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Description
This study presents the physical characterization and the development of a 3D reconstruction framework for a high-resolution muon tracking detector, originally designed for the NA64 experiment at CERN. The instrument consists of two detection planes, each featuring a grid of 16 horizontal (X) and 16 vertical (Y) overlapping plastic scintillating bars. The bars are arranged in an interleaved architecture with a symmetric readout of 8 channels per side, a configuration optimized for high-rate particle environments now being adapted for cosmic muon radiography.
To transition this technology from fixed-target particle physics to geophysical exploration, we implemented a modular processing pipeline. The detector's response is modeled by calculating the pixelated angular acceptance ($\mathcal{T}$) and the solid angle ($\delta\Omega$) based on the specific $16 \times 16$ bar geometry and the inter-planar separation. This geometric model is integrated with digital elevation data to perform ray tracing through a voxelized volume, utilizing a digital differential analyzer (DDA) approach to determine the intersection lengths ($F_{i,k}$) within the grid.
The 3D density reconstruction is formulated as a linearized inverse problem based on the density-length relationship ($\gamma = F\rho$). Due to the underdetermined and ill-conditioned nature of the muographic survey, the solution is stabilized using a Bayesian Maximum A Posteriori (MAP) estimation. This allows for the incorporation of geologically relevant prior information ($\rho^{(0)}$) and a weight matrix ($W_{\gamma}$) derived from the Poissonian statistics of the observed muon counts. We present simulation results evaluating the sensitivity of the $16X \times 16Y$ hodoscope to density anomalies such as mineral veins and cavities. By leveraging the high-precision tracking capabilities inherited from its original design for CERN experiments, the system demonstrates the ability to resolve subsurface structures with high spatial resolution and quantified uncertainty.
A defining characteristic of this instrument is its extreme compactness, designed for high-resolution imaging in highly confined spaces. The detection planes consist of scintillating bars with individual dimensions of $70 \text{ mm}$ in length, $3 \text{ mm}$ in width, and a thickness of $1 \text{ mm}$. By implementing a $1 \text{ mm}$ overlap between adjacent bars, the configuration of $16X$ and $16Y$ elements results in a precise active area of approximately $33 \times 33 \text{ mm}^2$. This miniaturized footprint, combined with the high-density readout, allows for the deployment of the hodoscope in narrow boreholes and small-scale mining galleries (typical of small and medium-sized mining operations) where conventional muographic equipment cannot be installed.