Calibration Plan for the SBC 10-kg Liquid Argon Detector with 100 eV Target Threshold

TL;DR

This work outlines a calibration plan for a 10 kg liquid-argon bubble chamber (SBC-LAr10) aiming at sub-keV nuclear recoil thresholds to enable competitive dark matter and CE$\nu$NS measurements. It combines photoneutron scattering with two novel low-energy calibrations—nuclear Thomson scattering and thermal neutron capture—implemented in a Geant4-based framework to predict NR spectra and nucleation efficiencies. Through mock datasets and MCMC-based fits, the authors demonstrate that standard-precision calibrations can reach ~20% threshold accuracy, while a high-precision program could achieve ~5% threshold precision, enabling robust DM sensitivity and reactor CE$\nu$NS studies. The methodology also provides a pathway to validate sophisticated simulations of low-energy nuclear recoils in noble liquids and to quantify systematic uncertainties essential for future low-threshold dark matter and neutrino experiments.

Abstract

The Scintillating Bubble Chamber (SBC) Collaboration is designing a new generation of low background, noble liquid bubble chamber experiments with sub-keV nuclear recoil threshold. These experiments combine the electronic recoil blindness of a bubble chamber with the energy resolution of noble liquid scintillation, and maintain electron recoil discrimination at higher degrees of superheat (lower nuclear recoil thresholds) than Freon-based bubble chambers. A 10-kg liquid argon bubble chamber has the potential to set world leading limits on the dark matter nucleon cross-section for $O(\mathrm{GeV}/c^{2})$ masses, and to perform a high statistics coherent elastic neutrino nuclear scattering measurement with reactor neutrinos. This work presents a detailed calibration plan to measure the detector response of these experiments, combining photoneutron scattering with two new techniques to induce sub-keV nuclear recoils: nuclear Thomson scattering and thermal neutron capture.

Figures (13)

• Figure 1: (Left) The SBC-LAr10 design shown as a schematic diagram, showing the warmer superheated liquid (argon doped with 10 ppm xenon) kept at 130 K between the fused silica jars and the colder 90 K stable liquid region. The schematic also highlights the cameras and piezo acoustic sensors for bubble detection in addition to the SiPMs for scintillation light collection. (Right) The labeled CAD model for the detector SBCsnowmass.
• Figure 2: The simulated geometry in the vicinity of the calibration tube and the active detector volume. The bottom of the calibration tube is 12.1 cm from the top of the active LAr. Particles from a calibration source must traverse the 0.6 cm stainless steel cap on the source tube, the 0.95 cm thick stainless steel pressure vessel wall, at least 10 cm of liquid CF$_{4}$, and the 0.51 cm thick fused silica outer vessel to reach the active detector fluid. The simulated geometry also features major detector components, such as the HDPE castle, that are not included in this figure but can be seen in Figure \ref{['fig:bubble-chamber']}.
• Figure 3: The simulated spectra of NRs from the photoneutron sources on argon in the detector. The simulated source is a gamma source in the center of a metallic beryllium cylinder. Source activities are chosen to give rates of around 50 bubbles/hour above a threshold of 80 eV.
• Figure 4: The simulated NR energy spectra from Thomson scattering sources on argon in the detector. The source activities were chosen to give rates of around 50 bubbles/hour above a threshold of 80 eV, but capped at 100 µ Ci. $^{228}$Th uses the highest activity easily available of 10 µ Ci.
• Figure 5: Argon nucleus recoil process after thermal neutron capture.