Ultrasensitive determination of long-lived radionuclides by accelerator mass spectrometry for applications from the Earth Sciences and cosmochemistry

Ultrasensitive determination of long-lived radionuclides by accelerator mass spectrometry for applications from the Earth Sciences and cosmochemistry

Merchel, S.; Akhmadaliev, S.; Pavetich, S.; Renno, A. D.; Rugel, G.; Several Dreams-Users

Concentrations of long-lived radionuclides in man-made material e.g. from decommissioning nuclear installations or unintended release are generally high enough to be directly measured by conventional mass spectrometry, decay counting or radiochemical neutron activation analysis. However, often samples need to be radiochemically processed before measurements to enrich radionuclides, eliminate the matrix and disturbing isobars or nuclides of similar decay characteristics.
Though, if the same radionuclides are produced in terrestrial and extraterrestrial matter by cosmic-ray induced nuclear reactions, concentrations are only in very rare cases measurable by other analytical techniques than accelerator mass spectrometry (AMS) after radiochemical separation. Samples from geomorphology applications can contain radionuclides at the level of several ten thousand atoms per gram mineral e.g. quartz or calcite, thus, asking for processing of 100 g starting material.
Experimental - AMS
For accelerator mass spectrometry negative ions (molecules or elements) are extracted from samples containing long-lived radionuclides (t1/2 > 100 a) in a Cs-sputter ion source. By inserting these ions in a tandem accelerator, they gain MeV-energies. Then by passing through matter (gas or foil) at the positively charged terminal in the middle of the accelerator, the negative ions lose outer electrons and convert into multiple-positively charged ions being then further accelerated towards the exit. Effectively all molecules are destroyed by this stripping process.
Generally, AMS is measuring isotope ratios, i.e., stable isotopes are usually detected in Faraday-cups and radionuclides in ionization chambers. Such a set-up of two mass spectrometers in one, namely the first with negative ions of keV-energy, the second with high-energy positive ions of MeV-energy, and combined with several magnetic and electrostatic analyzers (Fig. 1), allows analyzing isotope ratios as low as 10-16, thus, providing the ultimate detection limit of all mass spectrometry methods.
Figure 1: AMS set-up at DREAMS [1-3].
If using isotopically-enriched carrier such as 35Cl, AMS can also be applied to simultaneously measure stable chlorine by isotope dilution, i.e. ID-AMS.
Very recently, a new AMS facility has been installed at the Ion Beam Centre of the Helmholtz-Zentrum Dresden-Rossendorf: DREsden AMS (DREAMS) [1-3], which is capable of measuring 10Be, 26Al, 36Cl, 41Ca, and 129I (t1/2 = 0.1-16 Ma) as low as 10-14-10-16 (radionuclide/stable nuclide).
In 2013, the DREAMS set-up will be extended for measurements of actinides, and in 2014 for stable elements. Expected detection limits for stable isotope ratios are not as good as for radionuclide AMS, but still some orders of magnitude better than for traditional dynamic secondary ion mass spectrometry (SIMS), i.e. 10-9-10-12.
DREAMS applications are wide-spread e.g. meteoritics, astrophysics, geomorphology, climate research, hydrogeology, resource technology and risk assessment of natural hazards like rock falls.
[1] S. Merchel et al., GIT Labor-Fachzeitschrift 56(2) (2012) 88-90.
[2] S. Akhmadaliev et al., Nucl. Instr. Meth. B (2012) in print, doi:10.1016/j.nimb.2012.01.053.
[3] http://www.dresden-ams.de (Nov. 2012).

Keywords: AMS; radionuclide; cosmogenic; cosmochemistry

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