ProtoDUNE-SP Liquid Argon TPC

• ProtoDUNE-SP is a single-phase liquid argon time projection chamber that validates scalable design and key technologies for future multi-kton neutrino experiments.
• Its robust cryogenic and argon purification systems achieve electron lifetimes over 30 ms, ensuring minimal charge loss across meter-scale drifts.
• Integrated readout electronics, photon detection, and calibration systems deliver high-resolution 3D event reconstruction and inform DUNE far detector design.

The ProtoDUNE Single-Phase (ProtoDUNE-SP) Liquid Argon Time Projection Chamber (LArTPC) is a full-scale prototype for the Deep Underground Neutrino Experiment (DUNE) far detector, constructed and operated at CERN. Serving as a technical testbed and physics demonstrator, ProtoDUNE-SP has validated key technologies and operational strategies intended for multi-kton-scale neutrino and rare-event observatories, providing the LArTPC community with the first high-granularity, large-mass, single-phase TPC beam test data and benchmarks for next-generation experiments (Collaboration et al., 2021, Collaboration et al., 2020, Collaboration et al., 17 Nov 2025).

1. Detector Architecture and Active Volume

ProtoDUNE-SP employs a membrane cryostat with an internal volume of approximately 8.5 m × 8.5 m × 7.9 m, enclosing about 770 t of liquid argon. The TPC is segmented into two horizontal-drift regions by a central cathode plane assembly (CPA) biased to –180 kV, producing a uniform drift field of $E = 500$ V/cm over a maximum drift length of 3.6 m on each side (Collaboration et al., 2021, Manenti, 2017, Collaboration et al., 17 Nov 2025). The active TPC volume, defined by the anode and field-cage boundaries, measures 7.2 m (x) × 6.1 m (y) × 7.0 m (z) and contains three Anode Plane Assemblies (APAs) per side—each 6 m tall and 2.3 m wide (Collaboration et al., 2021, Collaboration et al., 11 Jul 2025). The field cage, composed of aluminum profile rings linked by resistor chains (ΔV ≈ 3 kV per step), ensures field uniformity and shields the LAr-gas interface.

The wire-readout configuration on each APA includes four layers: a shielding grid (not read out), two induction planes (U, V at ±35.7°), and a vertical collection plane (X), with wire pitches of 4.67–4.79 mm (for spatial resolutions ∼1–3 mm). Each APA carries 2560 readout wires, yielding 15,360 TPC channels in total (Collaboration et al., 2021, Majumdar et al., 2021). Induction planes are biased to ensure transparency to drifting electrons and provide bipolar signals; the collection plane records unipolar charge signals for calorimetry and event topology reconstruction (Manenti, 2017).

2. Cryogenics and Argon Purification

Reliable electron transport over meter-scale drift distances is contingent on achieving and maintaining extreme bulk liquid argon purity. The ProtoDUNE-SP cryogenic system is architected for continuous liquid recirculation (≈7.0 t/h), rapid purification with molecular sieves (for H₂O) and copper catalysts (for O₂), and efficient boil-off gas condensation (Collaboration et al., 2021, Collaboration et al., 11 Jul 2025). Purified LAr is returned at the bottom of the TPC and recirculated via in-cryostat plumbing designed for homogeneous mixing.

Liquid argon purity is monitored by both dedicated ICARUS-style purity monitors and in situ cosmic-ray muons, measuring the drift electron lifetime $\tau_e$ via exponential charge attenuation: $Q(t) = Q_0\,e^{-t/\tau_e}$ For optimized operation, ProtoDUNE-SP routinely achieved $\tau_e > 30$ ms (O₂-equivalent <10 ppt), supporting <5% charge loss across the maximum drift distance ($t_{max} \simeq 2.25$ ms) (Collaboration et al., 11 Jul 2025). Spatial gradients in purity (BL→BR, bottom→top) are tracked and correspond to cryogen flow patterns, with systematic multi-point monitoring validating negligible stratification on operational timescales.

3. Readout Electronics, Data Acquisition, and Photon Detection

Front-end analog processing occurs on cold ASICs directly mounted on the APA frames, each handling 128 channels with programmable gain and shaping times. Signals are digitized at 2 MHz with 12-bit resolution (Manenti, 2017, Collaboration et al., 2021, Borga et al., 2018). The continuous waveforms are optically multiplexed via Warm Interface Boards (WIBs) and transmission over 9.6 Gb/s optical fibers to the external data acquisition (DAQ).

The ProtoDUNE-SP DAQ leverages the FELIX system (Front-End LInk eXchange), an FPGA-based PCIe Gen3 I/O architecture that bridges TPC readout and a commodity server farm (Borga et al., 2018). A single FELIX PCIe card receives up to ten 9.6 Gb/s optical streams (payload 7.68 Gb/s), performing CRC verification and DMA transfer to host DRAM. BoardReader processes extract triggered 5 ms event slices (≈60 MB uncompressed per APA), perform lossless compression (mean factor 3.7–4.0, via Intel QAT hardware acceleration and DEFLATE), and forward the result over 10 GbE for event building. The system sustains 96 Gb/s aggregate input, trigger rate up to 25 Hz, with sub-12 ms event-building latency (Borga et al., 2018). Early operations demonstrated robust performance, with dead channel fractions <0.25% and negligible data loss.

Scintillation light collection is performed by Photon Detection System (PDS) modules embedded within the APAs, exploiting dip-coated and double-shift light guides (with TPB wavelength shifting) and ARAPUCA/X-ARAPUCA devices coupled to SiPMs (Gallice, 2021, Collaboration et al., 2021). The PDS delivers time resolution $\sigma_t \sim 14$ ns and achieves photon yields well above the DUNE requirement (1.9 PE/MeV at 3.6 m for ARAPUCA) (Gallice, 2021, Collaboration et al., 2020).

4. Calibration, Xenon Doping, and Operational Performance

Calibration utilizes CRT-tagged through-going muons and cathode-crossing tracks for drift velocity, electron lifetime, field-uniformity, and space-charge mapping (Collaboration et al., 11 Jul 2025, Collaboration et al., 2020). Energy loss normalization is set via stopping muons; field-dependent nonuniformities in recombination and dE/dx are corrected via 3D maps and the Modified Box model.

During extended operation, xenon doping up to 19 ppm was conducted to shift LAr scintillation from 128 nm to 178 nm and to compensate for N₂-induced light quenching (Gallice, 2021). Key findings indicate that even low Xe concentrations yield substantial increases in Rayleigh scattering length, markedly improving PDS response uniformity across the TPC. Fractional light collection rose >×2, uniformity within ~10%, and charge collection was unaffected (<1% deviation up to 20 ppm Xe) (Gallice, 2021).

Performance metrics include:

• Spatial resolution: ≲1.5 cm (wires), ≲1 mm (drift; dt = 0.5 μs).
• Energy resolution: 5% for muons (dE/dx), 10–15%/$\sqrt{E}$ for EM showers.
• Detection thresholds: ∼0.1 fC equivalent.
• Signal-to-noise (collection): >16:1 MIP (C-plane), >50:1 in some runs (Manenti, 2017, Collaboration et al., 2021, Collaboration et al., 2020).

5. Physics Program and Exclusive Hadron–Argon Scattering Measurements

A primary physics deliverable for ProtoDUNE-SP is the measurement of exclusive π⁺–argon cross sections in the 500–800 MeV energy range, critical for tuning intranuclear cascade (FSI) and secondary-interaction (SI) models in neutrino event generators (Collaboration et al., 17 Nov 2025). Utilizing fine-grained 3D tracking and calorimetry, absorption, charge exchange (π⁺→π⁰), and other inelastic channels are reconstructed and separated.

Methodological innovation includes a simultaneous binned-likelihood fit (with MC statistical error propagation and Barlow–Beeston treatment) to data and “thin-slice” MC modeling, beam–momentum reweighting, and comprehensive systematic error treatment. The following cross sections (mb, combined stat⊕syst) were measured:

Tₚ [MeV]	σ_abs	σ_cex	σ_other	σ_tot
550	160 ± 36	148 ± 52	276 ± 104	584 ± 122
650	161 ± 42	144 ± 44	253 ± 61	558 ± 86
750	131 ± 43	158 ± 46	301 ± 65	590 ± 90

Correlation matrices demonstrate significant shared systematics. These results, including the first-ever π⁺–Ar charge-exchange measurements, provide critical inputs for SBND and DUNE event generator tuning (Collaboration et al., 17 Nov 2025).

6. Engineering Lessons and Impact on DUNE Far Detector Design

ProtoDUNE-SP provides definitive operational validation for membrane cryostats, cold modular TPC assemblies, ultra-high purity LAr cryogenics, field-cage and cathode/HV architectures, cold digital readout, and hybrid photon detection (Collaboration et al., 2021, Majumdar et al., 2021). Specific lessons include:

• Membrane integrity and cryostat leak rates confirm scalability to 17 kt-class modules.
• Cold electronics in LAr deliver necessary noise performance and reliability.
• Modular APA/CPA/field-cage assembly approaches enable efficient installation and maintainability.
• SCADA-based unified controls, robust DAQ, and flexible trigger/run modes support sustained high-throughput physics operation.
• Novel approaches, such as xenon doping for light collection uniformity and hardware-accelerated lossless compression, are practically validated at scale (Gallice, 2021, Borga et al., 2018).

All principal system requirements for DUNE have been confirmed or exceeded in ProtoDUNE-SP, including drift electron lifetime (>30 ms), field uniformity (<1% deviation), and light yield. The ProtoDUNE-SP operational experience directly informed the engineering, QA/QC, and commissioning procedures now standard in DUNE construction and predicted for other megaton-scale LArTPCs (Collaboration et al., 2021, Majumdar et al., 2021, Collaboration et al., 2020).

7. Summary and Outlook

ProtoDUNE-SP stands as the benchmark implementation of single-phase LArTPC technology at the kton-scale, confirming that multi-meter drift fields, full cold-electronics integration, high channel-density readout, and advanced PDS subsystems can be scaled without compromise to physical or operational performance. In addition to delivering world-first exclusive hadron–argon scattering results, ProtoDUNE-SP has validated the architectural path for DUNE and established the experimental and engineering standards for the next generation of precision neutrino, rare-event, and astroparticle physics with liquid argon (Majumdar et al., 2021, Collaboration et al., 17 Nov 2025).

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References: (Borga et al., 2018, Manenti, 2017, Majumdar et al., 2021, Collaboration et al., 2021, Gallice, 2021, Collaboration et al., 2020, Collaboration et al., 11 Jul 2025, Collaboration et al., 17 Nov 2025)