Accelerator mass spectrometry in environmental geoscience. A review

Abstract

Using accelerator mass spectrometry (AMS), ratios as low as 10^–14 can be measured for rare compared with abundant isotopes of an element. Attaining such an extreme ratio can be achieved through the elimination of interfering molecular ions by high energy dissociation, avoidance of isobaric interference through negative ion discrimination, and reduction of detector background by high-energy ion counting. Carbon-14 is the best known nuclide routinely determined by AMS, because samples with as little as 10⁶ atoms of ¹⁴C (less than 1 mg of modern carbon) can be measured more rapidly and precisely than is possible by the beta-counting method. AMS can be applied to rare stable nuclides, but most of the applications have been with rare radionuclides the half-lives of which are so long as to preclude decay counting: ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl and ¹²⁹I belong to this group, whose half-lives range from 5000 to 16 000 000 years, and which are all produced naturally as well as anthropogenically. Chlorine-36 and iodine-129 have both been used in groundwater tracing studies, and ¹²⁹I is becoming a more widely applied radionuclide in this respect; one reason being its association with nuclear fuel reprocessing facilities. A radioactively benign nuclide, because of its long half-life, ¹²⁹I has been released in relatively large amounts into the atmospheric and oceanic environments, where backgrounds have risen several orders of magnitude over pre-bomb levels. The point sources (e.g., Sellafield, Cap la Hague) produce plumes in the North Sea, Barents Sea and Western Atlantic from which transport models can be built. Atomospheric dispersion of ¹²⁹I from Sellafield over the Lake District has been documented through analysing mosses down-wind from the plant. The immobilization of iodine over the limestone bedrock in the northeast of the region can be seen dramatically in the ¹²⁹I moss data. Beryllium-10 and aluminium-26, produced continuously by cosmic-ray spallation in the atmosphere, are delivered to the surface by precipitation. Their accumulation in ice cores yields paleo-cosmic-ray flux data, and their presence in soils may allow estimates of creep rates to be made. Additionally, ¹⁰Be and ²⁶Al are produced in surface rocks by in situ cosmic-ray bombardment, and this leads to a method of estimating exposure or erosion ages of landforms. There are more radionuclides in the environment, about which little is known because of their rarity. One by one, AMS is uncovering their secrets and opening up new frontiers in environmental geoscience.