Soil moisture and composition by surface Prompt Gamma Neutron Activation analysis (PGNA) 4th NAFE Workshop Chris Waring Craig Smith

Neutron Source Shielding Gamma Detectors NeutronsGamma rays MeV PC spectral analysis for elemental composition and depth PGNA soil moisture and composition scanning H Si Fe Al ~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) Carbon ( ) Iron (55.85) Silicon (28.09) Aluminium (26.98) Calcium (40.08) Sulphur (32.06) PGNAA RESPONSE FOR SELECTED ELEMENTS

252 Cf Neutron Source Shielding Gamma Detector Neutrons Gamma rays Cable winch, Data comms 0 % Si 100 %0 % Porosity 50 % MeV 1. Spectral analysis for elemental composition eg Si, H, Cl, + gives calculated parameters Porosity, Lithology, Salinity 2. Hydraulic Conductivity requires tracer injection + H

Cl 1.95 MeVCl 6.1 MeVCl 1.95 / 6.1 NaCl tracer injection & migration Steps 1.Log bore with PGNAA (black) 2.Mix 5% NaCl tracer through bore 3.Apply P (head) to tracer to move tracer into rock 4.Drain 2x bore volume freshwater from top of bore 5.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 Geoscience

Area = A e -(x-x 0 ) 2 / D 2 dx (MeV) Gaussian fit: f(x) = A e -(x-x 0 ) 2 / D 2 where A is peak amplitude X0 is peak energy at maximum D 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? ?

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 252 Cf neutron source Limitations are ; OHS handling radiation source ~ 10 7 n/s linear acquisition speed 2m / min BGO 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 ~ 10 9 n/s linear acquisition speed ~ 20 m / min high and low spectral resolution detectors matched to range of elements (major and trace elements) depth distribution for element trace elements to ppm range

Neutron Source Shielding Gamma Detectors NeutronsGamma rays MeV PC spectral analysis for elemental composition and depth PGNA soil moisture and composition scanning H Si Fe Al ~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 (LaBr 3 ), 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 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.

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

Recharge Indirect measures currently Hydraulic Conductivity Significantly improved technology via PGNAA Logging Residence Time 35 S, 3 H, 14 C 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 Geoscience