8/7/2019 Poster%20156

1/1

.00E-07

.00E-06

.00E-05

.00E-04

0 5 10 15 20 25 30 35 40 45 50

Depth (cm)

0% Water (1.4g/cc)

5% Water (1.47 g/cc)

10% Water (1.56 g/cc)

20% Water (1.75 g/cc)

50% Water (1.4 g/cc)

50% Water (2.8 g/cc)100% Water (1 g/cc)

(bulk dry) = 1.4 g/cc

1.00E-07

1.00E-06

1.00E-05

0 5 10 15 20 25 30 35 40 45 5

Depth (cm)

GammaYield(/cm

2/source)

0% Water (1.5 g/cc)

5% Water (1.47 g/cc)

10% Water (1.56 g/cc)

20% Water (1.75 g/cc)

50% Water (2.8 g/cc)

100% Water (1 g/cc)

(bulk dry) = 1.4 g/cc

1.00E-07

1.00E-06

1.00E-05

0 5 10 15 20 25 30 35 40 45

Depth (cm)

GammaYield(/cm

2/source)

0.25 g/cc

0.5 g/cc

0.75 g/cc

1 g/cc

1.25 g/cc

1.5 g/cc

2 g/cc

} bulk density range

1.00E-06

1.00E-05

1.00E-04

0 5 10 15 20 25 30 35 40 45 50

Depth (cm)

0.25 g/cc

0.5 g/cc

0.75 g/cc

1 g/cc

1.25 g/cc

1.5 g/cc

2.0 g/cc

}bulk density range

Bulk Density and Moisture Effects on Monte Carlo

Simulations of Soil Carbon Analysis for the INS

System

Oded Doron and Lucian Wielopolski,

Brookhaven National Laboratory,

Environmental Sciences Department,

Upton, NY 11973

AbstractInelastic neutron scattering (INS) is a new method for carbon analysis in soil, in the field, that is completely non-destructive and uniquely measures large volumes of ~0.3 m3. It is based on fast 14 MeV

dergoing inelastic neutron scattering with carbon nuclei and inducing the emission of 4.43 MeV gamma rays. The fast neutrons are produced by a (d,t) neutron generator (NG) with the induced gamma ray

ected by an array of NaI detectors. The transport of neutrons and gamma rays in the soil is affected by the bulk density and moisture content. While variations in the bulk density affect both the neutro

mma rays mass attenuation coefficients, the moisture affects mainly the transport of neutrons due to elastic scatterings with hydrogen nuclei. Changes in the transport properties of the soil might affect th

nal yield from various depths that would affect the calibration and sampled depth and volume. A probabilistic Monte Carlo method is utilized to model the system. The entire system is simulated, following t

particles from their creation to death thus enabling modeling accurately this complex system and economically replacing complicated experiments. The code used here is the Monte Carlo N-Particle trans

rsion 5 (MCNP5) developed at Los Alamos National Laboratory. This work assesses the magnitude of the expected change in the carbon signal yield due to variations in the soil parameters; bulk de

isture content.

2.50E-08

3.00E-08

3.50E-08

4.00E-08

4.50E-08

5.00E-08

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Bulk Density (g/cc)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.25 0.5 0.75 1 1.25 1.5 1.75Bulk Density (g/cc)

Volume(m

3)

20

25

30

35

40

45

50

0 0.25 0.5 0.75 1 1.25 1.5 1.75

Bulk Density (g/cc)

Depth(cm)

Detectors

Shielding

Shielding

Source

250c

m200cm

50 cm

System geometry and soil volume modeled in MCNP5.

5-14 MeV Neutron Flux versus Depth and Bulk Density

5-14 MeV Neutron Flux versus Depth and Moisture Content

4.43 MeV Gamma Ray Yield at the Detectors versus Depth

and Bulk Density

4.43 MeV Gamma Ray Yield at the Detectors versus Depth

and Moisture Content

An isotropic 14 MeV neutron point source was modeled to emit

nto a 250cm by 200cm by 50cm soil volume with a carbon

content of 2% by weight. The fast neutron flux, 5-14 MeV, in

the soil is plotted versus bulk density and moisture content

variations.

An isotropic 4.43 MeV gamma point source was mode

emit from different depths of a 250cm by 200cm by 50cvolume with a carbon content of 2% by weight. The c

region of interest gamma yield, 4.43 MeV, at the detec

plotted versus bulk density and moisture content variation

An isotropic 14 MeV neutron point source was modeled to emit

nto a 250cm by 200cm by 50cm soil volume with a carbon

content of 2% by weight. The carbon region of interest gamma

yield, 4.43 MeV, at the detectors is plotted versus bulk density

and moisture content.

4.43 MeV Gamma Ray Yield at the Detectors versus Bulk Density

4.43 MeV Gamma Ray Yield at the Detectors versus Moisture Content

The volumes contributing to 90% of the total carbon in

gamma ray flux intercepting the detectors and the ma

depth associated with the volumes are plotted versu

density variations.

Volumes contributing to 90% of the Total Carbon Gamma

Flux Intercepting the Detectors versus Bulk Density

Moisture Content and Bulk Density

The soil bulk density is affected by an increase in moisture

content. Assuming that a given soil volume remains constant with

the increase of moisture content, i.e. pore space allowing, and no

swelling occurs, the increase in the bulk density of the given soilvolume is dictated by equation 1.

(1)

Where: Bold = old bulk density previous to addition of

moisture

Bnew = new bulk density with added moisture

X = the weight fraction of moisture added

X

old

new

B

B

=1

Conclusions

Soil bulk density affects the neutron and gamma transport

through soil thus affecting the gamma yield at the detectors.

However in the average bulk density range the effect is minimal.

Moisture content affects the neutron and gamma transport

through the increase of bulk density, rather than the increased

presence of the hydrogen introduced into the soil volume.

The 4.43 MeV gamma ray yield at the detectors increases with

increasing bulk density, as a result of the increased number

density.

The volumes contributing to 90% of the total carbon gamma ray

flux and the maximum depth of these volumes decreases with

increasing bulk density. The decrease can be attributed to the

increased number density of the elements in the soil, as a larger

fraction of the gamma ray yield will come from a more shallow

depth.

Maximum Depth of Volumes contributing to 90% of the Total

Gamma Ray Flux Intercepting the Detectors versus Bulk De

.50E-08

.70E-08

.90E-08

.10E-08

.30E-08

.50E-08

.70E-08

0 5 10 15 20 25 30 35 40 45 50

Mositure Content (wt%)