Bubble formation and growth In superheated liquid bubble chambers

Abstract

Bubble chambers are a type of particle detector that make use of the superheated state to detect minute energy depositions. Early studies of nucleation in bubble chambers provide evidence that, if sufficient energy is deposited within a region of the superheated liquid, it triggers nucleation of a spherical protobubble that will grow and become a visually observable bubble whilst emitting an acoustic impulse. The acoustic emission of alpha particle-induced bubbles are readily distinguishable from those produced by lower-energy neutron scattering events. This thesis investigates the nucleation of bubbles and the associated acoustic emissions in superheated liquids, with a focus on understanding the nature of the differences in acoustic emissions produced by different types of particles. Using molecular dynamics simulations, we successfully replicated a superheated liquid state using a Lennard-Jones potential to model the C3F8 molecule. We also successfully triggered nucleation by low-mass high kinetic energy projectile from within the superheated liquid. This process mimics what is believed to occur in superheated liquid such as C3F8, which is used in PICO bubble chambers, where either a fluorine or carbon nucleus recoils with sufficient kinetic energy to trigger the nucleation process. The simulations show that bubbles nucleated in close proximity grow independently of each other throughout the coalescence. This supports a long-standing hypothesis that energy deposition from particles can be modeled as a track of non-interacting protobubbles that will grow into the singular observed bubble. Furthermore, within the limitations of particle transport software, we modeled the fragmentation of molecules, a process that is believed to occur during the energy deposition of particles, which could be used to explain the observed nucleation efficiencies in PICO detectors. Furthermore, from numerical simulations we were successful in using the total change in volume with respect to time to model the acoustic emissions and reproduce observations in PICO detectors. The protobubble model was used to further investigate partial energy depositions near the walls of the detector and from particulates. The acoustics of partial alpha energy depositions can be reduced to those of low energy nuclear recoils, which justifies removing events that occur near the walls of the detector from any analysis. In addition to studying nucleation, simulations of neutron exposures in the PICO-40L detector were compared with data, in agreement with the conventional nucleation model involving nucleation efficiencies of recoiling nuclei. The simulations predicted the ratio of the number of multiple bubble to the number of single bubble events (MSR) to be 3.0±0.2, in agreement with the observations. Finally, simulations of gamma exposures will be compared to experimental data to identify potential temperature profiles within the detector, as the nucleation model relevant to electrons produced by the passage of gamma in the superheated liquid is sensitive to variations in the nucleation threshold energy, which is influenced by temperature.