listen to a whisper through a mile of static-filled telephone wire), scientists decided to put the "hearing aids" inside the tank.

They designed custom microchips called ASICs that are built to live in a permanent deep freeze.

• The LArASIC is the "Microphone": It catches the tiny electrical spark from the neutrino hit and amplifies it.
• The ColdADC is the "Translator": It takes that raw electrical spark and turns it into digital code (1s and 0s).
• The COLDATA is the "Messenger": It bundles all that digital info together and zips it out of the tank through high-speed cables.

The Analogy: Imagine trying to record a delicate snowflake melting. Instead of trying to run a long, clumsy tube from the snowflake to a recorder in another room, you place a tiny, microscopic camera inside the freezer right next to the snowflake.

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2. The Motherboard: The Nervous System

These chips don't just float around; they are mounted on Front-End Motherboards (FEMBs). These boards act like a nervous system, connecting the "microphones" to the "messengers."

The paper explains how they managed to pack all this power into a tiny space without the electronics getting too hot. If the electronics get too warm, they would actually boil the liquid argon, creating bubbles that would ruin the experiment. It’s a delicate balancing act: staying powerful enough to work, but staying "cool" enough to keep the argon liquid.

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3. The "Warm" Interface: The Command Center

Once the signal leaves the freezing tank, it hits the Warm Interface Board (WIB). This is the bridge between the frozen underworld and the human world. It takes the massive flood of data coming from the tank and organizes it so that scientists' computers can actually read it.

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4. The Test Run: ProtoDUNE-HD

Before building the massive, multi-billion dollar DUNE detector, scientists built a "mini-version" called ProtoDUNE-HD at CERN in Switzerland.

Think of this like a test flight for a new jet engine. They ran this prototype for seven months to see if the electronics would break, if the noise was too loud, or if the "hearing aids" would fail in the cold.

The results were a massive success:

• Crystal Clear Hearing: Even though the environment is incredibly noisy, the electronics were able to distinguish the tiny "whispers" of particles from the background roar.
• Reliability: Out of over 10,000 channels, almost every single one worked perfectly. It’s like building a massive orchestra with 10,000 instruments and having every single one play in perfect tune.
• The "Power Surge" Hiccup: They did find one small quirk: if a signal is too big (like a sudden shout), it can momentarily "drain the battery" of the nearby chips, causing a tiny glitch in their neighbors. It’s like if someone in a crowded room screams so loud that everyone else momentarily loses their breath. The scientists already know how to fix this in their data analysis.

Summary

This paper proves that we have successfully built a "super-hearing" system that can survive in a deep-freeze environment for decades. This technology will allow DUNE to listen for the most elusive particles in the universe, helping us understand why the universe exists at all.

Technical Summary

Technical Summary: Design and Performance of the DUNE Horizontal Drift Far Detector Charge Readout Electronics

1. Problem Statement

The Deep Underground Neutrino Experiment (DUNE) requires massive Liquid Argon Time Projection Chambers (LArTPCs) to measure neutrino oscillations with high precision. To achieve the necessary spatial resolution and sensitivity, the detectors must resolve ionization charges of less than 10,000 electrons.

Traditional "warm" electronics (operating at room temperature) face significant challenges in such large-scale detectors, including high input capacitance from long cables and increased electronic noise. While "cold" electronics (operating at 87 K inside the liquid argon) offer lower noise and reduced cabling through multiplexing, they must be exceptionally reliable, as they are inaccessible for replacement for decades. The technical challenge lies in designing a custom, integrated, and highly stable cryogenic electronics chain capable of operating at scale under realistic detector conditions.

2. Methodology

The paper details the development and validation of a 3-ASIC cryogenic readout solution and its integration into a large-scale prototype, ProtoDUNE-HD, which operated at the CERN Neutrino Platform in 2024.

• Electronics Chain Design:
  • LArASIC (Analog Front-End): A 16-channel CMOS ASIC responsible for charge amplification and shaping. It features adjustable gain and peaking time to optimize dynamic range and noise.
  • ColdADC (Digitization): A 16-channel ASIC that performs 14-bit digitization using a 15-stage pipelined architecture with built-in self-calibration to ensure linearity despite cryogenic temperature shifts.
  • COLDATA (Communication): A control and data concentration ASIC that manages digital communications, performs 8b10b encoding, and serializes data for high-speed transmission (1.25 Gbit/s) via twinax cables.
• Hardware Integration: The ASICs are mounted on Front-End Motherboards (FEMBs), which are housed in metallic Cold Electronics (CE) boxes attached to the Anode Plane Assemblies (APAs). These connect to Warm Interface Boards (WIBs) via custom Samtec cables.
• Validation via ProtoDUNE-HD: The system was tested in a 770-ton LArTPC. The researchers used a combination of internal pulser scans (for gain, linearity, and crosstalk calibration) and cosmic-ray muon data (for real-world signal-to-noise ratio assessment).

3. Key Contributions

• Monolithic FEMB Design: Transitioned from a mezzanine-based design to a monolithic architecture where the COLDATA ASIC replaces the previous FPGA-based approach, significantly simplifying the cold electronics.
• Advanced Calibration Framework: Developed a sophisticated pulse-shape fitting and deconvolution model to correct for channel-dependent electronics distortions (overshoot/undershoot), enabling standardized gain and peaking-time extraction.
• Robust Power and Timing Architecture: Implemented a highly regulated power distribution scheme using cryogenic LDOs and a centralized Power and Timing Card (PTC) to ensure low-noise operation and global synchronization across the detector.
• Integrated Monitoring: Provided a comprehensive analog monitoring path to transmit LDO voltages, temperature, and reference voltages from the cold volume to the warm electronics for real-time diagnostics.

4. Results

• Reliability: Out of 10,240 channels, 10,207 functioned normally. Only a negligible fraction (<0.3%) were excluded due to mechanical issues or high-voltage discharges.
• Noise Performance: The electronics achieved excellent noise levels. After offline correlated noise filtering, typical noise was 450–600 $e^-$ ENC for induction channels and 350–500 $e^-$ ENC for collection channels.
• Signal-to-Noise Ratio (SNR): The most probable SNRs were 14 (1st induction), 17 (2nd induction), and 40 (collection), exceeding DUNE physics requirements.
• Linearity and Crosstalk: The system demonstrated high linearity (non-linearity < 0.1% for most of the range) and extremely low crosstalk (typically < 0.1%, with a maximum of ~0.3% due to ColdADC multiplexing architecture).
• Saturation Behavior: The study identified a "power rail sag" effect during extreme saturation, where high current surges in one channel could induce opposite-polarity responses in neighbors. However, the team determined this is manageable through offline software corrections.

5. Significance

This work represents the successful finalization of the charge readout electronics for the DUNE Horizontal Drift far detectors. By proving that custom CMOS ASICs can operate reliably and performantly in a massive-scale liquid argon environment, the DUNE collaboration has cleared a major technical hurdle. The results validate the transition from prototype to production, providing the high-fidelity data acquisition capabilities required to explore neutrino mass ordering, CP violation, and physics beyond the Standard Model.