Simulating the Impact of the Natural Radiation Background on Bacterial Systems: Implications for Very Low Radiation Biological Experiments

Abstract

At very low radiation dose rates, the effects of energy depositions in cells by ionizing radiation is best understood stochastically, as ionizing particles deposit energy along tracks separated by distances often much larger than the size of cells. We present a thorough analysis of the stochastic impact of the natural radiative background on cells, focusing our attention on E. coli grown as part of a long term evolution experiment in both underground and surface laboratories. The chance per day that a particle track interacts with a cell in the surface laboratory was found to be 6 × 10-5 day-1, 100 times less than the expected daily mutation rate for E. coli under our experimental conditions. In order for the chance cells are hit to approach the mutation rate, a gamma background dose rate of 20 μGy hr-1 is predicted to be required.

Fig 1. The 96-well microplate was simulated with 24 wells being filled with Davis minimal broth (blue).

The polypropylene border to each well was 1 mm in width and 2 cm in depth. Liquid filled each well to a depth of 1.5 cm. A central well (hatched) was used as to measure dosages and secondary particles created.

Fig 2. The top panel shows the total electron interaction cross section (σ_tot, given per volume, for ρ = 1 g cm³ solutions), which determines the likelihood of electron interactions occurring for a given electron energy, in both water and Davis broth.

The difference between these curves is shown in the bottom panel. Across the range of electron energies considered in our simulations, water approximates the Davis broth solution to within < 0.5%.

Fig 3. The repeating boundary condition allows the simulation area at the macroscopic level to be broken up into a series of microdomains with (x, y) ∈ [−100 μm, 100 μm]² (left).

Each microdomain contains the same geometry due to the repeating boundary, however each track stores the identity of its current macroscopic domain, here labeled in the bottom right of each small cube. When a particle leaves a microdomain (right), it re-enters at the other side, due to the periodic boundary. When this occurs, the track updates its stored position in the macroscopic view.

Fig 4. Each source deposits energy in cells according to different Landau-like distributions.

Energy depositions are normalized to the hit rate, indicating for each source the chance a specific amount of energy is deposited in it in a day. The peaks near 600 eV and 1.2 keV in the γ-background and β-electron spectra are related to the emission of one or two short-traveling Auger electrons emitted by Oxygen atoms within cells, in addition to the energy deposited by other processes.

Fig 5. When the energy depositions are normalized to 10⁶ primary events, the characteristics of each source become clearer.

Sources that travel further through the medium impact more cells, whilst the significantly higher LET from neutron-induced ions is reflected in the flatter distribution of energy deposits from this source.