arXiv subjects

Sanjay Bhattarai

Publications and source records attributed to Sanjay Bhattarai.

7 recordsLinked to original sources

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

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

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, $μ(t)=μ_{0}[1-\exp(-(t/τ)^β)]$, where $τ$ 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 $λ_{D}<τ\lesssim W_{0}$ directly links the apparent mobility reduction to long-range screening and near-surface electric fields. These results yield a simple design guideline: maintaining thicknesses $t\gtrsim 3τ$ 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$νββ$) 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, $|m_{ββ}|$, 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, $Σ= m_1 + m_2 + m_3,$ 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 $|m_{ββ}|$, which plays a central role in $0νββ$ decay. By combining $|m_{ββ}|$ with decay phase-space factors, nuclear matrix elements, and the absorption probability of the virtual Majorana neutrino, we estimate the $0νββ$ 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

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

We investigate how recent updates to neutrino oscillation parameters and the sum of neutrino masses influence the sensitivity of neutrinoless double-beta (0$νββ$) 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$νββ$ 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↗

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↗