Application of associated particle neutron techniques for soil carbon analysis

Authors: KAVETSKIY, ALEKSANDR - Auburn University; Yakubova, Galina; Prior, Stephen - Steve; Torbert, Henry - Allen

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/12/2019
Publication Date: 10/2/2019
Citation: Kavetskiy, A., Yakubova, G.N., Prior, S.A., Torbert III, H.A. 2019. Application of associated particle neutron techniques for soil carbon analysis. In: Proceedings of the 25th International Conference on Application of Accelerators in Research and Industry, AIP Conference Proceedings. 2160:050006-1 - 050006-11. https://doi.org/10.1063/1.5127698.
DOI: https://doi.org/10.1063/1.5127698

Interpretive Summary: The uptake of carbon from atmospheric in the form of CO2 by plants can help mitigate global change through soil carbon storage. This project covers the development of an in-situ method of measuring soil carbon without destructive soil sampling. Neutron based associated particle imaging (API) can detect hidden illicit materials such as explosives, fissile materials, and narcotics. This paper discusses an API experimental setup that can measure soil carbon in the field in a timely manner without destructive soils cores that disturb the soil profile.

Technical Abstract: Accurate soil carbon field mapping can benefit modern agriculture practices. For creating such maps, soil carbon measurements using neutron-gamma analysis were developed and applied as a better alternative to traditional chemical analysis. Main components of a neutron-gamma analysis system are a neutron generator, gamma detectors, and special electronics. The proposed application of associated particle imaging (API) neutron techniques for soil carbon measurement could improve metrological characteristics (e.g., minimal detectible level of soil carbon, MDL) of our currently used soil carbon analysis system (i.e., Prompt Fast Thermal Neutron Analysis, PFTNA). Neutron stimulated gamma rays in the API mode are acquired in a relatively narrow neutron flux cone defined by an alpha-particle registration cone (alpha particles are produced with neurons in the DT reaction in neutron generators). Runs increasing the signal-to-noise ratio (SNR) of measurements can decrease MDL. To test the applicability of the API technique for soil carbon analysis, an experimental setup including a DT neutron generator (with system alpha particle registration capabilities), sodium iodide gamma detectors (10 cm × 10 cm × 48 cm), and nanosecond operated electronics was constructed and tested. This API setup can measure alpha-gamma coincidence (timing) spectra, time correlated energy gamma spectra, and energy correlated timing spectra. Speed of 14.1 MeV neutrons were defined from measurement of carbon energy correlated timing spectra. The measured value (5.2 cm/ns) agreed with reference data that confirmed a proper working setup and authenticated experimental results. Test experiments with graphite samples demonstrated that the minimal detectible level (MDL) of carbon with the appropriate timing window in the API mode is 2.5 times less than in the continuous mode and no worse than PFTNA measurements. A series measurements of the timing spectra, time correlated energy gamma spectra, and energy correlated timing spectra of different samples (i.e., melamine, urea, ammonia nitrate, sand, sand with buried graphite brick, sand-carbon mixtures) at different source-to-samples distances were conducted and will also be discussed in this presentation. For instance, measurements demonstrated that the energy correlated timing spectra gives the possibility of defining the time window for time correlated energy spectra measurement for better accuracy than timing spectra for gamma rays in full energy spectra. The identification of sample content located at some distance from the source can be accomplished with such measurements. This can be useful for disclosure of hidden objects. Information on the location of a buried sample in some free-flowing matter (e.g., sand or soil) and the presence of some nucleus (i.e., Carbon-12) can be received from the energy (4.44 MeV) of the correlated timing spectra. This could be useful for the study of roots and root-crops. Measurements of sand-carbon mixtures in the API mode demonstrated a linear dependence of carbon signal versus sample carbon content which can be used for calibration dependence in soil carbon content measurements. Results and discussion covered in this presentation will clearly indicate that the API method is a quite promising method for agricultural applications, and in particular, for soil carbon analysis due to significantly improved MDL.