Comprehensive Study of the Lunar Energetic Particle Environment with LunPAN

TL;DR

The paper addresses the lack of comprehensive in situ measurements of the lunar energetic particle environment outside Earth's magnetosphere. It proposes LunPAN, a 3-year lunar orbiter payload integrating PixPAN (100 MeV–10 GeV) and NeuPix (10 MeV–100 MeV plus albedo neutrons and gamma rays) to map GCRs, SEPs, albedo neutrons and gammas. The approach combines a compact magnetic spectrometer (PixPAN) with a hybrid neutron/gamma sensor (NeuPix), and includes onboard data processing pipelines and an orbit-selection framework to optimize science return. Key findings include estimated data rates, onboard processing strategies, and an orbit plan favoring a 100 km LLO to maximize albedo particle statistics and achieve 5% Poissonian uncertainty in ~2 days for many species, enabling robust radiation environment models and lunar geology inferences.

Abstract

LunPAN (Lunar Particle Analyzer Network) is a three-year mission proposal designed to comprehensively map the particle spectra in the lunar radiation field. It aims to provide precise measurements of Galactic Cosmic Rays (GCR), Solar Energetic Particles (SEP), and albedo particles, including charged particles, neutrons, and gamma-rays, originating from the Moon's surface. Therefore it will contribute to fundamental space physics, lunar geology sciences, space weather prediction, and radiation risk assessment for future lunar explorations. This is achieved through two state-of-the-art instruments; Pix.PAN and NeuPix. Pix.PAN is a compact magnetic spectrometer designed for precise measurements of penetrating charged particles, ranging from 100 MeV to 10 GeV. Based on the Mini.PAN project, Pix.PAN employs thin silicon pixel sensors optimized for energy resolution and particle identification. NeuPix is a hybrid active pixel sensor system capable of detecting neutrons, gamma-rays, and lower-energy charged particles between 10MeV and 100 MeV. Utilizing innovative sensor-converter combinations, NeuPix will provide spectral measurements of lunar albedo neutrons and gamma-ray fluxes. Currently, the LunPAN mission is accepted by ESA's "Small Missions for Exploration - Destination the Moon" call for a pre-A phase study. We will discuss mission outline and expected scientific performance of the PixPAN and NeuPix.

Figures (4)

• Figure 1: a) Geant4 implementation of LunPAN with its PixPAN (left) and NeuPix payloads (purple and brown squares to the right). b) CAD of the LunPAN platform by AerospaceLab with the 2 payloads
• Figure 2: a) Preliminary design of the mechanical structure of the PixPAN spectrometer. b) 300$\mu m$ thick Si wafer with "long thin pixels" at Advafab. c) Schematic of the sensor layout and how the "long thin pixels" are connected to the standard Timepix4 square readout cells.
• Figure 3: a) Sketch of the NeuPix instrument showing its main components (not to scale). b) Sketch of top view and 2 side views of the converter layer (not to scale) of the Neutron unit. c) Sketch of the Gamma unit with its shielding and sensor layer.
• Figure 4: Estimated times needed to reach 5% Poissonian uncertainty for a) albedo $\gamma$, b) slow neutrons, c) fast neutrons and d) GCR protons. Estimated times for LLOs and NRHOs are compared in each figure.