Neutron-Stimulated Gamma Ray Analysis of Soil

Abstract

This chapter describes technical aspects of neutron-stimulated gamma ray analysis of soil carbon. The introduction covers general principles, different modifications of neutron-gamma analysis, measurement system configuration, and advantages of this method for soil carbon analysis. Problems with neutron-gamma technology in soil carbon analysis and methods of investigations including Monte-Carlo simulation of neutron interaction with soil elements are discussed further. Based on the investigation results, a method of extracting the "soil carbon net peak" from the raw acquired data was developed. The direct proportional dependency between the carbon net peak area and average carbon weight percent in the upper 10 cm soil layer for any carbon depth profile was shown. Calibration of the measurement system using sand-carbon pits and field measurements of soil carbon are described. Measurement results compared to chemical analysis (dry combustion) data demonstrated good agreement between the two methods. Thus, neutron-stimulated gamma ray analysis can be used for in situ determination of near-surface soil carbon content and is applicable for precision geospatial mapping of soil carbon.

Neutron-Gamma Analysis of Soil for Digital Agriculture

“…For testing our PFTNA system, four 150 cm  150 cm  60 cm pits with sand-coconut shell mixtures of known carbon content (0, 2.5, 5 and 10 w% of carbon) were used. Calibration measurements should be performed such that errors are negligible compare to field measurements [4].…”

“…To achieve identical energy calibration, the energy calibration for a reference detector of the same type was established under laboratory conditions. To accomplish this by using several known gamma lines, the neutron stimulated gamma spectra (due to both inelastic neutron scattering and thermal neutron capture) of wet and dry soil, and soil-carbon mixes were acquired (see [4]). This resulted in several well-identified gamma peaks in the created spectra (e.g., 0.847 MeV iron peak, 1.779 MeV silicon peak, 2.223 MeV hydrogen peak, 4.438 MeV carbon peak, and 6.129 MeV oxygen peak, 7.63 MeV iron peak).…”

“…where LT INS,r + 1,i , LT TNC,r + 1,i and LT INS,r,i , LT TNC,r,i are the live time (in s) for the r + 1th and rth record for the ith detector, and INS and TNC spectra, respectively. Live time for each spectrum is calculated as [4]:…”

“…This includes gamma spectra acquired during neutron pulses (i.e., from gamma rays appearing due to inelastic neutron scattering, INS spectra) and between pulses (i.e., from gamma rays appearing due to thermal neutron capture, TNC spectra). Details concerning this methodology have been previously described [4][5][6][7].…”

“…A custom mobile PFTNA system was developed and constructed for measuring soil carbon in agricultural fields in the scanning regime [4,6]. A GPS device and specially developed software were added to the mobile system for simultaneous acquisition of gamma signals and geographical positions.…”