**Dating of water with Atom Trap Trace Analysis (ATTA) of **^{39}Ar

^{39}Ar

In hydrology the age of water, i.e. the time between last equilibrium with atmosphere
and the sampling of the water, is of great interest. With ocean circulation, climate
reconstruction and groundwater management just a few possible applications are
named. By measuring the concentration of a radioactive isotope, such as ^{14}C, the
age of a water sample can be determined. The dating range of a radioactive isotope
is given by its half-life (see figure 1). For water that is between 50 and 1000 years
old, there is a dating gap for which only one radioisotope is available: ^{39}Ar.

##### Fig. 1: Dating ranges for radioactive isotopes commonly employed for dating water (left) defined by the corresponding half-lives (right)

^{39}Ar is an ideal tracer because it is of cosmogenic origin with negligible anthropogenic
contribution and had a constant atmospheric concentration over the last
1000 years. Moreover, as a noble gas it is conservative and not involved in geochemical
processes, which e.g. complicates the dating with ^{14}C. However, the routine
dating with ^{39}Ar in environmental sciences is hampered by its extremely low atmospheric
abundance of ^{39}Ar/Ar ~ 10^{-15} corresponding to merely ~ 8000 ^{39}Ar atoms in
a liter of modern water. So far, it has been accessible only by Low-Level Counting
(LLC) in the underground laboratory in Bern requiring a sample size of several tons
of water and a measuring time of 8-60 days. The latter requirement mainly limits
the applicability of ^{39}Ar dating with LLC to groundwaters, where degassing several
tons of water from a well is typically feasible.

##### Fig. 2: Picture of the ^{39}Ar-ATTA setup in Heidelberg.

Atom Trap Trace Analysis (ATTA) is an atom optical technique that has been developed
for rare krypton isotopes. It is now available for routine analysis and several
environmental studies based on radiometric krypton dating with ATTA have been
conducted. The method makes use of the high selectivity of resonant photon scattering
involved in laser cooling in order to distinguish the desired rare isotope from the
abundant isotopes. Since it is not bound to radioactivity, it allows for a fast analysis
while requiring only comparatively small sample sizes independent of the isotope’s
half-life. In the course of this project a setup for ATTA of ^{39}Ar (figure 2) has been
developed in our collaborating group of Prof. Markus Oberthaler at the Kirchhoff-Institute for physics (www.kip.uni-heidelberg.de/matterwaveoptics/research/atta/).

For ^{39}Ar-analysis of groundwater samples with ATTA, the dissolved argon gas
needs to be extracted from the water. Our group has developed an instrument
for degassing several tons of water in the field (see figure 3) which is based on
membrane contactors that are permeable for the dissolved gases in the water.

##### Fig. 3: Impression of groundwater sampling with the degassing instruments.

The gas extracted from the water is compressed into 9 l-containers for easy transport
to the laboratory. There, the argon is seperated from the other gases with a
gas-chromatic setup (figure 4), that has been built in the course of the project.
Together with the ATTA group at the Kirchhoff-Institute we could for the first time
date groundwater samples with ATTA of ^{39}Ar [Ritterbusch et al., 2014]. After having dated large groundwater
samples, the next challenge is ^{39}Ar-dating in small water and ice samples
of about 1 kg, which requires the development of facilities for the extraction of
argon from small samples of water and ice with high purity and high extraction
efficiency.

##### Fig. 4: Schematic (left) and picture (right) of the setup for the extraction of argon.

**Publications:**

- Aeschbach-Hertig, W., 2014. Radiokrypton dating finally takes off. Proc. Natl. Acad. Sci. 111: 6856-6857.
- Ritterbusch, F., S. Ebser, J. Welte, T. Reichel, A. Kersting, R. Purtschert, W. Aeschbach-Hertig, M. K. Oberthaler, 2014. Groundwater dating with Atom Trap Trace Analysis of
^{39}Ar. Geophys. Res. Lett., in press. doi: 10.1002/2014GL061120. - Welte, J., F. Ritterbusch, I. Steinke, M. Henrich, W. Aeschbach-Hertig, and M. K. Oberthaler, 2010. Towards the realization of atom trap trace analysis for
^{39}Ar. New J. Phys. 12, 065031(14pp) - Welte, J., I. Steinke, M. Henrich, F. Ritterbusch, M. K. Oberthaler, W. Aeschbach-Hertig, W. H. Schwarz, and M. Trieloff, 2009. Hyperfine spectroscopy of the 1s5-2p9 transition of
^{39}Ar. Rev. Sci. Instrum. 80, 113109, doi:10.1063/1.3257691. - Reichel, T., 2013. Groundwater Degassing and Separation of Argon from Air for
^{39}Ar Dating with ATTA. PhD thesis, University of Heidelberg, 142 pp. pdf (55.7 MB) - Kersting, A., 2013. A new method of krypton purication for groundwater dating with atom trap trace analysis. Diploma thesis, University of Heidelberg, 76 pp. pdf (11.8 MB)
- Schwefel, R., 2012. Methoden zur Probenaufbereitung von Eis- und Grundwasserproben zur
^{39}Ar-Datierung mittels "atom trap trace analysis". Diploma thesis, University of Heidelberg, 107 pp. pdf (7.0 MB) - Kollefrath, A., 2011. Test einer Entgasungsanlage. Bachelor thesis, University of Heidelberg, 65 pp. pdf (5.4 MB)

**Recommended Links:**

ATTA-pioneers in Argonne: http://www.phy.anl.gov/mep/atta/research/atta.html

ATTA at Kirchhoff-Institute Heidelberg: www.kip.uni-heidelberg.de/matterwaveoptics/research/atta/

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