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Uranium isotope ratios are not invariant, researchers show
October 24, 2007CHAMPAIGN, Ill. - For years, the ratio of uranium's two long-lived isotopes, U-235 and U-238, has been considered invariant, despite measurements made in the mid-1970s that hinted otherwise. Now, with improved precision from state-of-the-art instrumentation, researchers at the University of Illinois unequivocally show this ratio actually does vary significantly in Earth materials.
The new findings are in line with recent findings in other high-mass isotope systems - such as thallium or mercury - that had been assumed to be invariant. Additionally, the new measurements "could represent the first evidence of the nuclear field shift found in nature," said U. of I. graduate student Charles J. Bopp, who led the study.
What, exactly, causes the variance is not yet clear, though, Bopp said.
There are two basic types of uranium ore deposits: magmatic, which develop due to hydrothermal effects; and sedimentary, which develop by chemical reduction of uranium in groundwater in subsurface aquifers.
In 1976, scientists George Cowan and Hans Adler analyzed gas mass spectrometry results of uranium hexafluoride (before artificial isotopic enrichment processes took place) derived from uranium ores around the world. This assessment revealed a slight offset in the distribution of the ratio of U-235 to U-238, with magmatic-type deposits having on average higher U-235 percentage weight and sandstone-type deposits having lower.
However, the precision of individual analyses remained approximately 3 per mil (3 parts per thousand) while the average offset between deposit types was less than this.
With the higher precision now obtainable in the UI geochemistry laboratory, Bopp and UI geology professor Craig Lundstrom have observed the same offset between uranium ores from different geologic settings.
The researchers used a technique called multiple-collector inductively coupled plasma-mass spectrometry to measure the ratio of U-235 to U-238 in three sandstone-type and three magmatic-type uranium ores provided by the Smithsonian Institution.
"Repeated analysis of the ore samples shows the sandstone-type ores to be consistently depleted in U-235 relative to magmatic-type ores by approximately 1 per mil, which is a significant amount of variation," said Bopp, who will present the findings at next week's annual meeting of the Geologic Society of America.
The observed depletion of U-235 is most likely the result of a nuclear field shift effect as isotopes partition between the water and the reduced uranium ore mineral, Bopp said. But what uranium reduction process - biotic or abiotic - is responsible is not yet clear.
"We can't parse that apart at this stage," Bopp said. "We observe a depletion, and we know there are microbes present in these types of deposits, but we can't say for sure who's doing what without a much more in-depth study of a single locality."
University of Illinois at Urbana-Champaign
There are two basic types of uranium ore deposits: magmatic, which develop due to hydrothermal effects; and sedimentary, which develop by chemical reduction of uranium in groundwater in subsurface aquifers.
In 1976, scientists George Cowan and Hans Adler analyzed gas mass spectrometry results of uranium hexafluoride (before artificial isotopic enrichment processes took place) derived from uranium ores around the world. This assessment revealed a slight offset in the distribution of the ratio of U-235 to U-238, with magmatic-type deposits having on average higher U-235 percentage weight and sandstone-type deposits having lower.
However, the precision of individual analyses remained approximately 3 per mil (3 parts per thousand) while the average offset between deposit types was less than this.
With the higher precision now obtainable in the UI geochemistry laboratory, Bopp and UI geology professor Craig Lundstrom have observed the same offset between uranium ores from different geologic settings.
The researchers used a technique called multiple-collector inductively coupled plasma-mass spectrometry to measure the ratio of U-235 to U-238 in three sandstone-type and three magmatic-type uranium ores provided by the Smithsonian Institution.
"Repeated analysis of the ore samples shows the sandstone-type ores to be consistently depleted in U-235 relative to magmatic-type ores by approximately 1 per mil, which is a significant amount of variation," said Bopp, who will present the findings at next week's annual meeting of the Geologic Society of America.
The observed depletion of U-235 is most likely the result of a nuclear field shift effect as isotopes partition between the water and the reduced uranium ore mineral, Bopp said. But what uranium reduction process - biotic or abiotic - is responsible is not yet clear.
"We can't parse that apart at this stage," Bopp said. "We observe a depletion, and we know there are microbes present in these types of deposits, but we can't say for sure who's doing what without a much more in-depth study of a single locality."
University of Illinois at Urbana-Champaign
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Heavy metals accumulate more in some mushrooms than in others
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Report examines hidden costs of energy production and use
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Study in Spain and Romania confirms radon as second leading cause of lung cancer
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Superheavy Element 114 Confirmed: A Stepping Stone to the Island of Stability
Scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory have been able to confirm the production of the superheavy element 114, ten years after a group in Russia, at the Joint Institute for Nuclear Research in Dubna, first claimed to have made it.
Guide on lung cancer in 'never-smokers': A different disease and different treatments
A committee of scientists led by Johns Hopkins investigators has published a new guide to the biology, diagnosis and treatment of lung cancer in never-smokers, fortifying measures for what physicians have long known is a very different disease than in smokers.
MU researcher uses bacteria to make radioactive metals inert
The Lost Orphan Mine below the Grand Canyon hasn't produced uranium since the 1960s, but radioactive residue still contaminates the area.
Using waste to recover waste uranium
Using bacteria and inositol phosphate, a chemical analogue of a cheap waste material from plants, researchers at Birmingham University have recovered uranium from the polluted waters from uranium mines.
Hydrogen protects nuclear fuel in final storage
When Sweden's spent nuclear fuel is to be permanently stored, it will be protected by three different barriers. Even if all three barriers are damaged, the nuclear fuel will not dissolve into the groundwater, according to a new doctoral dissertation from Chalmers University of Technology in Sweden.
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