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

Summary

Here, we present the protocol for in situ measurement of soil carbon using the neutron-gamma technique for single point measurements (static mode) or field averages (scanning mode). We also describe system construction and elaborate data treatment procedures.

Abstract

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.

Introduction

Knowledge of soil carbon content is required for optimization of soil productivity and profitability, understanding the impact of agricultural land use practices on soil resources, and evaluating strategies for carbon sequestration^1,2,3,4. Soil carbon is a universal indicator of soil quality⁵. Several methods have been developed for soil carbon measurements. Dry combustion (DC) has been the most widely used method for years⁶; this method is based on field sample collection and laboratory processing and measurement that is destructive, labor intensive, and time consuming. Two newer methods are laser-induced breakdown spectroscopy, and near and mid infrared spectroscopy⁷. These methods are also destructive and only analyze the very near-surface soil layer (0.1 - 1 cm soil depth). In addition, these methods only yield point measurements of carbon content for small sample volumes (~60 cm³ for DC method, and 0.01-10 cm³ for infrared spectroscopy methods). Such point measurements make it difficult to extrapolate results to field or landscape scales. Since these methods are destructive, recurring measurements are also impossible.

Previous researchers at Brookhaven National Laboratory suggested applying neutron technology for soil carbon analysis (INS method)^7,8,9. This initial effort developed the theory and practice of using neutron gamma analysis for soil carbon measurement. Starting in 2013, this effort was continued at the USDA-ARS National Soil Dynamics Laboratory (NSDL). The expansion of this technological application over the last 10 years is due to two main factors: the availability of relatively inexpensive commercial neutron generators, gamma detectors, and corresponding electronics with software; and state of the art neutron-nuclei interaction reference databases. This method has several advantages over others. An INS system, placed on a platform, could be maneuvered over any type of field that requires measurement. This non-destructive in-situ method can analyze large soils volumes (~300 kg) that can be interpolated to a whole agricultural field using just a few measurements. This INS system is also capable of operating in a scanning mode that determines the average carbon content of an area based on scanning over a predetermine grid of the field or landscape.

Protocol

1. Construction of the INS system

1. Use the general INS system geometry shown in Figure 1.

Figure 1. INS System Geometry. Please click here to view a larger version of this figure.

2. Use the INS system design shown in Figure 2.¹⁰

Figure 2. Overview of the INS System.
A) first block contains neutron generator, neutron detector, and power system; B) second block contains three NaI (Tl) detectors; C) third block contains equipment for system operation; D) general view of the first block showing individual components; and E) close up view of the gamma detectors.¹⁰ Please click here to view a larger version of this figure.

3. Use three blocks in the INS system (see Appendix).
   1. For the first block (A), use a neutron generator (NG) and power system (Figure 2A and 2D). Pulsed neutron output of this generator will be 10⁷ - 10⁸ n/s with neutron energy of 14 MeV. The power system will consist of four batteries (12 V, 105 Ah), a DC-AC Inverter, and a charger. This block will also contain iron (10 cm x 20 cm x 30 cm) and boric acid (5 cm x 20 cm x 30 cm) shielding to protect the gamma detector from neutron irradiation.
      NOTE: A neutron detector is also included in this block for checking that the NG is functioning properly.
   2. For the second block (B), use gamma-ray measurement equipment (Figure 2B and 2E). This block will contain three 12.7 cm x 12.7 cm x 15.2 cm scintillation NaI(Tl) detectors with corresponding electronics. The exterior size of the detectors with electronics will measure 15.2 cm x 15.2 cm x 46 cm.
   3. For the third block (C), use a laptop computer that controls the neutron generator (with DNC software), detectors, and data acquisition system (Figure 2C).

2. Caution and Personal Requirements

1. Have each user of the INS system pass radiological training.
2. Ensure that each person operating the NG carries a radiation monitoring badge. During measurements, the restricted area boundary (>20 µSv/h) around the NG will have the radiation symbol with the words "CAUTION, RADIATION AREA." All edges of the restricted area will be no less than 4 m from the NG.
3. In an emergency, immediately push the "Emergency Interrupt" button on the NG, remove the key from the NG, and unplug the NG from the power source.

3. Preparation of the INS system for Measurement

1. Check the power system. The power level indicator on the charger will be green, or more than 3 red lamps must illuminate. If not, connect the charger to a power outlet and wait until batteries become fully charged (green lamp will illuminate) or until an acceptable power level is reached (≥3 red lamps will illuminate).
2. Turn on the inverter (green lamp illuminates) and laptop.
3. Run the data acquisition program on the laptop to operate the gamma detectors and check the required parameters for each detector. The values of these parameters will be defined and recorded previously at INS system testing.
   1. Place a Cs-137 control source (any type) within 5 - 15 cm from the detectors.
   2. Start spectra acquisition for 1 - 3 min; check the centroids of the 662 keV Cs-137 peak for all detectors. They must be at the same channel. If not, use the Energy Coefficient Scale of thedata acquisition program by changing the value to adjust the 662 keV peak centroids.
4. Turn on the NG by using the special key. The indicator lamp on the NG will illuminate green and yellow.

4. Calibration of the INS System

1. Prepare 4 pits sized 1.5 m x 1.5 m x 0.6 m with homogeneous sand-carbon mixtures (Figure 3). Carbon contents is 0, 2.5, 5 and 10 w%.
   NOTE: A concrete mixer is used to make synthetic soil composed of construction sand and coconut shell (100% carbon content, average granular diameter < 0.5 mm). Homogeneity of these mixtures is determined visually.

Figure 3. View of Pit with Sand and Pit with 10 Cw% Sand-carbon Mixture. Please click here to view a larger version of this figure.

2. Take measurements over the pits using the following steps.
   1. Position the INS system over the pit manually or by towing with a suitable vehicle. Position the INS system such that the projection of the neutron source is centered on the pit.
   2. Run the DNC software on the laptop that operates the NG generator. In the Faults column on the right side of the DNC program screen, all lamps will illuminate green; if not, click the Clear button. Insert the following parameters: for Pulse Parameters - frequency 5 kHz, duty cycle 25%, delay 0 µs, extension 2 µs; for Beam - high voltage 50 kV, beam current 50 µA (note that these parameters can be different depending on the particular INS system setup and task).
      1. Activate the switch on the DNC program screen and wait for the NG to enter the working regime where the High voltage and Beam current will come to stable values corresponding to the entered values; Reservoir current also will come to a stable value.
   3. Run the data acquisition software on the laptop to operate the gamma detectors. Start spectra acquisition by running the data acquisition program for 1 h. The two spectra acquisition processes (INS & TNC and TNC) will appear on the screen.
   4. After 1 h, stop the spectra acquisition and save spectra to the hard drive (File | Save MCA Data | choose the folder and enter the file name.
      NOTE: There will be two saved spectra (TNC and INS) with filename extensions .mca and _gated.mca, respectively).
   5. Select second detector (click the arrow in the top left corner) and save the spectra for this detector. Do the same for third detector.
   6. Click File | Exit to close the software.
   7. Turn off the DNC software by turning off the switch on the DNC program screen.
   8. Repeat steps 4.2.1 - 4.2.7 for the other pits.
   9. Turn off the NG by using the special key. The indicator lamp on the NG will dim.
3. Determine the INS system background spectra by elevating the whole INS system to a distance greater than 4 m above the ground surface and away from any large objects, and repeat data acquisition steps 4.2.2 - 4.2.9.
4. Data processing
   1. Use a spreadsheet program to open data files saved in step 4.2.4. Find values for output and input count rates (OCR and ICR) and real time (RT) in rows 28, 27, and 30, respectively.
   2. Calculate the life time (LT) for INS & TNC and TNC spectra for all measurements as
      LT_i = OCR_i/ICR_i·RT_i (1),
      where OCR_i and ICR_i are the output and input count rates for the i-th measurement and RT_i is real time of the i-th measurement.
   3. Calculate the gamma spectra in counts per second (cps) by dividing the spectra (rows 33-2080 in the spreadsheet) by the corresponding LT.
   4. Calculate the net INS spectra from the corresponding measurements for each pit as
      Net INS Spectrum = (INS&TNC - TNC)_Pit - (INS&TNC - TNC)_Bkg (2)
   5. Find the gamma peaks 1.78 MeV (²⁸Si) and 4.44 MeV (¹²C) in the Net INS spectrum for each pit, and calculate the peak areas (4.44 MeV C peak area, 1.78 MeV Si peak area) using IGOR software.
      1. Open the software by double clicking the icon. Insert first Net INS Spectrum into the Table.
      2. Click Windows | New Graph | From Target | "FileName" | Do It. The Spectrum appears in the Graph Window. Click Graph | Show Info. The windows with A and B markers appears under the graph window.
      3. Place mouse pointer on sign A, push the left mouse button, and drag the cursor to the spectrum on left side of the 1.78 MeV peak. Place mouse pointer on the sign B, push the left mouse button, and drag the cursor to the spectrum on right side of the 1.78 MeV peak.
      4. Click Analysis | Multi-peak Fit | Start New Multi-peak Fit | From Target | Continue. In the pop-up window marked Use Graph Cursor | Baseline Linear| Auto-locate Peaks Now | Do it | Peak Results. The area of the peak appears in the pop-up window.
      5. Repeat the same operations for 4.44 MeV peak.
      6. Repeat all previous operations with the remaining Net INS Spectra.
   6. Find the Net carbon peak areas for each pit by the equation
      Net C peak area_i = 4.44 MeV C peak area_i- 0.058 · 1.78 MeV Si peak area_i (3)
   7. Build the calibration line for the INS system as a direct proportional dependence of the Net carbon peak area vs. carbon concentration expressed in weight percent.
      1. Open new Table in IGOR software: click Window | New Table. Enter pit carbon concentration values in the first column, and the corresponding Net C peak area in the second column.
      2. Plot the Net C peak area vs pit carbon concentration: click Windows | New Graph. Choose Net C peak area as YWave, and carbon concentrations as XWave. Click Do it. The points appear on the Graph.
      3. Build the calibration line: Click Analysis | Curve Fitting | Function - line | From Target | Do it. The calibration line and the calibration coefficient (k) will appear in the window.

5. Conducting Field Soil Measurements in Static Mode

1. Prepare the INS system for measurement according to step 3.
2. Place the system over the site requiring soil carbon content analysis manually or by towing using suitable vehicle. Position the INS system such that the projection of the neutron source is centered over the site being measured.
3. Implement actions following steps 4.2.2 - 4.2.9 and 4.4.1 - 4.4.6 for determining Net C peak areas for the study sites.
4. Calculate the carbon concentration in weight % using the calibration coefficient as
   

6. Conducting Field Soil Measurements in the Scanning Mode

1. Estimate the path that the INS system will travel over the field while accounting for travel speed (≤ 5 km/h), field size, INS system footprint (radius ~1 m), and measurement time (1 h) such that the moving trajectory eventually covers the whole field area. For convenience, place flags at turn points along the field perimeter.
2. Prepare the INS system for measurement according to step 3.
3. Implement actions following steps 4.2.2 - 4.2.3.
4. Follow the predetermined travel path for 1 h.
5. Implement actions following steps 4.2.4 - 4.2.9 and 4.4.1 - 4.4.6 for determining Net C peak areas for the studied field.
6. Calculate the carbon concentration in weight % using the calibration coefficient by equation 4.

Results

Soil INS & TNC and TNC gamma spectra

A general view of the measured soil gamma spectra is shown in Figure 4. The spectra consist of a set of peaks on a continuous background. The main peaks of interest have centroids at 4.44 MeV and 1.78 MeV in the INS & TNC spectra. The second peak can be attributed to silicon nuclei contained in soil, and the first peak is an overlapping peak from carbon and silicon nucle...

Discussion

Building on the foundation established by previous researchers, the NSDL staff addressed questions critical to the practical and successful use of this technology in real world field settings. Initially, NSDL researchers demonstrated the necessity to account for the INS system background signal when determining net carbon peak areas.¹¹ Another effort showed that the net carbon peak area characterizes the average carbon weight percent in the upper 10 cm soil layer (regardless of carbon depth distri...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Neutron Generator	Thermo Fisher Scientific, Colorado Springs, CO DNC software	MP320
Gamma-detector:		na
- NaI(Tl) crystal	Scionix USA, Orlando, FL
- Electronics	XIA LLC, Hayward, CA
- Software	ProSpect
Battery	Fullriver Battery USA, Camarillo, CA	DC105-12
Invertor	Nova Electric, Bergenfield, NJ	CGL 600W-series
Charger	PRO Charging Systems, LLC, LaVergne, TN	PS4
Block of Iron	Any	na
Boric Acid	Any	na
Laptop	Any	na
mu-metal	Magnetic Shield Corp., Bensenville, IL	MU010-12
Construction sand	Any	na
Coconut shell	General Carbon Corp., Patterson, NJ	GC 8 X 30S
Reference Cs-137 source	Any	na