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

4th NAFE Workshop

Chris Waring Craig Smith

Neutron SourceShielding

Gamma Detectors

Neutrons Gamma rays

0 - 10 MeV

PC spectral analysis for elemental compositionand depth

PGNA soil moisture and composition scanning

H

Si

FeAl

~50cms

Neutron and Gamma depth penetration up to ~ 50cms

2D Surface Mapping

Plan View

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

Element(atomic mass)

Thermal neutron capture cross-section (barns)

Major gamma-rays (MeV)

Gamma-ray intensity(per 100 neutron

radiative captures)

Hydrogen(1.0079)

0.3326 2.223 100

Carbon(12.0107)

0.00337 1.263.684.94

29.532.167.6

Iron(55.85)

2.55 5.926.027.637.65

99

24.128.5

Silicon(28.09)

0.16 1.162.093.544.936.38

19.921.568.062.712.4

Aluminium(26.98)

0.23 7.72 27.4

Calcium(40.08)

0.43 1.944.426.42

72.615.038.9

Sulphur(32.06)

3.32 0.842.382.933.225.42

75.644.522.327.159.1

PGNAA RESPONSE FOR SELECTED ELEMENTS

252Cf Neutron Source

Shielding

Gamma Detector

Neutrons

Gamma rays

Cable winch, Data comms

0 % Si 100 % 0 % Porosity 50 %

0 - 10 MeV

1. Spectral analysis for elemental compositioneg Si, H, Cl, + givescalculated parametersPorosity, Lithology, Salinity

2. Hydraulic Conductivity requires tracer injection

+ H

Cl 1.95 MeV Cl 6.1 MeV Cl 1.95 / 6.1 NaCl tracer

injection & migrationSteps1. Log bore with PGNAA

(black)2. Mix 5% NaCl tracer

through bore3. Apply P (head) to tracer to

move tracer into rock4. Drain 2x bore volume

freshwater from top of bore5. Log bore with PGNAA

(red)

HC

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

•Fe response

•Si response

•H response (used for stabilisation)

PROMPT GAMMA NEUTRON ACTIVATION ANALYSIS (PGNAA)

Channel (Energy)

Hole Depth

Mining Mining GeoscienceGeoscience

Area = A e -(x-x0

) 2/ D

2

dx∫2

1

x

x

(MeV)

Gaussian fit:

f(x) = A e -(x-x0

) 2 / D

2

whereA is peak amplitudeX0 is peak energy at maximumD is the characteristic peak width

(MeV)

Si (4.934 MeV)

Escape peak from Si

Cl (6.111 MeV)

Cl (7.414 MeV)Cl (7.790 MeV)

Cl (6.619 MeV)

?

Escape peak?Escape peak?

?

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?• 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

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

Neutron SourceShielding

Gamma Detectors

Neutrons Gamma rays

0 - 10 MeV

PC spectral analysis for elemental compositionand depth

PGNA soil moisture and composition scanning

H

Si

FeAl

~50cms

Depth measurement by 1. Differential gamma energy attenuation (similar to HC measurement of

injected tracer)2. Geometric arrangement of multiple detectors each with a depth bias

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, ++

Elemental Analysis

• There are three major types of nuclearreactions that are used in elementalanalysis:

– Prompt inelastic scattering reactions (n, n’g)– radiative capture reactions (n, g)– long half life activation

• These three types of reactions havedifferent temporal characteristics.

• This fact may be exploited by usingpulsed neutron generators andappropriate timing of detector circuitry.

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 AppliedActivated 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

RechargeIndirect measures currently

Hydraulic ConductivitySignificantly improved technology via PGNAA Logging

Residence Time35S, 3H, 14C measured. Each requires care and corrections for application

Groundwater Sustainability

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

Timing

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.

Institute for Environmental Research

Mining Mining GeoscienceGeoscience