Abel Inversion Comparison of Geant4 Simulation and Ozone Production Using Cavity Ringdown Spectroscopy in Nitrogen/Oxygen Mixtures in the Presence of Alpha Radiation

The effects of radioactive materials on atmospheric gases have been a topic of interest for years. Radioactive materials ionize the surrounding air, and subsequent reactions lead to molecules such as ozone and nitrogen oxides. The presence of these species above background levels can be used as a marker for radioactive materials which has desirable defense applications like remote detection of radioactive materials. The molecules created in the presence of radioactive materials have been quantified in literature using G-values, which is the number of molecules of a product produced per 100 eV of deposited energy. In this work, Cavity Ringdown Spectroscopy (CRDS) is used to measure ozone at different positions above a 5 mCi 210Po alpha source that is shielded with a stainless-steel lid for 60 minutes and then unshielded. Abel Inversion is used to transform straight-line path measurements of ozone above the source from CRDS taken at various lateral offsets into a radial profile of ozone centered over the source. A Geant4 radiation transport Monte Carlo model of the experimental system is used to model spatial distributions of energy deposition, assess particle energy spectra and trajectories, and create stopping power plots that are comparable with published NIST data. The number of ozone molecules measured experimentally is used with the computationally modeled spatial distributions of deposited energy for 1.0 second of activity to investigate ozone G-values, G(O3), when looking at 1 mm3 voxels, straight-line paths of 1 mm3 voxels, and an entire layer of 1 mm3 voxels. Based on the model’s spatial predictions and the physical system’s constraints, measurements were conducted 1.6 cm above the source. G(O3) values were calculated across individual and ranges of voxels but were all found to be much higher than anticipated, especially where deposited energy trends towards zero causing G(O3) values to diverge. Previous literature shows G(O3) of roughly 10, which is roughly a factor of 10 lower than this work’s values. The main takeaway of this work is that G-values are reasonable when looking at averaged behaviors but have issues with the approach of taking measurements along different beam paths or when subdivided into individual voxels.