Molecular Dynamics Simulations of Bubble Nucleation in a Liquid-Noble Scintillator

TL;DR

This work addresses how scintillation in liquid argon alters bubble nucleation thresholds in a WIMP-search bubble chamber. Using HOOMD-blue molecular dynamics, the authors model energy deposition, photon production, and delayed de-excitation, comparing nucleation thresholds with and without scintillation. They find that scintillation raises the average threshold by a factor of 2.16, primarily due to photon losses and, to a lesser extent, time-delayed energy release; energy deposited after the initial rapid growth phase does not contribute to stable bubble formation. The results have implications for detector design and background rejection in scintillating bubble chambers and motivate future exploration of xenon-doped argon mixtures.

Abstract

The Scintillating Bubble Chamber collaboration is searching for Weakly Interacting Massive Particles using a novel bubble chamber with intended thresholds as low as 100eV. Existing molecular dynamics simulations of bubble formation in bubble chambers were conducted with non-scintillating target materials and therefore do not account for the energy transfer to photons or time-delayed releases that occur in atomic de-excitation. In this study, we use the HOOMD-blue molecular dynamics framework to simulate bubble formation in liquid argon, including photon creation, ionization, and direct nuclear recoils. A multi-stage bubble growth process similar to that reported in the literature was observed. When comparing simulated thresholds with and without scintillation effects, we found that scintillation raises the average energy required to form a bubble by a factor of 2.16. This is larger than the fraction of energy lost to photon creation, and demonstrates that energy stored in excited molecular states with lifetimes longer than the rapid growth phase of nucleation (~250 ps) does not contribute significantly to bubble formation. This conclusion was further supported by simulations showing increased bubble nucleation thresholds when the excited molecular state lifetimes were increased, even under identical thermodynamic conditions.

Figures (7)

• Figure 1: Ar-Ar de-excitation pathways of ionized and excited atoms in solid argon. Figure from Grosjean_solidAr_1997.
• Figure 2: Proportional breakdown of energy deposition mechanisms for an electron in LAr. The values assume a 100 eV event, but proportions are independent of event energy.
• Figure 3: Evolution of a nucleation event (90K, 920 eV) in multiple views. Top: Density (atoms$/\sigma^3$) of vertical slices in the x-y plane. Bottom: Cross-sectional render in the x-z plane using OVITO ovito. Colouring by particle velocity highlights the initial heat spike in the first frame (t = 1.35 ps), and several de-excitations occurring in the second frame (t = 45.73 ps).
• Figure 4: Nucleation energy threshold ranges as a function of temperature at 20 psia. Predicted thresholds for the scintillation points, determined by scaling the "scintillation disabled" points by the fraction of energy lost to photon emission, are marked in gray.
• Figure 5: Simulation cell volume over time at 90 K and 20 psia for successful (920 eV) and failed (820 eV) bubble formation with scintillation effects enabled. Beginning of different simulation phases are marked with dashed lines.