Application of associated particle neutron techniques for soil carbon analysis

Abstract

Accurate soil carbon field mapping can benefit modern agricultural 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). This measurement mode tends to increase the signal-to-noise ratio (SNR) of measurements, which in turn tends to decrease MDL. To test applicability of the API technique for soil carbon analysis, an experimental setup including a DT neutron generator (with 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.The proper working setup and authenticated experimental results were confirmed by definition of the 14.1 MeV neutrons speed from measurement of carbon energy correlated timing spectra. The measured value (5.2 cm/ns) agreed with reference data.Series measurements of timing spectra, time correlated energy gamma spectra, and energy correlated timing spectra of different samples (i.e., ammonia nitrate, sand, graphite bricks, sand-carbon mixtures) at different source-to-samples distances were conducted and are discussed. Measurements demonstrate that the energy correlated timing spectra gives the possibility of defining the time window for time correlated energy spectra measurement with better accuracy than timing spectra for gamma rays in the full energy range. The identification of sample content located at some distance from the neutron source can be accomplished with such measurements. This can be useful for disclosure of hidden objects.From the API mode measurements of sand-carbon mixtures, the carbon MDL of this method was defined and compared with carbon MDL of different PFTNA devices. This comparison demonstrates better carbon MDL for the API mode.Findings from this work clearly indicate that the API method is a quite promising method for agricultural applications, and for soil carbon analysis due to significantly improved MDL.