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Investigating Ultra-Low Energy Ionization Yield from Nuclear Recoils in Semiconductor Detectors via Molecular Dynamics Simulations

Chang-Hao Fang

Abstract

Nuclear recoil ionization yield constitutes a critical uncertainty source in low-energy detection for dark matter (DM) and coherent elastic neutrino-nucleus scattering (CE$ν$NS) experiments. We present a novel methodology employing molecular dynamics simulations to assess ionization yields in crystalline semiconductor detectors. This non-parameterized approach resolving inherent limitations of traditional Lindhard model through explicit incorporation of crystal condensed matter effects, facilitating a seamless reliability from high-energy ($E>10$\,keV) to electron-hole pair (EHP) regimes. Our model achieves the best agreement with experimental data in silicon to date, especially at the minimal energy level of a single EHP. Meticulously consideration of ion transport mechanisms reveals fundamental ionization yield distributions, superseding conventional single-value models. The distributional paradigm extends the DM-nucleon elastic scattering exclusion limit to 0.29\,GeV/$c^2$ under single-EHP sensitivity. We further report advancements in modeling quantum effects and channeling phenomena affecting ionization yields in high-purity germanium detectors.

Investigating Ultra-Low Energy Ionization Yield from Nuclear Recoils in Semiconductor Detectors via Molecular Dynamics Simulations

Abstract

Nuclear recoil ionization yield constitutes a critical uncertainty source in low-energy detection for dark matter (DM) and coherent elastic neutrino-nucleus scattering (CENS) experiments. We present a novel methodology employing molecular dynamics simulations to assess ionization yields in crystalline semiconductor detectors. This non-parameterized approach resolving inherent limitations of traditional Lindhard model through explicit incorporation of crystal condensed matter effects, facilitating a seamless reliability from high-energy (\,keV) to electron-hole pair (EHP) regimes. Our model achieves the best agreement with experimental data in silicon to date, especially at the minimal energy level of a single EHP. Meticulously consideration of ion transport mechanisms reveals fundamental ionization yield distributions, superseding conventional single-value models. The distributional paradigm extends the DM-nucleon elastic scattering exclusion limit to 0.29\,GeV/ under single-EHP sensitivity. We further report advancements in modeling quantum effects and channeling phenomena affecting ionization yields in high-purity germanium detectors.
Paper Structure (7 sections, 1 equation, 4 figures)

This paper contains 7 sections, 1 equation, 4 figures.

Table of Contents

  1. Introduction
  2. Molecular Dynamic Approach
  3. Results for Silicon and Germanium
  4. Mean values of QF
  5. Intrinsic Randomness and Distribution of QF
  6. Impact on Dark Matter Searches
  7. Summary and Future Prospects

Figures (4)

  • Figure 1: [left] Ionization production scenario: primary recoil atom (red), displaced atoms (orange), and lattice-bound atoms (blue). Grey fog and shadows denote electronic gas and ionization, respectively; dashed circles show an ideal lattice without thermal relaxation. [right] Energetic atom trajectories from a recoil event in MD simulation.
  • Figure 2: Comparison of QFs from MD simulations (red dots) with experimental measurements (colored crosses) for (a) silicon and (b) germanium detectors.
  • Figure 3: (a) Inherent distributions of sub-$\mathrm{keV_{nr}}$ QFs. (b) Angular dependence of QF at recoil energy of $20\,\mathrm{eV_{nr}}$.
  • Figure 4: 90% C.L. upper limits for the $\chi$-N interaction. Red line is obtained from recent SENSEI result adariFirstDirectDetectionResults2025 using distribution perspective.