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| Titel |
Thoron (220Rn) in spring water |
| VerfasserIn |
S. Huxol, E. Höhn, H. Surbeck, R. Kipfer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021049
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| Zusammenfassung |
| Thoron (220Rn, half-life 55.6 s) is a shorter-lived isotope of the radioactive noble-gas radon
(the longer-lived isotope is 222Rn; half-life 3.6 d). Both radionuclides are part of a natural
radioactive decay chain, thoron from the 232Th and radon from the 238U series. They
can be found in soil-near air and soil-gas, and, in case of radon, its occurrence
in ground water is well known. We expected to find also thoron in groundwater.
But, as radon and thoron result from different decay chains, the geochemical and
geophysical behaviour of their precursors differs, too. The emanation of thoron out
of solid material that contains the thoron precursor 224Ra and the occurrence in
aquatic systems are not well known. To assess the thoron emanation, we formulated
two working hypotheses. The first one is based on the low solubility of the thoron
precursors in oxic ground waters: 232Th and its daughter nuclides will remain located at
almost the same positions in the crystal lattice as their precursors. In that case, the
thoron concentration in groundwater depends on the distribution of the precursors
in the aquifer material (“primary emanation”). The second hypothesis is based
on the enhanced mobility of the radium isotopes, the precursors of 220,222Rn, in
anoxic ground water of springs. If the anoxic spring water gets in contact with
oxygen, Ra tends to co-precipitate with Fe and Mn oxide/hydroxides and accumulates
at surface coatings. From the decay of 224Ra thoron emanates to the water phase
(“secondary emanation”). We measured radon and thoron with a Rad7 solid-state detector
coupled to a RadAQUA unit (closed gas loop, in contact with sprayed flowing water),
which allows continuous measurement of radon and thoron in water. In order to
test our working hypothesis, several springs containing oxygen were analysed -
none of them showed any detectable thoron. At an anoxic mineral spring with Fe
and Mn oxide/hydroxide precipitations at its outlet we have measured a thoron
concentration of around 0.2 |
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