1 keV electron track evolution in 150 nm liquid water sphere

Geant4-DNA released for the first time in 2014 (in Geant4 10.1) features to simulate water radiolysis, following the modeling of physical interactions described in the
Physics section. These features include the production of chemical species, the simulation of their diffusion and chemical reactions.

Today, Geant4-DNA adopts several approaches for the simulation of water radiolysis:
  • A Step-By-Step approach (SBS): Brownian transport of molecules is simulated using the Smoluchowski model. Chemical species are represented as point objects diffusing in the (continuum) liquid medium. Chemical reactions are « controlled by diffusion ». This approach enables chemical species to be tracked in space and time, but requires computer resources. See references: 1, 2, 3, 4, 5.
  • The Independent Reaction Time method (IRT): in this approach, the « independent pairs » approximation is assumed: it calculates the reaction times between all possible pairs of reactive species, as if they were isolated. Then, reactions occur one by one, starting with the pairs with the shortest reaction times. It is no longer necessary to diffuse the chemical species and calculate the possible reactions between the species at each time step. As this approach excludes the diffusion of chemical species, it is significantly faster. See references: 6, 7, 8
  • A synchronized version of the IRT (IRT-sync): this approach synchronizes the positions of chemical species with each shortest reaction time calculated by the IRT method. This implementation provides users with spatio-temporal information on the species, which can then be coupled with information on the geometric boundaries or geometry of a biological target. See reference 9.
  • A mesoscopic approach: this approach uses a compartment-based representation which describes the evolution of species through the alteration of species concentrations in different compartments. These compartments are created by a voxelization of the simulation volume where species can react with each other within the same voxels, and diffusion is modeled by jumps between adjacent voxels. This approach is suitable for a larger number of species in a limited volume. See reference 10.

The examples section includes several examples for the simulation of water radiolysis:
  • chem1 to chem5: SBS-based, including calculation of radiochemical yields (G-values) versus time (chem4, chem5)
  • chem6: IRT-based, including calculation of radiochemical yields (G-values) versus time or LET. SBS and IRT-sync can also be used.
  • scavenger: simulation of scavenging using IRT
  • UHDR: mesoscopic approach (in combination with SBS)
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