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| Titel |
Quality assurance and 36Cl program at SUERC: Implications to landscape evolution research |
| VerfasserIn |
Klaus Wilcken, Stewart Freeman, Richard Phillips, Christoph Schnabel, Steven Binnie, Andrew Dougans, Tibor Dunai, Patience Cowie, Gerald Roberts |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042902
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| Zusammenfassung |
| In situ-produced cosmogenic nuclei, made by cosmic ray induced nuclear reactions
cumulatively on exposed surfaces, are natural chronometers and valuable tools for
environmental and geological research. Cosmogenic 36Cl (t1-2=3e5 yr) is dominantly
produced in spallation reactions on Ca and K, and via neutron capture on 35Cl, and hence is
applicable to a range of lithologies for studying events within the last 1 Myr or so. The
different 36Cl production mechanisms result in versatility but also challenging data
interpretation when unravelling the measured 36Cl concentrations.
The main difficulty in utilising 36Cl for environmental and geological research arises
from the stable isobar 36S. However, if high enough ion energies are available, these two
isotopes can be separated based upon their different rate of energy loss in matter. This has
typically required large (10-15 MV) legacy nuclear physics particle accelerators but recently
it has been shown that sufficient separation can be achieved with much lower ion energies
than before (~30 MeV); the detector resolution being improved by using uniform thin (~30
nm) Silicon rich Nitride membranes as a detector window to minimise energy losses and peak
broadening.
As a consequence, measurements can now be done with 5 MV, or even smaller, modern
accelerator mass spectrometers utilising gas stripping to produce the highest possible quality
beams. Accordingly a new class of commercial purpose-build 5-6 MV 36Cl-capable
spectrometers is being deployed around the globe with additional measurement capacity
greater than that of the installed base. This should increase accessibility and promote wider
and more varied 36Cl use. However, laborious sample preparation chemistry and production
rate uncertainties remain difficulties.
An example 36Cl programme utilising the 5 MV accelerator mass spectrometer at
SUERC will be presented. Our internal quality assurance program shows that no external
uncertainty beyond 3% counting statistics is observed and Purdue PRIMELab Z93-0005
(nominally 1.20e-12 36Cl/Cl) AMS primary normalization standard is long-term consistent
with K. Nishiizumi-provided secondary standard (nominally 5.0e-13 36Cl/Cl) used for
monitoring. In addition to analytical quality of the analysis, monitored with the fore
mentioned standards, we present results from an internal chemical standard, used to monitor
sample preparation, as well as example data from a recent study of carbonate bedrock
exhumation via active faulting. |
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