A molecular dynamics simulation framework for investigating ionizing radiation-induced nano-bubble interactions at ultra-high dose rates

Abstract: We present a microscopic formalism that accounts for the formation of nano-scale bubbles owing to a burst of water molecules after the passage of high energy charged particles that lead to the formation of “hot” non-ionizing excitations or thermal spikes (TS). We construct amorphous track structures to account for the formation of TS by ionizing radiation in liquid water. Subsequently, we simulate sudden expansion and collective motion of water molecules by employing a molecular dynamics (MD) simulation that allows computation of O(10⁶) particle trajectories and breaking/forming of chemical bonds on the fly using a reactive force field, ReaxFF. We calculate the fluctuations of thermodynamic variables before and after TS formation to model the macroscopic abrupt changes in the system, possibly the occurrence of a first-order phase transition, and go beyond the accessible simulation times by engaging fluid dynamic equations with appropriate underlying symmetries and boundary conditions. We demonstrate the coexistence of a rapidly growing condensed state of water and a hot spot that forms a stable state of diluted water at high temperatures and pressures, possibly at a supercritical phase. Depending on the temperature of TS, the thin shell of a highly dense state of water grows by three to five times the speed of sound in water, forming a thin layer of shock wave (SW) buffer, wrapping around the nano-scale cylindrical symmetric bubble. The stability of the bubble, as a result of the incompressibility of water at ambient conditions and the surface tension, allows the transition of supersonic SW to a subsonic contact discontinuity and dissipation to thermo-acoustic sound waves. Thus, TS gradually decays to acoustic waves, a channel of deexcitation that competes with the spontaneous emission of photons, and a direct mechanism for water luminescence. We further study the mergers of nanobubbles that lead to jet-flow structures at the collision interface. We introduce a time delay in the nucleation of nano-bubbles, a novel mechanism, responsible for the growth and stability of much larger or even micro-bubbles, possibly relevant to FLASH ultra-high dose rate (UHDR). Graphical abstract: Molecular dynamic simulation of the interaction between two ionizing radiation-induced nano-bubble formed simultaneously. (Figure presented.)