NeuPix: Lunar Particle Sensor

• NeuPix is a hybrid active pixel sensor system that detects and spectrally analyzes lunar neutrons, gamma-rays, and 10–100 MeV charged particles near the surface.
• Its design integrates specialized converter layers with high-resolution pixel matrices to distinguish particle types through event topology and energy deposition patterns.
• In the LunPAN mission, NeuPix enhances lunar radiation mapping, supports space weather prediction, and informs radiation risk assessments for future exploration.

NeuPix is a hybrid active pixel sensor system developed for the LunPAN (Lunar Particle Analyzer Network) mission, designed to detect and spectrally characterize neutrons, gamma-rays, and lower-energy charged particles (10 MeV–100 MeV) in the unique particle environment near the lunar surface. NeuPix’s mission is integral to mapping the lunar radiation field, quantifying albedo particles, and addressing challenges in space physics, lunar geology, space weather prediction, and radiation risk assessment for future lunar exploration (Hulsman et al., 12 Nov 2025).

1. Scientific Objectives

The principal aim of NeuPix is to acquire precise spectral measurements of lunar albedo neutrons and gamma-ray fluxes, as well as lower-energy charged particles, specifically in the 10 MeV to 100 MeV range. This complements the higher-energy, charge-resolved measurements provided by the companion Pix.PAN instrument, thereby enabling a comprehensive characterization of the full lunar energetic particle environment. Measurement of these fluxes directly underpins studies of lunar surface composition, particle transport phenomena, and the secondary radiation hazards relevant to both crewed and robotic missions (Hulsman et al., 12 Nov 2025).

2. Detection Principles and Sensor Architecture

NeuPix utilizes a hybrid active pixel sensor approach, wherein incoming particles interact with custom converter materials layered atop high-resolution pixel detector arrays. These converter elements are selected to optimize neutron and gamma-ray conversion efficiency while preserving the system’s capability to register lower-energy charged particles. The hybrid configuration allows NeuPix to disentangle particle species via distinctive energy deposition topologies and temporal coincidence patterns.

Key elements of the NeuPix system architecture include:

• Sensor-Converter Combinations: Specialized converter layers (e.g., boron or gadolinium for neutron capture, high-Z for gamma interactions) are integrated onto silicon-based pixel matrices.
• Event Topology Discrimination: The pixel-level granularity enables the identification of unique interaction signatures for neutrons, gamma-rays, and charged particles, based on cluster shape, spatial extent, and energy deposition per pixel.
• Active Pixel Readout: Fast, low-noise readout electronics facilitate time-stamping and correlated coincidence analysis, allowing for spectral reconstruction and real-time background suppression.

3. Spectrometry and Performance Characteristics

NeuPix enables spectrometric measurements by leveraging the differential energy deposition patterns of the detected particles:

• Spectral Measurement: Energy-dependent response functions are established via calibration, allowing deconvolution of detected cluster signals into incident energy spectra for each particle species.
• Flux Resolution: The system is optimized to resolve both the continuum of solar and cosmic neutron/gamma flux incident on the lunar surface and the distinct secondary albedo spectra reflected from or emitted by the regolith (Hulsman et al., 12 Nov 2025).
• Simulations and Expected Sensitivity: Mission simulations confirm that NeuPix can robustly cover the lunar albedo neutron and gamma-ray flux regime, closing a critical observational gap below 100 MeV that is inaccessible to traditional, charge-only spectrometers.

4. Role within LunPAN Mission

Within the LunPAN mission, NeuPix’s coverage of low- to intermediate-energy particles is essential for:

• Complete Lunar Particle Mapping: By jointly analyzing data from NeuPix and Pix.PAN, the mission achieves a unified view of charged, neutral, and electromagnetic particle spectra across a wide energy span.
• Complementarity with Magnetic Spectrometry: Whereas Pix.PAN directly measures particle rigidity and energy via trajectory curvature in a magnetic field (100 MeV–10 GeV), NeuPix provides sensitivity to neutral (neutron, gamma) and low-energy charged particles, leveraging events with no or minimal track signatures (Hulsman et al., 12 Nov 2025).
• Enabling New Science: NeuPix enables studies of neutron-induced lunar surface activation, secondary production mechanisms, and spatial/temporal flux variations tied to solar activity, all of which inform both fundamental physics and operational risk models.

5. Calibration, Operation, and Data Management

NeuPix system calibration is performed both pre-launch and in-flight to ensure spectrometric fidelity:

• Energy Calibration: Laboratory procedures utilize mono-energetic neutron and gamma sources to map sensor-converter response functions over the operational energy range.
• In-Flight Alignment and Monitoring: Regular calibration runs and cross-correlation with Pix.PAN event streams enable dynamic adjustment of detector response models and real-time health monitoring.
• Data Reduction and Transmission: Selected clustered events, tagged by particle topology, are time-stamped and stored in an event buffer for downlink. Ground analysis pipelines reconstruct spectra and perform joint event analyses with Pix.PAN.

6. Impact and Scientific Significance

NeuPix will fill a critical measurement niche by directly resolving the spectral characteristics of neutrons, gamma rays, and low-energy charged particles originating from or near the lunar surface. These capabilities directly inform the following domains:

• Space Physics: Detailed characterization of secondary particle production and transport in the lunar environment.
• Lunar Geology: Mapping of neutron and gamma flux can be correlated with elemental surface composition, supporting remote sensing and subsurface prospecting.
• Space Weather and Exploration Risk: Quantitative risk models for human and robotic assets in lunar orbit or on the surface are improved by accurate knowledge of ambient and albedo radiation components (Hulsman et al., 12 Nov 2025).

7. Future Directions

NeuPix technology sets a precedent for future lunar, deep space, or planetary particle studies requiring high spatio-temporal resolution for neutral and low-energy charged species. Ongoing phases of ESA’s “Small Missions for Exploration” program are expected to benchmark NeuPix’s in situ performance. A plausible implication is the adaptation of the NeuPix architecture for broader classes of planetary missions, targeting energetic particle environment mapping and radiation field assessment in diverse astrophysical contexts (Hulsman et al., 12 Nov 2025).