Simulation Study on the Discrimination of $0νββ$ Events from Single-Electron Events Using Orthogonal-Strip HPGe Detectors

Abstract

Neutrinoless double beta decay ($0νββ$) offers a sensitive probe of neutrino mass and its Majorana nature. Orthogonal-strip high-purity germanium (HPGe) detectors with high spatial resolution provide a promising approach for distinguishing $0νββ$ events from single-electron backgrounds. In this work, a simulation framework was developed to evaluate the discrimination performance of these detectors. The framework combined Geant4 simulations with a hybrid numerical-analytical approach to model charge cloud dynamics. A dual-branch convolutional neural network (CNN) was implemented to extract topological features for event classification. The impact of detector geometry on discrimination performance was quantitatively assessed. For a fixed crystal thickness of 15 mm, the background rejection efficiency decreased from 79.5\% to 59.0\% as the strip pitch increased from 0.1 mm to 0.5 mm. For a strip pitch of 0.25 mm, a crystal thickness of 20 mm was found to be optimal, balancing full-energy peak (FEP) efficiency with discrimination capability. These results demonstrate that orthogonal-strip HPGe detectors can effectively suppress single-electron backgrounds, and provide quantitative guidance for detector design in $^{76}$Ge $0νββ$ decay searches.

Figures (11)

• Figure 1: Schematic diagram of the orthogonal-strip HPGe detector. The crystal features an 80-mm diameter and a 15-mm thickness, with orthogonal strip electrodes fabricated on the top and bottom surfaces. The number of strips and their pitch are illustrative and not to scale.
• Figure 2: Simulated energy spectrum of individual electrons (left) and the angular distribution of the two electrons (right) for $0\nu\beta\beta$ events generated by BxDecay0.
• Figure 3: Spatial distributions of energy deposition for a $0\nu\beta\beta$ event (left) and a single-electron event (right). The red star denotes the initial vertex at (0,0,0). Marker sizes and colors (from purple to yellow) scale with the energy deposited ($\Delta E_i$) at each interaction step.
• Figure 4: Schematic of the charge cloud evolution and collection process in an orthogonal-strip HPGe detector. Electron and hole clouds drift toward opposite electrodes, expanding through Coulomb repulsion and thermal diffusion. When the cloud size becomes comparable to the strip pitch, charge sharing occurs across multiple electrodes. The cathode and anode strips are depicted non-orthogonally here for clarity.
• Figure 5: Validation of the hybrid numerical-analytical approach against full numerical simulations. Energy profiles projected along the x-axis for electrons collected by the top electrode (left) and along the y-axis for holes collected by the bottom electrode (right). The top row shows a typical $0\nu\beta\beta$ event, while the bottom row shows a single-electron event. In each panel, the initial energy deposition from Geant4 (light purple dashed line) is compared with the hybrid approach (orange curves) and full numerical simulations (blue points with error bars representing mean and standard deviation from multiple runs). The relative standard deviation remains within 10%, reflecting the statistical stability of charge collection for these MeV-scale events. Residuals (black dots) and ±10 keV bands (shaded) are shown in the lower panels.