The effect of non-ionizing excitations on the diffusion of ion species and inter-track correlations in FLASH ultra-high dose rate radiotherapy

TL;DR

This paper introduces a microscopic model explaining how non-ionizing excitations influence ion species diffusion and inter-track correlations in ultra-high dose rate radiotherapy, highlighting the role of heat transfer via phonons.

Contribution

It presents a novel mechanism involving phonon-mediated energy transfer that accounts for increased ion diffusion and altered reaction rates in radiation track structures.

Findings

Hot non-ion species significantly increase ion diffusion constants.

The model predicts higher inter-track chemical reaction rates.

Lower intra-track reaction rates are suggested by the diffusion behavior.

Abstract

We present a microscopic mechanism that accounts for the outward burst of "cold" ion species (IS) in a high-energy particle track due to coupling with ''hot" non-ion species (NIS). IS refers to radiolysis products of ionized molecules, whereas NIS refers to non-ionized excitations of molecules in a medium. The interaction is mediated by a quantized field of acoustic phonons, a channel that allows conversion of thermal energy of NIS to kinetic energy of IS, a flow of heat from the outer to the inner core of the track structure. We demonstrate the coexistence of "hot" NIS with "cold" IS in the radiation track structures right after their generation. NIS, concentrated within nano-scales volumes wrapping around IS, are the main source of intensive heat-waves and the outward burst of IS due to femto-second time scale IS-NIS coupling. By comparing the transport of IS coupled to NIS with identical configurations of non-interacting IS in thermal equilibrium at room temperature, we demonstrate that the energy gain of IS due to the surrounding hot nanoscopic volumes of NIS significantly increases their effective diffusion constants. The much higher diffusion constants predicted in the present model suggest higher inter-track chemical reaction rates at FLASH-UHDR, as well as lower intra-track reaction rates.