Tagged neutron technique for in-situ soil composition determination

Authors: Kavetskiy, Aleksandr; Yakubova, Galina; GAUTAM, SIDHARTH - Auburn University; Prior, Stephen - Steve; Torbert, Henry - Allen

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 5/31/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: The tagged neutron technique (TNT) was applied for soil composition determination, utilizing a TNT Mobile system equipped with a portable neutron generator (API-120) with a built-in alpha detector, a 7.62×25.4 cm LaBr(Ce) gamma detector, a 4-channel digital pulse processor with a Linux operating system (Pixie-Net) for data acquisition, radiation shielding, GPS, an autonomous power system, and an operational laptop. This system was used for field measurements to assess soil composition. During field measurements, results were recorded as binary files (~1-1.5 GB for 20-30 minutes of measurements, depending on soil type). These files contained alpha and gamma pulses detected by respective detectors during soil neutron irradiation. Time-of-flight (TOF) spectra (number of gamma ray pulses registered by the detector versus time after the alpha particle's detection), were generated using IGOR software from these binary files. Alpha particles act as "tags" for neutrons. These alpha particles travel from the neutron generator target to the entrance window of the alpha detector that is positioned above the generator target with the soil or other samples located beneath it. The TOF spectra features a high, narrow peak (~10 ns wide) of gamma ray pulses from the soil, superimposed on a relatively low and wide band (~100 ns) of pulses from the TNT Mobile system equipment. The gamma-ray pulses in the TOF peak are specifically due to inelastic neutron scattering (INS) in the soil. The gamma spectrum, representing the number of gamma ray pulses versus their energy within a 7 ns window around the TOF peak, can be derived from binary files using IGOR software. This spectrum characterizes gamma rays produced by INS in the soil, from which soil composition information can be extracted. Soil can be approximated as a mixture of oxides of its main components: SiO2, Al2O3, Fe2O3, CaO, H2O, and carbon (C). The INS soil gamma spectra is thus a sum of the INS gamma spectra of these oxides and C. A deconvolution procedure of the TNT soil gamma spectrum into the TNT gamma spectra of these oxides and C provides the content of each oxide and C in soil. Reference samples of oxides and C were sized (~ diameter 100×50 cm) ensuring that the intensity of the TNT gamma spectra reached a steady state and did not increase with further sample size. Monte Carlo simulations of neutron-stimulated gamma spectra (using MCNP6.2 code) were employed for this purpose. The TNT gamma spectra of reference oxides and C were measured in a box with the aforementioned dimensions using the TNT Mobile system. The distance from the neutron generator target to the sample surface was maintained the same as in soil measurements. The TNT gamma spectra of reference samples within the 7 ns time window were extracted from the saved binary files as previously described. These spectra were then used in the deconvolution procedure, which accounted for neutron and gamma-ray attenuation during their propagation in samples and soil. The TNT method for soil composition determination and results of field measurements will be discussed in more detail during the presentation. These results showed good agreement with those obtained by other methods, such as chemical analysis, dry combustion method, weight method, moisture determination by time domain reflectometry, and nuclear methods. Consequently, TNT can be recommended as a non-destructive, in-situ, less labor-intensive, and time-efficient method for soil content determination, beneficial for agricultural and soil science purposes.