Low-Energy Calibration of SuperCDMS HVeV Cryogenic Silicon Calorimeters Using Compton Steps

TL;DR

This work demonstrates a novel 0 V Compton-step calibration method for SuperCDMS HVeV silicon calorimeters, validating low-energy calibration via L- and K-shell Compton features up to ~2 keV and cross-checking with HV LED-based calibration. By combining Geant4 simulations with FEFF-derived dynamic structure factors, the authors model low-energy Compton scattering and extract detector-specific calibration factors, unveiling a ~30% weaker 0 V response relative to HV for the same phonon energy. The approach yields important insights into detector response modeling at sub-keV energies and has implications for optimizing low-mass dark matter searches with SuperCDMS SNOLAB, while highlighting areas for improving core-hole treatments in FEFF and cross-talk/offset effects in LED-based calibrations. Overall, the work establishes Compton-step calibration as a viable, low-background alternative to optical-photon calibration for ultra-low energy cryogenic calorimeters.

Abstract

Cryogenic calorimeters for low-mass dark matter searches have achieved sub-eV energy resolutions, driving advances in both low-energy calibration techniques and our understanding of detector physics. The energy deposition spectrum of gamma rays scattering off target materials exhibits step-like features, known as Compton steps, near the binding energies of atomic electrons. We demonstrate a successful use of Compton steps for sub-keV calibration of cryogenic silicon calorimeters, utilizing four SuperCDMS High-Voltage eV-resolution (HVeV) detectors operated with 0 V bias across the crystal. This new calibration at 0 V is compared with the established high-voltage calibration using optical photons. The comparison indicates that the detector response at 0 V is about 30% weaker than expected, highlighting challenges in detector response modeling for low-mass dark matter searches.

Figures (13)

• Figure 1: Schematic of the detector tower configuration during data acquisition without the LED modules. The top lid is not drawn. The top detector box contains the NF-H and NF-C2 detectors, while the bottom box houses the NF-E and NF-C1 detectors, which are not visible in the schematic.
• Figure 2: The spectrum of energy depositions in HVeV detectors based on the Geant4 simulation. A Gaussian smearing with an energy-dependent width has been applied to the spectrum. The smearing corresponds to 2% of the deposited energy, approximating the detector performance that is observed in previous studies HVeV_characterization.
• Figure 3: Dynamic structure factors (DSF) computed using FEFF calculations for electrons in various shells. The energy and momentum transfer values are shown on the $\it{x}$ and $\it{y}$ axes, respectively. The color shows the magnitude of the DSF cp_correction.
• Figure 4: Differential cross sections for Compton scattering between 50 eV and 500 eV, computed using Eqn. \ref{['differential_xsection']} and DSFs from FEFF. The blue curve includes the effect of the vacated core-hole potential, while the orange curve excludes it.
• Figure 5: The distribution of the average pre-pulse region values within the processing window for events in the calibration data. The fit of a Gaussian function to the left side of the distribution determines the mean value and standard deviation of the distribution. Events in the shaded regions were three standard deviations away from the mean of the Gaussian and were rejected.