Some notes on the geochemistry of radon and radium applied to uranium exploration
· Radon and radium occur about midway in the uranium decay series.
· Once separated from the parent uranium it takes a million years for radon and radium to grow back into equilibrium with uranium. Therefore, any correlation with uranium in younger material is due to similar chemistry, not radioactive production.
· The parent-daughter pair radium-radon grow into equilibrium with the half-life of the daughter radon, 3.8 days. This provides the classic and EPA technique for radium analysis: let the radon grow into equilibrium with radium and then measure the radon with the Lucas cell.
· Radon is soluble in water and, given the extremely low abundance of radon in the earth (due to its short half-life), water is never saturated with radon and radon bubbles never form; however, if another gas phase is present radon will enter the gas phase according to Henry’s law.
· With half-lives of 3.8 days and 1,622 years respectively all the radon and radium in the earth came from radioactive decay of uranium or thorium. Therefore, there is no reason to suggest that radon and radium are any less direct than uranium as geochemical indicators of uranium ore.
· In some cases radium is a more direct indicator than uranium because in the surficial environment uranium is more likely to be enriched by organic matter in sediments and soils leading to false anomalies.
· Variations in radon content on a day-to-day or hour-to-hour basis can be smoothed out by repeated readings.
· Radon does not come out of solid rock. A radon anomaly in soil gas cannot be explained by the proximity of granite or pegmatite boulders. Likewise, although radon can be detected in snow overlying frozen clay-rich soil it cannot be detected in snow overlying highly radioactive bare rock.
· Variations of content of the two main isotopes of radon, radon-222 and radon-220 (thoron), with half-lives of 3.8 days and 55 seconds respectively, reflect soil permeability and can be used to fine tune the results and weed out false soil-gas anomalies. Of all the radon measuring instruments and techniques Lucas cells provide the only method of resolving these two isotopes. The gamma-ray spectrometer will not do it.
· Radon results are available in five minutes in the field for immediate follow-up.
· For more details on principles see www.finderschoice.com/radon/principles.php .
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