arXiv subjects

Dongming Mei

Publications and source records attributed to Dongming Mei.

At least 19 recordsLinked to original sources

arXiv2026

Fabrication and characterization of lithium-diffused inverted coaxial point-contact HPGe detectors

High-purity germanium (HPGe) detectors with low capacitance, low electronic noise, and stable high-voltage operation are important for high-resolution gamma-ray spectroscopy and rare-event searches. We report the fabrication and characterization of two compact p-type inverted coaxial point-contact (ICPC) HPGe detectors, AK01 and AK02, produced from crystals grown at the University of South Dakota. Both devices use lithium-diffused $n^{+}$ outer contacts together with amorphous-Ge/Al point-contact electrodes. The principal advance is the implementation of this hybrid contact process on two independently fabricated USD-grown ICPC prototypes and the demonstration of stable electrical and spectroscopic operation. Both detectors exhibited picoampere-level leakage currents, operational depletion-voltage ranges of approximately 200--250~V (AK01) and 240--260~V (AK02), and estimated effective plateau capacitances of approximately 0.84 and 0.87~pF, respectively. Measurements with a $^{137}$Cs source yielded archived full-energy-peak FWHM values of 1.71 and 1.62~keV near 662~keV for AK01 and AK02, while AK02 yielded 1.04~keV FWHM at 59.5~keV with $^{241}$Am. Electric-field calculations reproduce the expected ICPC field configuration for the measured detector geometry. A Geant4 model of AK02 is used as a qualitative response study; its 0.8-mm Li-diffused inactive layer is an illustrative model assumption rather than a measured thickness, and its Gaussian broadening is constrained by the experimental resolution. The work therefore establishes the fabrication and operation of compact lithium-diffused ICPC prototypes while identifying quantitative inactive-layer characterization as an important next step.

physics.ins-det↗

arXiv2026

Post-diffusion cooling effects on Hall-derived active lithium donor profiles in high-purity germanium

Lithium (Li) diffusion is commonly used to form $n^{+}$ contacts in high-purity germanium (HPGe) detectors, but the final electrically active donor profile can be sensitive to the post-diffusion thermal history. Li was introduced into HPGe coupons using a lithium-in-oil suspension and diffused for $30~\mathrm{min}$ at nominal temperatures of $240$--$310~^{\circ}\mathrm{C}$. Short- and long-cooling protocols were documented by measured witness-Ge cooling histories. Sequential material removal combined with Hall-effect measurements at $77~\mathrm{K}$ was used to reconstruct difference-derived apparent Hall donor profiles. Because Hall response in a nonuniform conducting layer is mobility weighted, these profiles are operational electrically active-donor metrics rather than direct local or total-Li concentration profiles. Complementary-error-function fits were used to obtain the extrapolated apparent intercept $N_{s,\mathrm{app}}$ and the apparent profile-width parameter $D_{\mathrm{app}}$. For the coupons studied, short cooling was associated with larger $N_{s,\mathrm{app}}$ and sharper profiles, whereas long cooling was associated with lower $N_{s,\mathrm{app}}$ and broader low-concentration tails. Fit-derived concentration-threshold depths likewise extended farther into the Ge bulk for the long-cooling coupons. These results show that the complete post-diffusion thermal history should be considered when parameterizing Hall-active Li-diffused $n^{+}$ contacts for HPGe detector fabrication.

physics.ins-det↗

arXiv2026

Temperature-Dependent Charge Transport in USD-Grown High-Purity Germanium: Interplay Between Freeze-Out and Multi-Scattering Mechanisms

We report temperature-dependent charge transport measurements in p-type high-resistivity germanium crystals grown at the University of South Dakota. Hall-effect and four-probe resistivity measurements were performed on five planar samples over the temperature range of 2-300 K. The apparent Hall mobility exceeds 10$^6$ cm$^2$ V$^{-1}$ s${^-1}$ at cryogenic temperatures and decreases systematically with increasing temperature, while the effective Hall carrier concentration exhibits strong carrier freeze-out behavior at low temperatures. The combined evolution of Hall mobility, effective Hall carrier concentration, and resistivity reveals distinct transport regimes associated with carrier freeze-out, extrinsic conduction, and phonon-limited scattering. The transport behavior is interpreted using a Matthiessens-rule-inspired phenomenological mobility model motivated by the combined influence of ionized impurity, neutral impurity, and acoustic phonon scattering. Variations among samples are correlated with differences in effective Hall carrier concentration and transport behavior. These measurements establish a transport baseline for USD-grown high-resistivity germanium crystals and provide guidance for future material optimization toward detector-grade high-purity germanium for low-background rare-event detector applications.

physics.app-ph↗

arXiv2026

Preparing Students for AI-Powered Materials Discovery: A Workflow-Aligned Framework for AI Literacy, Equity, and Scientific Judgment

Artificial intelligence (AI) is reshaping education, scientific training, and materials discovery. In materials science, AI models increasingly support property prediction, experiment prioritization, and hypothesis generation; however, the limiting factor is no longer only algorithmic capability but also whether students and educators can use AI with domain-specific scientific judgment. This workshop-informed white paper and curriculum-oriented position article argues that AI education for AI-powered materials discovery must move beyond tool access and surface-level interaction with generative AI systems toward a workflow-aligned model of AI literacy. We connect AI literacy to materials-informatics competencies: data provenance, domain-specific featurization, model validation, uncertainty quantification, physics informed reasoning, reproducibility, and experimental feedback. We also emphasize outcome-oriented equity: institutions should evaluate not only access, participation, and engagement, but also whether AI-enabled instruction produces comparable learning gains, transfer of learning, confidence calibration, defined as the alignment with students confidence and the quality or correctness of their work, persistence, and research readiness across student subgroups. The paper synthesizes relevant evidence, identifies risks for learners such as cognitive off-loading and cognitive surrender, and provides a dual-track curriculum model and implementation recommendations such as curriculum guides and an assessment plan for courses, bootcamps, workshops, and program-level reform. The central goal is to prepare students to become better scientists, not merely more efficient users of AI tools.

physics.ed-ph↗

arXiv2026

Process Development and First Cryogenic Operation of Compact Germanium Ring-Contact HPGe Prototypes

Rare-event experiments such as LEGEND-1000 require high-purity germanium (HPGe) detectors with excellent energy resolution, low electronic noise, and scalable low-background packaging. The germanium ring-contact (GeRC) concept addresses this need through a recessed ring-and-groove electrode geometry intended to preserve point-contact-like low-capacitance signal formation in larger crystals. However, reliable GeRC fabrication has remained unproven because the non-planar groove geometry complicates machining, surface recovery, conformal passivation, and especially the eventual formation of a robust lithium-diffused outer contact. We report the fabrication and first cryogenic operation of two compact n-type GeRC process-validation prototypes produced from in-house HPGe crystals at the University of South Dakota. An optimized workflow was developed for core drilling, groove cutting, non-planar polishing, conformal amorphous-germanium (a-Ge) encapsulation, Al patterning, and GeRC-specific cryogenic mounting. Two independent sputtering systems were used to test whether the thin-film sequence remains operable across substantially different deposition environments. At 77~K, both devices biased stably, showed an inferred depletion onset near 340~V from a pulser-based capacitance proxy consistent with electrostatic modeling, and produced identifiable full-energy peaks from $^{241}\mathrm{Am}$ and $^{137}\mathrm{Cs}$. These results establish a proof-of-principle process and readout baseline for geometry-specific GeRC development. They do not yet constitute a deployment-ready large-mass GeRC technology, but they define the foundation for the next step: integrating conformal lithium-paint deposition and controlled diffusion on the ring-and-groove topology.

physics.ins-det↗

arXiv2026

Internal Charge Amplification in Germanium at 77K and 4K: From Single-Free-Flight Bounds to a Physics-Informed Ionization Model

Internal charge amplification (ICA) in cryogenic high-purity germanium (HPGe) can lower detection thresholds by providing gain inside the detector crystal, but reliable operation requires a predictive estimate of the avalanche-onset \emph{critical electric field} $E_{\mathrm{crit}}$. We present a compact framework for $E_{\mathrm{crit}}$ at 77~K and 4~K (typical HPGe operating temperatures) that bridges (i) a mobility-based single-free-flight (SFF) upper bound with (ii) a physics-informed impact-ionization model incorporating energy-dependent scattering, nonparabolic (Kane) dispersion, intervalley transfer, and the high-energy ``lucky-drift'' tail. This unified treatment yields closed-form, design-useful relations, including $E_{\mathrm{crit}}^{(\mathrm{PI})}=B(T)/\ln[A(T)d]$, and a practical calibration workflow that maps measured low-field mobility $\mu(T)$ and gain curves $M(V)$ (Chynoweth analysis) to device-level bias targets with propagated uncertainty bands. Example electron and hole estimates indicate that realistic transport typically lowers $E_{\mathrm{crit}}$ relative to SFF and increases the predicted change in $E_{\mathrm{crit}}$ between 77~K and 4~K. The resulting portable formulas connect materials/transport inputs to geometry, excess noise, and field shaping, providing design-ready guidance for stable, unipolar-favored ICA with controlled quenching in Ge and other cryogenic semiconductors.

physics.ins-det↗

arXiv2026

Machine-Learning Optimization of Detector-Grade Yield in High-Purity Germanium Crystal Growth

High-purity germanium (HPGe) crystals underpin some of the most sensitive detectors used in fundamental physics and other high-resolution radiation-sensing applications. Despite their importance, the supply of detector-grade HPGe remains limited because achieving high yield in Czochralski growth (CZ) depends on tightly coupled, nonlinear processes, impurity incorporation, thermal gradients, and dynamic control settings that are largely mastered by only a handful of companies with decades of experience. Here we present a data-driven prediction framework based on a Bidirectional Long Short-Term Memory (BiLSTM) neural network with multi-head attention, trained on time-resolved growth parameters (e.g., heater power, pull rate, and impurity indicators) from 48 independent crystal runs. The model predicts the final detector-grade fraction for each growth and, using SHAP feature-importance analysis, identifies impurity concentration and growth rate as the dominant factors governing yield, consistent with empirical understanding. By providing a quantitative, interpretable link between in-process signals and post-growth detector quality, this framework offers a practical path toward improving yield, reducing dependence on trial-and-error tuning, and scaling HPGe production for next-generation rare-event detectors.

physics.app-ph↗

arXiv2026

Hybrid-Contact Planar HPGe Process Vehicle Toward Ring-Contact Designs

Rare-event searches including dark matter, coherent elastic neutrino--nucleus scattering (CE$\nu$NS), and neutrinoless double-beta decay (0$\nu\beta\beta$) require high-purity germanium (HPGe) detectors with ultralow noise, stable backgrounds, and electrode geometries that can scale to larger single-crystal masses. Ring-contact (ring-and-groove) designs address scalability by shaping the electric field to preserve low-capacitance readout, but their nonplanar topology motivates a lithium-contact process that is compatible with conformal deposition and robust high-voltage operation. As a process demonstration toward future ring-contact prototypes, we fabricate and characterize a hybrid-contact planar HPGe device, KL01. Here, ``hybrid'' denotes an $n^{+}$ contact formed by an in-house lithium-suspension paint followed by controlled thermal diffusion, combined with an AJA-developed a-Ge/Al $p^{+}$ contact and a-Ge sidewall passivation. At 77~K the device exhibits pA-scale leakage current under kV bias, a depletion plateau near $V_{\mathrm{dep}}\approx 1300$~V, and energy resolutions of 1.57~keV FWHM at 59.5~keV and 2.57~keV FWHM at 662~keV. These results validate the compatibility of the paint-and-diffuse lithium process with thin-film a-Ge/Al contacts and establish a practical fabrication workflow to be extended to ring-and-groove electrodes for next-generation rare-event HPGe modules.

physics.ins-det↗

arXiv2025

Depletion-limited Effective Hall mobility in Micrometer-Scale High-Purity Germanium Crystals

Electrostatic effects can strongly constrain charge transport in thinned high-purity germanium (HPGe), with direct implications for radiation detectors and Ge-based electronic and quantum devices. We report a systematic experimental characterization of the thickness-dependent effective Hall mobility in bulk-grown, detector-grade HPGe at room temperature using Hall-effect measurements on n- and p-type samples sequentially thinned from 2.7~mm to 7~\textmu m. The intrinsic bulk carrier mobility remains thickness independent in this regime; the observed reduction in Hall-extracted mobility arises from electrostatic surface depletion that reduces the electrically active conducting thickness. The thickness-dependent data are accurately parameterized by an empirical extended-exponential relation, $\mu(t)=\mu_{0}[1-\exp(-(t/\tau)^{\beta})]$, where $\tau$ is a characteristic electrostatic length scale. Comparison with boundary-scattering and depletion-based models shows that Fuchs--Sondheimer scattering is negligible, while electrostatic depletion dominates the transport behavior. The hierarchy $\lambda_{D}<\tau\lesssim W_{0}$ directly links the apparent mobility reduction to long-range screening and near-surface electric fields. These results yield a simple design guideline: maintaining thicknesses $t\gtrsim 3\tau$ preserves near-bulk transport, whereas thinner structures operate in a depletion-controlled regime with strongly reduced effective conductivity.

physics.app-ph↗

arXiv2025

Virtual Majorana Neutrinos and the Minimum Neutrino Mass Scale in Neutrinoless Double-Beta Decay

Virtual Majorana neutrinos are indispensable for neutrinoless double-beta (0$\nu\beta\beta$) decay. In this study, we demonstrate that the overlap of the virtual Majorana neutrino wavefunction, predominantly composed of a right-handed antineutrino component with a strongly suppressed left-handed component (with amplitude proportional to the effective Majorana neutrino mass, $|m_{\beta\beta}|$, is crucial for triggering this decay process. This effective mass, derived from the minimum neutrino mass, offers valuable insights into the absolute neutrino mass scale. Using best-fit parameters from neutrino oscillation experiments, the minimum neutrino mass is determined from the sum of the three neutrino mass eigenstates, $\Sigma = m_1 + m_2 + m_3,$ which is represented by two narrow bands centered at approximately 0.06 eV/c$^2$ for the normal hierarchy (NH) and 0.102 eV/c$^2$ for the inverted hierarchy (IH). Under these constraints, the minimum neutrino mass is found to be 0.001186 eV/c$^2$ for NH and 0.002646 eV/c$^2$ for IH, thereby establishing a potential absolute neutrino mass scale for both scenarios. From these values, we calculate $|m_{\beta\beta}|$, which plays a central role in $0\nu\beta\beta$ decay. By combining $|m_{\beta\beta}|$ with decay phase-space factors, nuclear matrix elements, and the absorption probability of the virtual Majorana neutrino, we estimate the $0\nu\beta\beta$ half-life for key isotopes, namely, $^{76}$Ge, $^{130}$Te, and $^{136}$Xe, using two independent methods. The results are in good agreement, and we also discuss the uncertainties in the nuclear matrix elements that may affect these calculations.

hep-ph↗

arXiv2024

Enhancing Sensitivity in Ge-Based Rare-Event Physics Experiments through Underground Crystal Growth and Detector Fabrication

The cosmogenic production of long-lived isotopes such as $^{3}$H,$^{55}$Fe, $^{60}$Co, $^{65}$Zn, and $^{68}$Ge poses a significant challenge as a source of background events in Ge-based dark matter (DM) and neutrinoless double-beta decay ($0\nu\beta\beta$) experiments. In the pursuit of DM, particularly within the largely unexplored parameter space for low-mass DM, new detector technologies are being developed with extremely low-energy thresholds to detect MeV-scale DM. However, isotopes like $^{3}$H, $^{55}$Fe, $^{65}$Zn, and $^{68}$Ge, produced cosmogenically within the detector material, emerge as dominant backgrounds that severely limit sensitivity in these searches. Similarly, efforts to detect $0\nu\beta\beta$, especially under a neutrino normal mass hierarchy scenario, require a sensitivity to the effective Majorana mass of $\sim$1 meV. Achieving this level of sensitivity necessitates stringent suppression of background signals from isotopes such as $^{60}$Co and $^{68}$Ge, which impose critical detection limits. To reach the targeted sensitivity for these next-generation experiments and to unlock their full discovery potential for both low-mass DM and $0\nu\beta\beta$, relocating Ge crystal growth and detector fabrication to underground environments is crucial. This approach is the most effective strategy to significantly reduce the production of these long-lived isotopes, thereby ensuring the experimental sensitivity required for groundbreaking discoveries.

physics.ins-det↗

arXiv2024

Impact of recent updates to neutrino oscillation parameters on the effective Majorana neutrino mass in 0$\nu\beta\beta$ Decay

We investigate how recent updates to neutrino oscillation parameters and the sum of neutrino masses influence the sensitivity of neutrinoless double-beta (0$\nu\beta\beta$) decay experiments. Incorporating the latest cosmological constraints on the sum of neutrino masses and laboratory measurements on oscillations, we determine the sum of neutrino masses for both the normal hierarchy (NH) and the inverted hierarchy (IH). Our analysis reveals a narrow range for the sum of neutrino masses, approximately 0.06 eV/c$^2$ for NH and 0.102 eV/c$^2$ for IH. Utilizing these constraints, we calculate the effective Majorana masses for both NH and IH scenarios, establishing the corresponding allowed regions. Importantly, we find that the minimum neutrino mass is non-zero, as constrained by the current oscillation parameters. Additionally, we estimate the half-life of 0$\nu\beta\beta$ decay using these effective Majorana masses for both NH and IH. Our results suggest that upcoming ton-scale experiments will comprehensively explore the IH scenario, while 100-ton-scale experiments will effectively probe the parameter space for the NH scenario, provided the background index can achieve 1 event/kton-year in the region of interest.

hep-ph↗

arXiv2024

Exploring the Potential of Residual Impurities in Germanium Detectors for Low-Mass Dark Matter Detection

The direct detection of MeV-scale dark matter (DM) particles hinges on achieving an exceptionally low energy detection threshold. Germanium (Ge) detectors, meticulously tailored with precise impurity compositions, hold the potential to enhance sensitivity to energy levels below the sub-electronvolt (sub-eV) range. This study explores the behavior of residual impurities inherent to Ge detectors at helium temperatures, unveiling a captivating freeze-out phenomenon leading to the formation of excited localized states known as dipole states. Using compelling evidence from relative capacitance measurements obtained from two detectors, we elucidate the transition of impurity atoms from free charge states to these dipole states as the temperature drops from 11 K to 6.5 K. Our investigation comprehensively covers the intricate formation of these dipole states in both n-type and p-type impurities. Furthermore, we shed light on the electric field generated by these dipole states, revealing their ability to trap charges and facilitate the creation of cluster dipole states. Confirming findings from previous measurements, we establish that these excited dipole states exhibit a binding energy of less than 10 meV, offering an exceptionally low detection threshold for MeV-scale DM. Building upon this concept, we propose the development of a 1-kg Ge detector with internal charge amplification, an innovative approach poised to surpass electrical noise and enable the detection of MeV-scale DM with unprecedented sensitivity.

physics.ins-det↗

arXiv2023

Development of low-threshold detectors for low-mass dark matter searches with a p-type germanium detector operated at cryogenic temperature

This study investigates new technology for enhancing the sensitivity of low-mass dark matter detection by analyzing charge transport in a p-type germanium detector at 5.2 K. To achieve low-threshold detectors, precise calculations of the binding energies of dipole and cluster dipole states, as well as the cross-sections of trapping affected by the electric field, are essential. The detector was operated in two modes: depleted at 77 K before cooling to 5.2 K and cooled directly to 5.2 K with various bias voltages. Our results indicate that the second mode produces lower binding energies and suggests different charge states under varying operating modes. Notably, our measurements of the dipole and cluster dipole state binding energies at zero fields were $8.716\pm 0.435$ meV and $6.138\pm 0.308$ meV, respectively. These findings have strong implications for the development of low-threshold detectors for detecting low-mass dark matter in the future.

physics.ins-det↗

arXiv2023

Development of Low-Threshold Detectors for Low-Mass Dark Matter Searches Using an N-Type Germanium Detector at 5.2 K

We investigated charge transport in an n-type germanium detector at 5.2 K to explore new technology for enhancing low-mass dark matter detection sensitivity. Calculations of dipole and cluster dipole state binding energies and electric field-dependent trapping cross-sections are critical to developing low-threshold detectors. The detector operates in two modes: depleting at 77K before cooling, or directly cooling to 5.2 K and applying different bias voltages. Results indicated lower binding energy of charge states in the second mode, at zero field and under an electric field, suggesting different charge states formed under different operating modes. Measured cluster dipole and dipole state binding energies at zero field were 7.884$\pm$0.644 meV and 8.369$\pm$0.748 meV, respectively, signifying high low-threshold potential for low-mass dark matter searches in the future.

physics.ins-det↗

arXiv2023

The Decay Q Value of Neutrinoless Double Beta Decay

An earlier publication "The implication of the atomic effects in neutrinoless double beta (0$νββ$) decay" written by Mei and Wei has motivated us to compare the decay Q value ($Q_{ββ}$) derived from the decay of the parent nucleus to the daughter nucleus with the two ejected beta particles in the final state to the $Q_{ββ}$ directly derived from the decay of the initial neutral atom to the final state of double-ionized daughter ion with the two ejected beta particles in the final state. We show that the results are the same, which is the mass-energy difference ($ΔMc^2$) subtracted by the total difference of the atomic electron binding energy ($ΔE_{b}$) between the ground states of initial and final neutral atoms. We demonstrate that $ΔMc^2$ is the sum of $Q_{ββ}$ and the atomic relaxation energy ($ΔE_{b}$) of the atomic structure after the decay. Depending on the atomic relaxation time, the release of the atomic binding energy may not come together with the energy deposition of the two ejected beta particles.

nucl-ex↗

arXiv2022

Evaluation of cosmogenic production of $^{39}Ar$ and $^{42}Ar$ for rare-event physics using underground argon

Underground argon (UAr) with lower cosmogenic activities of $^{39}Ar$ and $^{42}Ar$ has been planned as a detector in detecting scintillation light and charge collection using time projection chambers for dark matter searches and as a veto detector in suppressing backgrounds for neutrinoless double beta decay (0$νββ$) experiments. Long-lived radioactive isotopes, $^{39}Ar$ and $^{42}Ar$, can also be produced on the surface when UAr is pumped out from a deep well. Understanding the production of long-lived isotopes in Ar is important for utilizing UAr for dark matter and 0$νββ$ experiments in terms of its production, transportation, and storage. Ar exposure to cosmic rays at sea-level is simulated using Geant4 for a given cosmic ray muon, neutron, and proton energy spectrum. We report the simulated cosmogenic production rates of $^{39}Ar$, $^{42}Ar$, and other long-lived isotopes at sea-level from fast neutrons, high energy muons, and high energy protons. Total production rates of 938.53/kg$_{Ar}\cdot$day and 5.81$\times$10$^{-3}$/kg$_{Ar}\cdot$day for $^{39}$Ar and $^{42}$Ar are found from our simulation. Utilizing these production rates, we set a time limit of 954 days constrained by the production of $^{39}$Ar for UAr to be on the surface before it compromises the sensitivity for a dark matter experiment. Similarly, a time limit of 1702 days constrained by the production of $^{42}$Ar is found for a 0$νββ$ experiment.

physics.ins-det↗

arXiv2015

Crystal growth and detector performance of large size high-purity Ge crystals

High-purity germanium crystals approximately 12 cm in diameter were grown in a hydrogen atmosphere using the Czochralski method. The dislocation density of the crystals was determined to be in the range of 2000 - 4200 cm-2, which meets a requirement for use as a radiation detector. The axial and radial distributions of impurities in the crystals were measured and are discussed. A planar detector was also fabricated from one of the crystals and then evaluated for electrical and spectral performance. Measurements of gamma-ray spectra from Cs-137 and Am-241 sources demonstrate that the detector has excellent energy resolution.

physics.ins-det↗