Long-lived cosmogenic radionuclides: Determination by accelerator mass spectrometry and model applications

Introduction: Long-lived radionuclides with half-lives of 0.1-16 Ma (Tab. 1) have nowadays thousands of exciting applications, especially within environmental and geosciences. In nature, the so-called cosmogenic nuclides (CNs) are products of nuclear reactions induced by primary and secondary cosmic rays. Hence, they can be found in extraterrestrial material such as meteorites - originating from the asteroid belt, the Moon or Mars - and lunar samples in higher concentrations (e.g. ~10^{10 10}Be atoms/g or < 0.5 mBq/g). A combination of several CNs is used to reconstruct the exposure history of this unique material while in space (irradiation age) and on Earth (terrestrial age).
Though, in terrestrial material the concentrations are typically only on the order of 10⁴-10⁹ atoms/g (i.e. μBq/g - nBq/g) for ¹⁰Be produced in the Earth’s atmosphere, so-called atmospheric or meteoric ¹⁰Be, transported to the surface and further absorbed and incorporated at and in, e.g. sediments or ice. Some of the lowest ¹⁰Be concentrations (~10³ atoms/g), produced in-situ by neutron- and muon-induced nuclear reactions from e.g. oxygen and silicon in quartz, can be found in samples taken from the Earth’s surface. The concentrations of atmospheric or in-situ produced CNs record information to reconstruct sudden geomorphological events such as volcanic eruptions, rock avalanches, tsunamis, meteor impacts, earthquakes and glacier movements. Additionally, glacier movements and data from ice cores give hints for the reconstruction of historic climate changes and providing information for the validation of climate model predicting future changes. Slower processes such as sedimentation, river incision and erosion rates can also be investigated and last but not least, indirect dating of bones as old as several Ma’s is possible.
Anthropogenic production by release from nuclear reprocessing, accidents and weapons testing led to increased levels of CNs in surface water and soil (¹²⁹I,…), ice (³⁶Cl,…) and of course, material from nuclear installations themselves (⁴¹Ca,…).
Some of the CNs can be further used as natural or artificial tracers to follow pathways in oceanography, to date and identify sources of groundwater, to perform retrospective dosimetry and to study aspects in radioecology, phytology, nutrition, toxicology and pharmacology.
Method: Today, the analytical method of choice for long-lived cosmogenic radionuclides – especially non-gamma-active ones - is accelerator mass spectrometry (AMS). In contrast to decay counting, AMS scientists do not wait for the disintegration of the radioactive nucleus. In fact, the not-yet-decayed radionuclides are identified more efficiently by mass spectrometry. The main advantage of using a high-energy accelerator for mass spectrometry is the nearly complete elimination of background and interfering signals, resulting from molecular ions and ions with similar masses e.g. isobars. Thus, AMS generally provides much lower detection limits in comparison to conventional mass spectrometry. Our DREAMS (DREsden AMS) system (Akhmadaliev et al., 2013) offers excellent measurement capabilities also for external users.

Table 1. Radionuclides measured by AMS at DREAMS.
Nuclide t1/2 [Ma] Nuclide ratios of samples [10^-12]
(machine blank level)
¹⁰Be 1.387 0.01-300 (5x10^-16) ¹⁰Be/⁹Be
²⁶Al 0.705 0.001-60 (8x10^-16) ²⁶Al/²⁷Al
³⁶Cl 0.301 0.007-700 (2x10^-16) ³⁶Cl/³⁵Cl
⁴¹Ca 0.104 0.02-9000 (8x10^-15) ⁴¹Ca/⁴⁰Ca
¹²⁹I 15.7 artificial samples (3x10^-14) ¹²⁹I/¹²⁷I
actinides under development

The benefits from using AMS are obvious and manifold: Smaller sample sizes, easier and faster sample preparation, higher sample throughput and the redundancy for radiochemistry laboratories are largely reducing costs. Lower detection limits widen applications to shorter and longer time-scales and to sample types that could never be investigated before. Nevertheless, basic but accurate radiochemical sample separation is an essential prerequisite for AMS measurements.
Model applications: Some of the first successful CN-projects performed at DREAMS had been:

• Dating of marine sediments (with ANU, ETH, TANDAR, TUM & VERA) by ¹⁰Be & ²⁶Al and search for supernova-origin ⁶⁰Fe (by AMS at ANU & TUM)
• Growth rates of deep-sea manganese nodules by ¹⁰Be and ²⁶Al (with Senckenberg)
• ⁴¹Ca-determination in water and concrete from a nuclear power plant by LSC and AMS (with VKTA)
• Reconstruction of meteorites’ history by ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca (with U Poznan & Bern, MPI Mainz,…).

Acknowledgments
Thanks to all brave DREAMS-users working with a newly installed AMS-facility and for help from colleagues at other AMS-facilities (ANSTO, ANU, ASTER, ETH, VERA…) with cross-measurements and setting-up the time-of-flight-system for future actinide measurements.

References
Akhmadaliev et al. (2013) Nucl. Instr. Meth. Phys. Res. B 294, 5-10.