Neutron-gamma technologies for carbon sequestration assessments

Authors: Kavetskiy, Aleksandr; Yakubova, Galina; Prior, Stephen - Steve; Torbert, Henry - Allen

Submitted to: International Conference on the Application of Accelerators in Research and
Publication Type: Abstract Only
Publication Acceptance Date: 11/2/2022
Publication Date: 11/2/2022
Citation: Kavetskiy, A.G., Yakubova, G.N., Prior, S.A., Torbert III, H.A. 2022. Neutron-gamma technologies for carbon sequestration assessments [abstract]. 26th International Conference on the Application of Accelerators in Research and Industry and 53rd Symposium of Northeastern Accelerator Personnel (CAARI-SNEAP 2022), Denton, TX. October 30-November 3. In Book of Abstracts: https://caari-sneap.com/home

Interpretive Summary:

Technical Abstract: Methods of rapid in-situ soil carbon determination are desirable for assessing land management decisions that could enhance carbon sequestration and have potential relevance in emerging carbon credit markets. One promising technology is neutron gamma analysis, which registers gamma rays appearing from neutron irradiation of soil to determine soil carbon content. Since this method produces a complicated mixture of gamma rays appearing from neutron interactions with various soil elements, extraction of gamma rays directly connected with soil carbon is the main difficulty in this analysis. Different modes of measurement can be utilized to overcome this issue. Pulsed neutron soil irradiation and gamma spectra acquisition during and between neutron pulses is used in Pulsed Fast Thermal Neutron Analysis (PFTNA). This method allows separation of the inelastic neutron scattering (INS) spectrum from the thermal neutron capture (TNC) spectrum. In the net INS spectra, the peak with a centroid at 4.44 MeV is attributable to carbon and can be used for determining soil carbon levels. Based on this PFTNA method, a mobile system for determining soil carbon was designed and constructed to be suitable for towing across agricultural fields. This mobile inelastic neutron scattering system (MINS) consists of a MP320 pulsed neutron generator (Thermo Fisher Scientific Inc.), sodium iodide gamma detectors (Scionix) with thermostatic housing, neutron detector(s), GPS, a data acquisition system (Vega board, XIA LLC), a power system to provide autonomous work, and an operating computer. The MINS system design, spectral acquisition processes, and strategies for acquiring carbon data in the field will be discussed in this presentation. Soil carbon determination results and mapping of actual agricultural fields will also be shown and discussed. Associated particle imaging (API) or tagged neutron (TN) technique is another possible neutron-gamma method for detecting soil carbon in-situ or in bulk soil samples (~30-50 kg). Using this technique, measurements in alpha-gamma coincidence mode produce a neutron stimulated INS gamma response primarily from the sample. For this reason, this carbon measurement method has a high signal-to-noise ratio and sensitivity comparable to PFTNA. Our system for TN mode measurements was constructed using an API120 (Thermo Fisher Inc.) neutron generator (with built-in alpha gauge), LaBr(Ce) gamma detector (Ø7.62cm × 25.4 cm crystal size; Saint-Gobain), and a DAQ system with a Pixie-Net (XIA LLC) module working in the coincidence mode. This setup is currently being tested on large samples prior to further testing in field applications. The main advantage of TN mode is that the measured INS spectra are from the sample with no gammas from surrounding objects and measurement equipment. Because of this advantage, the amount of each component in the sample (based on decomposition of measured spectra components) can be defined. For sand-carbon mixture analysis, spectra of components [silicon, oxygen (water), carbon (graphite or coconut shell)] and spectra of mixtures were measured in the TN mode. Assuming that gamma spectra of mixtures are the sum of the gamma spectra of components with weight coefficients (proportional to fraction of components in mixture), the content of mixtures can be found using the least square method. Good agreement was found when comparing these sand-carbon mixture results with PFTNA carbon determinations (based on peak area calculations and previously created calibration dependence) and with chemical analysis results. This agreement supports the possible application of the TN method for field carbon determinations as a promising alternative to the current PFTNA method. Other advantages and disadvantages of the TN method in field applications will also be discussed.