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 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.

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 is difficult to measure directly by field gamma spectrometry. Geiger counters and most 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 most borehole logging tools.

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

U238 decay series
U235 decay series

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 will 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. In theory, secular isotopic equilibrium is attained in uranium deposits after approximately 1.7 million years if mineralization behaves as a closed geochemical system. This theoretical result assumes introduction of U238 as the only uranium isotope. In uranium deposits that have formed from uranium precipitation from groundwater, U238, U235, and most significantly U234 will be present in approximate ratios of their natural isotopic abundance. As a consequence, U234 is the rate limiting parent to be considered for calculations of secular equilibrium in the U238 series in most natural systems. The time to secular equilibrium then is reduced to about 540,000 years. In that high energy, high intensity gamma radiation is only emitted by progeny near the end of the decay chain, gamma radiation only reaches easily measured values as secular equilibrium is approached. Estimates of uranium ore grade from total gamma counts and spectral gamma counts all assume a state of near secular equilibrium.

Younger uranium deposits, refined uranium metal, uranium fuel, and depleted uranium all have highly attenuated gamma signals because they have not reached secular equilibrium. Significant gamma activity in natural uranium deposits only begins to increase after about 10,000 years before reaching a maximum between 540,000 and 1.7 million years. Relatively young, naturally occuring uranium mineralization, will emit gamma from progeny in proportion to their activity calculated from the period of ingrowth. 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 no longer in secular equilibrium. The practical result for uranium exploration is the introduction of error into uranium concentration calculations based on gamma measurement.

In the case of depleted uranium (nearly pure U238), virtually all of the gamma activity will originate from U238, Th234 and Pa234. Enriched uranium will have somewhat higher gamma activity proportional to the contribution of U235 and Th231.

Secular equilibrium is disturbed if the system is not closed. Uranium dissolved in oxygenated groundwater can be transported away, depleting the site. Radon produced by radioactive decay can escape to the atmosphere. In both cases, the decay chain is spatially separated and error is introduced into uranium concentration calculations that are based on late stage nuclides like Bi214.

Secular isotopic equilibrium

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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Alpha, beta, and gamma radiation have greatly different abilities to escape the soil or rock where they originate. Alpha measurements are from the surface only. Beta measurements test only the first centimeter of the surface. Gamma measurements range from ten cm to a meter in depth.

Radiation penetration

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.

Some isotopes have characteristic gamma peaks which can be clearly seen in this sample of oxidized uranium ore. Radium 226, bismuth 214, and lead 214 are well defined in this sample.

Uranium ore spectrum

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 made 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 proxies for uranium and thorium. If specific energy windows are not used, potassium in soil or rock is a potential source of noise. Thallium 208 (proxy for Th232), Bi214 and Pb214 (proxies for U238) have energies in the less noisy, high energy neighborhoods of the gamma spectrum and high intensity lines. 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 U238 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. This geochemical fractionation prevents or disrupts secular equilibrium of the uranium decay chain resulting in overestimation of the uranium concentration.

Shallow borehole gamma measurements are superior to surface measurements. With the appropriate instrumentation, a larger soil volume can be measured than a surface measurement, activity from atmospheric fallout concentrated at surface reduced, and to a lesser extent activity from cosmic sources reduced as well. The symmetrical geometry of a well drilled soil borehole also lends itself to more precise calibration of a gamma probe so that better correlation between counts and uranium concentration is possible. Though most portable systems have a "uranium ppm" setting, the accuracy claims of their manufacturers are dubious at best. The correlation algorithm in these handheld instruments does not account for the user's angle of operation, distance to the target, or other non-standard geometries and conditions that are possible.

Typical NaI(Tl) detectors with a prismatic or cylindrical geometry are designed to detect gamma photons at energies higher than about 150 KeV. 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. This effectively restricts their use to progeny products of U238 far removed from the original parent. Strong gamma emitters like Bi214 and Pb214 and to a lesser extent Ra226 are all well down the uranium decay chain and significant departures from secular equilibrium are possible and even likely in the pedogenic environment. Uranium content is thus infered from the presence of these progeny products but not directly measured.

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.

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