Neutron gamma analysis of soil carbon: Post-irradiation physicochemical effects

Abstract

In-situ soil carbon measurements would be beneficial when assessing carbon (C) sequestration practices and associated C credits. Neutron gamma analysis, which registers gamma rays that appear due to neutron irradiation, is a tool that can be used for such assessments. However, questions regarding post-effects of neutron irradiation on soil remain unexplored. Temporal post-irradiation effects (neutron flux 2⋅107 neutron/s, neutron energy 14 MeV) on soil chemical and physical attributes were investigated using a previously constructed pulsed fast-thermal neutron-gamma analysis system. Neutron and gamma dose rate distributions during stationary irradiation using this system and Monte-Carlo computer simulations were found to be in agreement; a stationary 1 h irradiation period (conservative or worse-case scenario) was utilized in these scenarios. Physical effects of activating new radioactive isotopes were experimentally determined by assessing changes in the post-irradiated soil gamma spectra (“hot background”) over time; additional soil radioactivity decreased to natural background levels within ∼1 h. Radiolytic decomposition estimates of soil water and soil organic material (primarily cellulosic residue), based on received dose loads and known radiation-chemical yields of water and organic material, were practically negligible. Results indicate that adsorbed radiation doses in scanning mode would be 500 to 1000 times less than in static mode. Thus, neutron gamma analysis does not impact physicochemical aspects of soil health and can be used for soil elemental content determinations without additional radiation safety concerns.