Opaque Hemispheric Detector Array

• Opaque Hemispheric Detector Arrays are advanced radiation detectors that use an intentionally opaque scintillator medium and dense fiber arrays to localize scintillation light with sub-millimeter accuracy.
• They achieve over 80–90% light confinement within a few centimeters by engineering a short scattering length, enhancing topological event reconstruction and background rejection.
• The modular hemispheric design enables scalable configurations for neutrino studies, CP violation research, and rare event searches, offering improved particle discrimination and efficient solid-angle coverage.

An Opaque Hemispheric Detector Array is a radiation detection system in which an opaque scintillating medium is instrumented with a dense array of wavelength-shifting optical fibers or equivalent light collectors arranged to capture and localize the scintillation light produced by particle interactions. Unlike traditional transparent detectors, where scintillation light propagates through the volume and is detected at distant photosensors, the opaque medium confines light near its creation point, enabling high-resolution spatial imaging and topological event reconstruction. This detection concept, realized in the LiquidO technique, introduces novel capabilities for neutrino detection, particle identification, and low-background rare event searches.

1. Physical Principles and Detector Architecture

The underlying principle of an opaque hemispheric detector array is the stochastic confinement of scintillation light within an optically thick ("opaque") scintillator. The scintillator is engineered to have a short scattering length (on the order of 0.5–5 mm) and an absorption length much longer than the detector size (meters scale), ensuring that optical photons undergo multiple random scatterings and remain localized in the vicinity of their creation before eventual absorption or capture by fibers (Cabrera et al., 2019, Navas-Nicolás et al., 14 Mar 2025, Collaboration et al., 18 Jul 2025).

Embedded within this volume is a three-dimensional grid of wavelength-shifting fibers. These fibers absorb the local scintillation light and re-emit it at a wavelength matched to the photosensor's peak sensitivity, guiding the photons by total internal reflection to silicon photomultipliers (SiPMs) or photomultiplier tubes (PMTs) at the fiber ends. The array can be arranged with pitches as fine as a few millimeters (3.2 mm demonstrated in a cubic 8×8 × 8 array (Collaboration et al., 18 Jul 2025)) or up to about 1 cm pitch for larger volumes (Cabrera et al., 2019).

The "hemispheric" attribute refers to the modular deployment of many such detector elements—each with their own self-contained opaque scintillator and fiber array—in a configuration that covers a solid angle approaching 2π or more, as considered for accelerator-based neutrino experiments (Tang et al., 2020). This supports directional reconstruction, uniform coverage, and efficient solid-angle utilization.

2. Scintillator Composition and Light Confinement

The scintillator media utilized in these detectors are customized to be intentionally opaque, usually by doping standard organic scintillators (e.g., LAB+PPO) with waxes or polymers. Typical formulations involve 10–20% paraffin wax or similar compounds, tuned to achieve sub-millimeter-to-millimeter scattering lengths (Navas-Nicolás et al., 14 Mar 2025, Collaboration et al., 18 Jul 2025). In the case of NoWaSH (a wax-loaded liquid scintillator), the scattering length can be further adjusted in situ by temperature, providing a tunable degree of opacity.

The highly scattering medium does not significantly degrade the intrinsic light yield; rather, it redirects the photons to follow a random walk, creating a "light ball" spatially coincident with the energy deposition. Only a small fraction of light escapes to distant fibers or detectors, with the majority collected locally. For example, with a few-millimeter scattering length, at least 80–90% of the light is confined within a radius of 4–5 cm from the interaction vertex in a 10-liter prototype (Navas-Nicolás et al., 14 Mar 2025).

For comparison:

Medium	Scattering Length	Light Confinement	Temperature Control
LAB+PPO (standard)	transparent	Light spreads throughout	No
NoWaSH–20 (opaque)	~mm	90% within ≤5 cm	Yes (via temp)

3. Topological Event Imaging and Resolution

The spatial mapping of the scintillation light as collected by the fiber array enables the reconstruction of detailed event topology at millimeter or sub-millimeter scale. The number of photoelectrons detected in each fiber provides a sub-pixel projection of the deposited energy distribution:

• For traversing charged particles, such as cosmic muons, the profile of the light collection as a function of fiber row yields a reconstructed track. In a 30-mm cube with an 8×8 fiber grid, muon tracks are reconstructed with an average position resolution of 450 μm per row for opaque media, improving upon the transparent case by a factor of 1.6 (Collaboration et al., 18 Jul 2025).
• Electrons (or positrons) below a few MeV produce spatially compact light balls, easily distinguishable from gamma-induced Compton cascades, which generate a string or halo of isolated light balls (Cabrera et al., 2019, Navas-Nicolás et al., 14 Mar 2025).
• The preserved event topology permits efficient background rejection and particle identification, including the possibility of neutron-gamma separation and positron/electron discrimination.

Combining the spatial pattern with pulse-shape discrimination, enabled by sub-100 ps timing electronics, allows the separation of Cherenkov and scintillation components, further enhancing particle identification (Navas-Nicolás et al., 14 Mar 2025).

4. Implementation in Hemispheric Arrays: Large-Scale and Modular Detectors

The opaque hemispheric array concept can be scaled to large fiducial masses for accelerator or reactor neutrino projects. Each module ("unit cell") is self-segmented by virtue of the light-confining medium—removing the requirement for mechanical segmentation or optically isolated cells commonly used in transparent scintillator arrays (Cabrera et al., 2019, Tang et al., 2020).

Key implementation requirements for future experiments include:

• Fiducial mass: Simulations indicate that to reach high-sensitivity physics goals, such as CP violation at 3σ for 75.6% of $\delta_{CP}$ values, modules of 120–220 kton are needed, with efficiency for electronlike events exceeding 50% (Tang et al., 2020).
• Detection efficiency and background suppression: The intrinsic imaging yields high particle ID (electron/muon), allowing suppression of beam and neutral-current backgrounds. Signal-to-background ratios greater than 10–20 are feasible in simulation for accelerator experiments (Tang et al., 2020).
• Near-far array configuration: Modularity allows strategic placement of near detectors (~1 kton, even as low as 10–20 tons for specific sterile neutrino tests) at short baselines (~250 m from the source), assisting in systematics control and background subtraction (Tang et al., 2020).

Integration with magnetized modules or enhanced dopant loading (e.g., gadolinium for neutron tagging, indium for CC event tagging) is facilitated by the tolerance of the opaque medium for high dopant concentrations.

5. Physics Capabilities and Applications

The opaque hemispheric detector array, especially via the LiquidO approach, has demonstrated or projected advantages in multiple domains:

• Neutrino detection: The combination of topological imaging and flexible dopant chemistry enables both antineutrino inverse beta decay detection (with optimized background rejection and neutron capture tagging) and charged-current tagging for solar electron neutrino detection below traditional thresholds (Cabrera et al., 2019, Navas-Nicolás et al., 14 Mar 2025).
• CP violation and oscillation studies: Simulated use in long-baseline experiments finds that, compared to conventional water Cherenkov detectors, opaque arrays offer superior energy resolution (e.g., $$\frac{\sigma_E}{E} \simeq \frac{5\%}{\sqrt{E\,(\mathrm{MeV})}}$$) and allow more of the $\delta_{CP}$ parameter space to be probed at high significance (e.g., 75.6% at 3σ for suitable masses and coverage) (Tang et al., 2020).
• Active-sterile neutrino searches: The high imaging fidelity and low backgrounds permit probing of regions motivated by the gallium and reactor anomalies at 2σ or higher confidence, with modest near detector masses (Tang et al., 2020).
• Rare event searches and medical applications: The technique's fine segmentation and background rejection potential extend its use to double beta decay, dark matter, and medical imaging modalities where localization of low-energy depositions is critical (Navas-Nicolás et al., 14 Mar 2025, Collaboration et al., 18 Jul 2025).

6. Theoretical and Quantum Measurement Considerations

The "one-and-only-one" detection feature intrinsic to the opaque hemispheric array shares a conceptual analogy with the "ideal detector array" studied in quantum measurement theory (Zirpel, 2022). In the idealized formalism, the array enforces that exactly one detector indicates a detection for each measurement due to the exclusive mapping of system eigenstates to detector states via a unitary interaction. Similarly, in practical opaque hemispheric arrays (provided geometrical completeness and no gaps), each event can be uniquely mapped to a single localized light ball and hence a single (or small number of adjacent) detector elements, minimizing ambiguity and supporting clear event counting.

Feature	Opaque Hemispheric Array	Ideal Detector Array (Zirpel, 2022)
Physical realization	Optical confinement in 3D geometry	Abstract tensor product Hilbert space
Detection exclusivity	Localized light ball, event-by-event	Exactly one pointer state per event
Measurement outcome	Single module/fiber(s) per event	Single detector "on" per event

A key distinction remains: whereas the ideal detector array is a formal construct with strict orthogonality, real opaque arrays must account for possible overlaps or partial detection due to edge effects or imperfections.

7. Experimental Prototypes and Performance Benchmarks

Proof-of-principle prototypes have empirically validated the opaque array concept:

• Small-scale cylinder (0.25 L): Demonstrated local confinement of light in a LAB-based scintillator made opaque by paraffin loading; near-entry fibers measured a 2× increase in collected light for localized events as compared to the transparent case (Cabrera et al., 2019).
• 10 L Mini-LiquidO: NoWaSH–20 medium (wax-loaded LAB+PPO) tunable by temperature; at low temperatures, 90% of light confined within 5 cm. Time resolution <100 ps enabled clear separation of Cherenkov and scintillation light, validating particle identification via pulse-shape discrimination (Navas-Nicolás et al., 14 Mar 2025).
• 30 mm Cube (LiquidO): 8×8 fiber grid with 3.2 mm pitch, using a wax-based opaque scintillator with ∼0.5 mm scattering length. For cosmic muons, achieved 450 μm per-row position resolution, 25% higher light yield compared to the transparent reference (Collaboration et al., 18 Jul 2025).

Outcomes confirm that the opaque hemispheric approach can provide fine-grained imaging, support high-efficiency detection, and tolerate significant dopant loading—each observed both theoretically and in experiment.

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In summary, the opaque hemispheric detector array introduces an experimentally validated architecture for localizing and imaging particle interactions in optically thick scintillator media. Through stochastic light confinement and dense fiber readout, it enables sub-millimeter scale spatial resolution, enhanced particle identification, high background rejection, and versatile event reconstruction. These features directly address longstanding limitations of transparent detectors and offer a scalable path for a broad suite of applications in neutrino physics, rare event searches, and beyond (Cabrera et al., 2019, Tang et al., 2020, Navas-Nicolás et al., 14 Mar 2025, Collaboration et al., 18 Jul 2025, Zirpel, 2022).