B.E. Lehmann

81Kr and85Kr in groundwater, Milk River aquifer, Alberta, Canada

Abstract The 85 Kr activity of well No. 9 is (2.1 ± 0.3) mBq/cm 3 STP ofKr or ≈0.3% of the modern atmospheric activity indicating that no young water component is present in this groundwater and that no contamination occurred in extracting a Kr gas sample for 81 Kr analysis. The 81 Kr concentration was measured by laser resonance ionization spectroscopy to be 82 ± 18% of the modern atmospheric concentration. Only 5000 atoms of 81 Kr were used in the final analytical step which represent ≈7% of the initial number present in a water sample of 50 1. An upper limit of 140 ka is calculated for the age of the water from a simple 81 Kr decay model. Of key importance for the future use of a 81 Kr dating technique is the conclusion that subsurface production of 81 Kr appears to be unimportant.

Resonance ionization spectroscopy and the detection of 81Kr

Abstract Development of resonance ionization spectroscopy (RIS) has made the concept of single-atom counting, whether radioactive or stable, practical for most elements of the periodic table. Tunable narrow-band lasers are used to efficiently and selectively excite and ionize a chosen element without interference from the much more abundant background, thereby achieving sensitivity at the level of a few atoms. Applications of the selectivity and sensitivity of RIS are numerous, including the direct analysis of solid, chemically processed liquid and noble gas samples. For example, 81 Kr, a 2.1 × 10 5 year half-life cosmogenic radioisotope was detected with less than 400 81 Kr atoms in the RIS analysis system.

Underground production of radionuclides in the Milk River aquifer, Alberta, Canada

Abstract Most cosmogenic radionuclides that may be used to estimate groundwater ages in excess of 100 ka are also produced underground by in situ nuclear reactions. Radioactive equilibrium between in situ production and decay would be approached within such long residence times. Radionuclide production by in situ nuclear reactions within the groundwater and within its host rock has been calculated theoretically and compared with some measured radionuclide contents of the groundwater. The fractional transfer of in situ produced 37 Ar, 39 Ar and 222 Rn from rock to groundwater necessary to account for the total measured groundwater contents were found to be similar, although site specific. The implications for radionuclide release mechanisms and for groundwater dating by the use of cosmogenic radionuclides, including those which are applicable for very old groundwaters ( 36 Cl, 81 Kr, 129 I), are discussed.

Development of an atom buncher

An ‘‘atom buncher’’ for controlling the concentration of gaseous samples has been conceptualized, evaluated theoretically, fabricated, and tested with excellent results. In effect, the atom buncher greatly increases the probability that a free atom will be in a small detector volume at a desired time. This was accomplished by using cryogenic techniques to condense atoms on a small spot and a pulsed laser to momentarily heat the spot to release the atoms at the desired time. Our work on noble gas atom counting by using resonance ionization spectroscopy is discussed as one example of the applications of the atom buncher.

Tunable VUV light generation for the low-level resonant ionization detection of krypton

High-power tunable VUV light pulses with energies up to 0.7 μJ were generated in the 115.7–116.9-nm region by use of a two-photon resonant four-wave mixing scheme in a Xe–Ar gas mixture. This is the highest reported pulse energy that has been produced in this wavelength region using a four-wave mixing process. Efficient detection of krypton isotopes at densities as low as 10 atoms/cm3 was demonstrated by resonantly ionizing the atom through its one-photon allowed state at the vacuum wavelength of 116.49 nm.

Application of isotopic methods to dating of very old groundwaters: Milk River aquifer, Alberta, Canada

The aim of this joint project was to evaluate the usefulness of available geochemical and isotope techniques for dating very old groundwater. This paper represents a synthesis of an IAEA sponsored study for which purpose the Milk River aquifer groundwaters were sampled from 16 wells during 1985 and subsequent years for the following measurements:2H,3H,13C,18O,14C,39Ar,81Kr,85Kr, noble gases including222Rn,36Cl,129I and U isotopes. The Milk River aquifer was selected for this study because several preceding investigations had established that this groundwater system contains waters whose ages range from recent to 1Ma. The present study has established that the Milk River aquifer system is very complex both in terms of groundwater origin and in terms of the evolution of its chemical and isotopic contents. However, this apparent complexity proved to be an interesting challenge with respect to the use of different and complementary approaches to interpretation of the geochemical and isotope data in terms of groundwater residence time. Thus, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater age in the Milk River aquifer (∼0.25Ma) while those based on the36Cl/Cl data uncorrected for any dilution by dead Cl should be considered as an upper limit (<2Ma). The Milk River Aquifer International Project is an excellent illustration of the variety of insight and increased confidence possible in flow-system analysis when multiple, independent dating methods are combined with detailed hydrogeological studies. The conclusion of the present study is that the potential for dating of very old groundwaters by a variety of isotope techniques is very high, provided a combination of methods is applied (never a simple method by itself).

Radioisotope dynamics — the origin and fate of nuclides in groundwater

The use of radionuclides as clocks for groundwater dating and as probes to investigate the geometry and spatial extent of the contact area between rocks and water is reviewed. Subsurface production rates for 222 Rn, 37 Ar, 55 Kr, 39 Ar, 36 Cl, 3 He, 4 He and 40 Ar in various rock types are listed. Measured Rn fluxes from the surface of sandstone grains and from pieces of granite point to scale-dependent diffusion coefficients. The temporal evolution of subsurface-produced 222 Rn-, 37 Ar-, 85 Kr- and 39 Ar-activities in groundwaters yields radionuclide escape factors between 0.1% and 9% for the Stripa granite (Sweden) and between 1% and 4% for the Milk River sandstone (Canada). The combination of 3 H, 85 Kr, 39 Ar, 14 C, 36 Cl, 4 He in the UK Triassic sandstone aquifer allows groundwater dating up to 40 000 a. Very old groundwaters can be studied using Cl, 36 Cl and 4 He evolution as demonstrated in the Milk River aquifer in Canada.

An isotope separator for small noble gas samples

Abstract A Wien filter isotope enrichment system has been combined with a small turbomolecular pump to form a closed isotope separator for small noble gas samples. Atoms which leave the exit aperture of the plasma discharge ion source without being ionized are circulated back into the source through a feedback line. The system can be operated for several hours in a closed mode to collect up to 50% of the total number of atoms of a selected isotope (e.g. 81 Kr) out of a small gas sample of only 2 × 10 −3 cm 3 STP. Ions are implanted at 10 kV into an aluminized Kapton foil after a flight distance of 150 cm. A beam stabilization system centers the ion beam in two perpendicular directions onto a target aperture to maintain a high enrichment factor of at least 10 3 over extended periods of time. Calibration of the enrichment process is achieved by isotope dilution. The system is a key part of the sample processing for 81 Kr and 85 Kr analysis by laser resonance ionization spectroscopy for applications in isotope geophysics.

Analysis of 81Kr in groundwater using laser resonance ionization spectroscopy

A new analytical technique based on resonant ionization of krypton with a vacuum ultraviolet (VUV) laser source was used to determine low-level /sup 81/Kr concentrations in groundwater. The long half-life (210,000 years) and low concentration (1.3 x 10/sup 3/ /sup 81/Kr atoms per liter of modern water at 10/sup 0/C) make the detection of /sup 81/Kr by radioactive counting techniques extremely difficult. In this method, krypton gas was removed from water taken from an underground Swiss aquifer using standard cryogenic and chromatographic techniques. Stable krypton isotopes were then reduced by a factor of 10/sup 7/ by a two-stage isotopic enrichment cycle using a commercially available mass spectrometer. The enriched gas containing about 10/sup 8/ stable krypton atoms and about 10/sup 3/ atoms of /sup 81/Kr was implanted into a silicon disc. This disc was then placed in the high vacuum final counting chamber and the krypton was released by laser annealing. This chamber contained a quadrupole mass spectrometer which used a pulsed VUV laser source as the ionizer. The measured signal indicated that the sample contained 1200 (+-300) atoms of /sup 81/Kr.

81Kr detection using resonance ionization spectroscopy

A resonance ionization ion source, in conjunction with a mass spectrometer, was used to count 1000 individual 81Kr atoms. In this method, a vacuum ultraviolet beam was used to resonantly excite krypton atoms in order to obtain atomic species selectivity and a small quadrupole mass spectrometer provided isotopic selectivity. After selection, the krypton ions were counted as they were implanted into a target. The method will also work for other isotopes.