Soil moisture and composition by surface prompt gamma neutron activation analysis pgna
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Soil moisture and composition by surface Prompt Gamma Neutron Activation analysis (PGNA). 4th NAFE Workshop Chris Waring Craig Smith. PGNA soil moisture and composition scanning. PC spectral analysis for elemental composition and depth. 0 - 10 MeV. Shielding. Neutron Source.

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Soil moisture and composition by surface Prompt Gamma Neutron Activation analysis (PGNA)

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Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Soil moisture and composition by surface Prompt Gamma Neutron Activation analysis (PGNA)

4th NAFE Workshop

Chris Waring Craig Smith


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

PGNA soil moisture and composition scanning

PC spectral analysis for

elemental composition

and depth

0 - 10 MeV

Shielding

Neutron Source

Gamma Detectors

Neutrons

Gamma rays

H

Si

Al

Fe

~50cms

Neutron and Gamma depth penetration up to ~ 50cms


2d surface mapping

2D Surface Mapping

Plan View


Quantitative soil moisture by pgna is easy

Quantitative soil moisture by PGNA is easy

  • Prompt Gamma Neutron Activation directly measures elemental abundance (H) by gamma spectral analysis

  • Other neutron soil moisture methods use a proxy relationship with back-scattered thermalised neutrons (affected by soil compaction and compositional variation)

Quantitative soil composition by PGNA is possible

  • Soil (Si, Al, Fe, Na, K, Ca, Mg, S, N, P, C, ) can be measured by PGNA with variable sensitivity due to abundance and neutron cross-section


Pgnaa response for selected elements

PGNAA RESPONSE FOR SELECTED ELEMENTS


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

0 % Si 100 %

0 % Porosity 50 %

Cable winch,

Data comms

0 - 10 MeV

+ H

1. Spectral analysis for

elemental composition

eg Si, H, Cl, + gives

calculated parameters

Porosity, Lithology, Salinity

2. Hydraulic Conductivity

requires tracer injection

Gamma Detector

Gamma rays

Shielding

252Cf Neutron Source

Neutrons


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Cl 1.95 MeV

Cl 6.1 MeV

Cl 1.95 / 6.1

HC

NaCl tracer injection & migration

Steps

Log bore with PGNAA (black)

Mix 5% NaCl tracer through bore

Apply P (head) to tracer to move tracer into rock

Drain 2x bore volume freshwater from top of bore

Log bore with PGNAA (red)


Prompt gamma neutron activation analysis pgnaa

PROMPT GAMMA NEUTRON ACTIVATION ANALYSIS (PGNAA)

Hole Depth

  • Coloured to display count rate (yellow = high, brown = low)

  • Fe response

  • Si response

  • H response (used for stabilisation)

Channel (Energy)

Mining Geoscience


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Gaussian fit:

f(x) = A e -(x-x0) 2 / D2

where

A is peak amplitude

X0 is peak energy at maximum

D is the characteristic peak width

Area = A e -(x-x0) 2/ D2 dx

(MeV)


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Si (4.934 MeV)

Escape peak from Si

Cl (6.111 MeV)

Cl (6.619 MeV)

Escape peak?

Escape peak?

Cl (7.414 MeV)

Cl (7.790 MeV)

?

?

(MeV)


What can we do now

What can we do now?

  • Drag logging tool over surface to gain surface test spectra

  • Build a surface system for H, Si, Cl, + with a LaBr3 or BGO detector, shielding and 252Cf neutron source

  • Limitations are ;OHS handling radiation source ~ 107 n/slinear acquisition speed 2m / minBGO spectral resolution limits range of elements


What is possible

What is possible?

  • Design surface system for rapid soil moisture and composition

  • Build a surface system for H, Si, Al, Fe, Na, K, Ca, Mg, S, N, P, C, + with multiple BGO, LaBr3, HPGe detectors, shielding and neutron generator (fusion source)

  • Capability ;no radiation when switched off, less OHS radiation concern source ~ 109 n/slinear acquisition speed ~ 20 m / minhigh and low spectral resolution detectors matched to range of elements (major and trace elements)depth distribution for elementtrace elements to ppm range


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

http://www.nsd-fusion.com/


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

PGNA soil moisture and composition scanning

PC spectral analysis for

elemental composition

and depth

0 - 10 MeV

Shielding

Neutron Source

Gamma Detectors

Neutrons

Gamma rays

H

Si

Al

Fe

~50cms

Depth measurement by

Differential gamma energy attenuation (similar to HC measurement of injected tracer)

Geometric arrangement of multiple detectors each with a depth bias


What do you want

What do you want?

CSIRO Exploration & Mining is building

  • a surface system with isotope neutron source and high res detector (LaBr3)

  • next generation borehole logging tools with higher res detector (LaBr3), spectral analysis software, ± neutron generator

    CSIRO Exploration & Mining can

  • Design and build a surface system for rapid soil moisture and composition H, Si, Al, Fe, Na, K, Ca, Mg, S, N, P, C, + with multiple BGO, LaBr3, HPGe detectors, shielding and neutron generator (fusion source)

    ANSTO Environmental Research is applying PGNA logging

  • to measure hydrogeology parameters (water, porosity, & hydraulic conductivity, salinity, lithology)

  • Is interested in a surface system to measure soil moisture and composition for groundwater recharge, C accounting, nutrient movement, ++


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Elemental Analysis

• There are three major types of nuclear

reactions that are used in elemental

analysis:

– Prompt inelastic scattering reactions (n, n’g)

– radiative capture reactions (n, g)

– long half life activation

• These three types of reactions have

different temporal characteristics.

• This fact may be exploited by using

pulsed neutron generators and

appropriate timing of detector circuitry.


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Hydraulic Conductivity Calculation

To calculate Hydraulic Conductivity a NaCl or KCl tracer is injected and relative distance moved under an applied hydraulic gradient is measured

Principle Applied

Activated gamma emissions at low energies are attenuated more by transmission through rock & water than high energies

Hence changes in the ratio of the tracer emissions is a function of the distance the tracer has moved away from the injection bore ie Cl 1.95 / 6.1 MeV

This distance function is established by experiments


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Groundwater Sustainability

Key components of any groundwater flow model necessary to predict Groundwater sustainability

Recharge

Indirect measures currently

Hydraulic Conductivity

Significantly improved technology via PGNAA Logging

Residence Time

35S, 3H, 14C measured. Each requires care and corrections for application


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Timing


Prompt gamma neutron activation technique

Prompt Gamma Neutron Activation Technique

  • The 252Cf source emits neutrons that are thermalised after interaction with hydrogen (in water and polyethylene). Source is 7.0 micrograms (equivalent to 140 MBq)

  • A thermal neutron enters the nucleus and produce an unstable compound nucleus (in an excited state), which decays by emission of one or more gamma-rays.

  • The gamma-rays emitted have energies characteristic of the target nucleus. If the intensity and energy of these gamma-rays are measured by means of a suitable spectrometric detector, the type and amount of an element present in ore/rock can be estimated.

  • The intensity of a given response is directly proportional to:

    • the abundance of that element,

    • the thermal neutron flux,

    • the thermal neutron capture cross section.


Soil moisture and composition by surface prompt gamma neutron activation analysis pgna

Mining Geoscience

Institute for Environmental Research


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