Carbon analysis of large soil samples using the tagged neutron method: Accounting for radiation attenuation.

Non-destructive methodology for determining carbon content in large or semi-infinite (soil) samples is discussed. This methodology is based on deconvoluting the sample's gamma spectra (received by tagged neutron method) on the sample component's spectra by accounting for neutron and gamma radiation attenuations. This algorithm was tested with both Monte-Carlo simulations and experimental gamma spectra. Good agreement was found between defined and actual sample component content. Application of this method for soil carbon determinations in agricultural fields is discussed.

Tagged neutron method for carbon analysis of large soil samples.

Laboratory determination of carbon content in 30-50 kg soil samples is described. The method is based on the tagged neutron technique. Procedure for carbon determination in such samples was developed based on a physical model and Monte-Carlo simulations (Geant4) of neutron stimulated gamma spectra. Measurement results of samples with different density and moisture demonstrate good agreement with standard dry combustion analysis. Thus, this method can be recommended as an alternative for laboratory determination of carbon in 30-50 kg soil samples.

Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes.

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of gamma rays created when neutrons interact with soil elements. The main parts of the INS system are a pulsed neutron generator, NaI(Tl) gamma detectors, split electronics to separate gamma spectra due to INS and thermo-neutron capture (TNC) processes, and software for gamma spectra acquisition and data processing. This method has several advantages over other methods in that it is a non-destructive in situ method that measures the average carbon content in large soil volumes, is negligibly impacted by local sharp changes in soil carbon, and can be used in stationary or scanning modes. The result of the INS method is the carbon content from a site with a footprint of ~2.5 - 3 m2 in the stationary regime, or the average carbon content of the traversed area in the scanning regime. The measurement range of the current INS system is >1.5 carbon weight % (standard deviation ± 0.3 w%) in the upper 10 cm soil layer for a 1 hmeasurement.

Applying Monte-Carlo simulations to optimize an inelastic neutron scattering system for soil carbon analysis.

Computer Monte-Carlo (MC) simulations (Geant4) of neutron propagation and acquisition of gamma response from soil samples was applied to evaluate INS system performance characteristic [minimal detectible level (MDL), sensitivity] for soil carbon measurement. The INS system model with best performance characteristics was determined based on MC simulation results. Measurements of MDL using an experimental prototype based on this model demonstrated good agreement with simulated data. This prototype will be used as the base engineering design for a new INS system.

Application of Geant4 simulation for analysis of soil carbon inelastic neutron scattering measurements.

Inelastic neutron scattering (INS) was applied to determine soil carbon content. Due to non-uniform soil carbon depth distribution, the correlation between INS signals with some soil carbon content parameter is not obvious; however, a proportionality between INS signals and average carbon weight percent in ~10cm layer for any carbon depth profile is demonstrated using Monte-Carlo simulation (Geant4). Comparison of INS and dry combustion measurements confirms this conclusion. Thus, INS measurements give the value of this soil carbon parameter.

"Hot background" of the mobile inelastic neutron scattering system for soil carbon analysis.

The problem of gamma spectrum peak identification arises when conducting soil carbon analysis using the inelastic neutron scattering (INS) system. Some spectral peaks could be associated with radioisotopes appearing due to neutron activation of both the measurement system and soil samples. The investigation of "hot background" gamma spectra from the construction materials, whole measurement system, and soil samples over time showed that activation of (28)Al isotope can contribute noticeable additions to the soil neutron stimulated gamma spectra.

Alpha indirect conversion radioisotope power source.

Advantages of radioisotope-powered electric generators include long service life, wide temperature range operation and high-energy density. We report development of a long-life generator based on indirect conversion of alpha decay energy. Prototyping used 300 mCi Pu-238 alpha emitter and AlGaAs photovoltaic cells designed for low light intensity conditions. The alpha emitter, phosphor screens, and voltaic arrays were assembled into a power source with the following characteristics: Isc=14 microA; Uoc=2.3 V; power output -21 microW. Using this prototype we have powered an eight-digit electronic calculator and wrist watch.