URANIUM EXPLORATION - GAMMA SOIL SURVEY

FULL SOIL SURVEY SERVICES

Stratamodel designs and executes gamma surface and shallow soil surveys. We also provide lab services for isotopic identification

Total gamma and spectral gamma provide a direct measurement of uranium and uranium progeny isotopes in the soil and at surface. Stratamodel uses highly sensitive gamma detection technology mounted on all terrain vehicles or carried in backpacks for linear traverses. We have portable spectrometers with either isotope specific probes or general purpose large volume crystal probes for grid point measurement, or shallow soil isotopic analysis.

Contact Stratamodel for more information or to schedule a gamma surface or soil survey.

CLIENTS

• Epsilon Energy Ltd, Subiaco, Western Australia
• Uranium Energy Corp, Vancouver, Canada
• Niger Uranium Limited, Sandton, South Africa
• Global Uranium, Toronto, Canada
• Homeland Uranium, Toronto, Canada
• Future Energy, Utah, USA
• Target Exploration and Mining, Vancouver, Canada
• Thelon Ventures, Vancouver, Canada
• International Ranger, Vancouver, Canada
• Magnum Uranium Corp, Vancouver, Canada

THEORY OF SOIL GAMMA SURVEYS

Uranium undergoes radioactive decay to lead via a series of radioactive elements called progeny or daughter radionuclides. Nuclides that emit alpha radiation (a helium nucleus) decay to isotopes of smaller atomic mass. Nuclides that emit beta radiation (an electron) decay to isotopes of larger atomic number with no change in atomic mass. Some nuclides also emit gamma radiation as the nucleons and electrons reconfigure to a more stable form during or shortly after an alpha or beta decay. Gamma photons are radiated at energies and intensities that are specific to each nuclide. The uranium progeny that emit gamma radiation can be identified in many cases by their characteristic spectrum if the gamma intensity is sufficiently high. Uranium 238 itself emits a single very low energy-low intensity gamma photon when it decays thus it cannot be measured directly by field gamma spectrometry. Geiger counters and other gamma detectors do not directly measure uranium. Most gamma detectors measure the radiation from nuclides that are far down the decay chain thus uranium concentration or activity can only be inferred assuming the sample or sample site is in secular isotopic equilibrium. In outcrop or soil, this assumption is rarely the case. All airborne and ground radiometric methods rely upon this assumption as do all borehole logging tools.

U²³⁸ and U²³⁵decay series. Isotopes with diagnostic gamma radiation are colored from low intensity (blue) to high intensity (red).

Secular equilibrium is the steady state condition where all progeny nuclides have the same decay activity as their parent. If U²³⁸ is present at a concentration sufficient to produce one bequerel (one decay/second), all lighter progeny must also be present in sufficient concentrations to produce one bequerel of activity. At equilibrium, the activity ratio of all nuclides in the U²³⁸ and U²³⁵ decay chains is equal to one. Secular isotopic equilibrium is attained in uranium deposits after a few million years if mineralization behaves as a closed geochemical system. If progeny nuclides escape or progeny nuclides remain behind when uranium escapes, the activity ratios of some or all remaining nuclides depart from unity and the deposit is not in secular equilibrium. The practical result for uranium exploration is the introduction of error into uranium concentration calculations based on gamma measurement.

Devices that measure gamma radiation sample a larger volume of rock or soil than either alpha or beta detectors. Alpha particles have a mass of four atomic mass units and are stopped by collisions with other atoms. Few alpha particles escape from the soil or rock and are stopped within a few micrometers of their origin by rock and within a few centimeters by air. Beta particles are electrons which have a much lower mass but are still stopped easily by collisions with solid matter. Gamma photons have no mass and in general more energy than either alpha or beta particles emitted by the nuclides in the uranium decay series thus they can penetrate tens of centimeters of rock before being attenuated.

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Total counts of gamma photons indicate the gross level of activity at a site or within a sample. However, uranium and its progeny are far from the only sources of gamma radiation. Cosmic rays are highly energetic gamma photons which contribute to total gamma counts. Potassium is a common element in soil and rock and the isotope K⁴⁰ can contribute a significant proportion of the total gamma radiation in some instances. Several other naturally occurring isotopes such as thorium are also commonly found in rocks and soils. Geiger counters and simple scintillation counters can only quantify total gamma counts. Some scintillation counters can isolate a few user defined gamma peaks and are a little more informative than total gamma counts. Even more sophisticated instruments divide gamma photons of different energy into hundreds or thousands of channels thus a very detailed record of a gamma spectrum can be analyzed and individual nuclides identified.

PRACTICE OF GAMMA SURVEYS

The simplest type of gamma survey is measurement of the total gamma activity of the surface. Gamma photons of the nuclides used for uranium exploration are not very energetic and even the best instruments can only detect gamma photons originating in the upper meter of soil or rock. Surface measurements are widely used either from the air or on the surface itself. With proper interpretation these can be a valuable first exploration pass. Ore grade uranium mineralization at surface is relatively easy to detect by this type of survey.

Continuous measurement or gridded sampling of total gamma radiation using standard techniques and instrumentation are widely used in uranium exploration. Isotopic ratios and specific energy 'windows' provide additional detail to a survey and in the former case, normalization. Despite this, all rely upon the gamma energies of just a few isotopes as either proxies for uranium and thorium or as a direct measure of potassium, a potential source of noise. Thallium 208 (proxy for Th²³²), Bi²¹⁴ (proxy for U²³⁸), and K⁴⁰ have energies in the less noisy, high energy neighborhoods of the EM spectrum and high intensity lines. Large crystal detectors are necessary to adequately quantify these nuclides but there is a tradeoff. The internal noise in such large crystal detectors overwhelms potentially more useful low energy-low intensity gamma signals from nuclides higher in the U²³⁸ decay chain and thus closer to the uranium one wants to measure. The problem with relying on these high gamma energy and intensity nuclides lies with their parents. Before Bi²¹⁴ is produced in the U²³⁸ decay chain, four potentially mobile nuclides can be separated from their primary uranium parents. Uranium ²³⁴, produced early in the ²³⁸ decay chain, and primary U²³⁵ can be solubulized then transported by moderately oxidized groundwater. The result is mineralization with a strong gamma signal but uranium depletion. Radium 226 though not geochemically mobile in dilute groundwater is solubulized at moderate to high salinity and can travel significant distances from its point of origin. Moreover, Ra²²⁶ is preferentially removed from the soil by some plants. Radon gas is potentially far more mobile and can also be transported away from its source. The nuclides Pb²¹⁰, Th²³⁴, U²³⁴, and U²³⁵ though more difficult to measure are far more indicative of uranium at surface and at depth than Bi²¹⁴ that is commonly used as a poxy for direct uranium measurement.

Shallow borehole gamma measurements are superior to surface measurements. With the appropriate instrumentation, a larger soil volume can be measured than a surface measurement. Measurement of Pb²¹⁰, U²³⁵, and Th²³⁴ avoid signals from Bi²¹⁴ and Pb²¹⁴ which fluctuate with time of day and season, soil moisture, etc resulting in over estimates of uranium equivalent concentration. The source of the fluctuation of Pb²¹⁴ and Bi²¹⁴ is the fluctuating concentration of Rn²²² in soil and at surface in still air conditions. Lead 210 measurement yields a long term average Rn²²² flux through the rock or soil. With its relatively long half life of 22 years and inert geochemical behavior, Pb²¹⁰ acts as a natural integrator of Rn²²² flux. U²³⁵, U²³⁴, and Th²³⁴ are a direct or nearly direct means of measuring uranium concentration bypassing the assumption of secular equilibrium.

Spectral gamma measurement, either along traverses or at grid points, can be more effective than total gamma alone. The instrumentation is more sophisticated and the data processing is more elaborate. However, by distinguishing the individual isotopes present, it is often possible to develop a more detailed picture of a prospective site. A level of geochemical reasoning is often useful in the interpretation of gamma spectra. This technique can be applied by taking soil samples and sending them to a lab for high precision gamma spectroscopy. With the proper field instrumentation, a less precise but nearly as effective process can be accomplished at far less cost and in far less time.

GAMMA LINKS

RADIATION FACTS

• U238 decay series
• U235 decay series
• Th232 decay series
• Radiation penetration
• Periodic table of the elements with isotopic data
• Glossary of nuclear terms

GAMMA INSTRUMENTATION

• Measuring radiation
• Scintillation probe basics
• Scintillating Crystals (NaI, CsI, BaF2 Crystal) and NaI Detector

GAMMA IN THE WILD

• Continental radiometric mapping
• Regional radiometric mapping
• District radiometric mapping
• Uranium exploration
• Detailed radiometric and secular equilibrium analysis
• Uranium exploration primer
• Surface gamma soil survey
• Radioactive minerals

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