Electron Cyclotron Resonance (ECR) ion sources and the production of multiple-charge positive ions with high efficiency in combination with a heavy-ion accelerator have opened the way to an alternative and complementary version of accelerator mass spectrometry (AMS). A notable strength of positive-ion over traditional AMS is the capability of ultra-high sensitivity detection of radioactive isotopes of noble gases, in particular Ar-37 (t(1/2) =3D 35 d) and Ar-39 (269 y). The complete dissociation of molecular ions in the ECR and in particular of hydride ions of neighboring stable isotopes results in superior isotopic separation. However, the use of high charge states, necessary for acceleration to high energy, entails the existence of severe transmission degeneracies with stable ions having nearly equal mass-to-charge ratios, in addition to that of stable isobaric ions. Separation or discrimination of these parasitic ions require powerful and sophisticated dispersive systems at detection stage. We review here work performed and in progress at the ATLAS facility of Argonne National Laboratory (ANL) where an ECR ion source, a Radio-Frequency Quadrupole (RFQ), a superconducting linear accelerator and a Gas Filled Magnet (GFM) are used as an AMS setup.

Systematic investigations and experience from several application projects on small carbon samples over a number of years have resulted in measuring the radiocarbon content of 10 mu g C samples with an overall precision of typically 1%. A substantial reduction of the carbon contamination during graphitization was achieved, resulting in 31 +/- 30 ng modern and <100 ng C-14-free carbon. Thus, graphitization is no longer the limiting factor because earlier sample preparation steps usually introduce much larger contamination. The method has been extended to a variety of materials and applied to various projects. Realistic conditions for procedure development can only be achieved in the context of applications on true samples; methods developed are the lyophilization of samples in solution, combustion, ultraviolet oxidation, or carbonate hydrolysis with phosphoric acid, which allows to prepare samples for a wide range of applications. Insights gained from systematic investigations and from real applications are presented.

This paper summarizes the present knowledge on the variation of summer temperatures in the European Alps throughout the Holocene by combining the results of an extraordinary archaeological find with the information gathered from glacier and tree-line movements. As it turns out, there were several distinct periods were the glaciers were smaller than today, allowing in some periods the growth of trees in areas, which even now are still covered with ice. On average, the first half of the Holocene was warmer than the second half, with temperatures starting to decrease around the time of the Iceman some 5000 yr ago. One of the coldest periods during the Holocene, the so-called Little Ice Age (LIA), lasted from about AD 1300 to 1850. It is well known that since then the Alpine glaciers have been receding, most likely amplified by anthropogenic impact. The study of temperature variations before human influence may help to eventually disentangle natural and anthropogenic causes for the global warming of our time.

Accelerator mass spectrometry (AMS) is sometimes called 'the art of counting atoms one by one'. In addition to counting individual atoms, AMS is also capable to determine both mass number (A) and atomic number (Z). Since an atom (also called a nuclide) is unambiguously characterized by the proton number (Z) and the neutron number (N = A-Z), a rare nuclide can be well separated from possible background events, and extremely low abundances of specific nuclides can be measured. In general, the use of an accelerator system as a mass spectrometer improves the isotope abundance sensitivity by many orders of magnitude as compared to standard mass spectrometry (without an accelerator). In particular, AMS provides the means to measure minute traces of long-lived radioisotopes of cosmogenic and/or anthropogenic origin in essentially every domain of our environment at large. This allows one to use AMS for performing research in many different areas, ranging from archaeology to astrophysics. In this review, an update of the current status of AMS and an outlook to future developments in both technical and applied aspects will be presented.

Recent progress with compact ionization chambers has opened new possibilities for isobar suppression in accelerator mass spectrometry (AMS). Separation of Cl-36 (t(1/2) = 0.30 Ma) at natural isotopic levels from its stable isobar S-36 became feasible at particle energies of 24 MeV, which are also accessible for medium-sized tandem accelerators with 3 MV terminal voltage like VERA (Vienna Environmental Research Accelerator). Investigations with an ionization chamber revealed how physics favors isobar separation even at energies below the maximum of the Bragg curve. The strong energy focusing effect at high energy losses reduces energy straggling significantly and isobar separation steadily increases up to almost full energy loss. With an optimized detection setup, sulfur suppression factors of 2 x 10(4) have been achieved. Refraining from the additional use of degrader foils has the benefit of high transmission to the detector (similar to 16%), but requires a low sulfur output from the ion source. Therefore several backing materials have been screened for sulfur content. The dependence of the sulfur output on the AgCl sample size has been investigated as well. Precision and accuracy have been thoroughly assessed over the last two years. Since drifts in the spectra are efficiently corrected by monitoring the position of the S-36 peak, the reproducibility for high ratio samples (Cl-36/Cl > 10(-12)) is better than 2%. Our blank value of Cl-36/Cl approximate to (5 +/- 5) x 10(-16) is competitive to other labs. Cl-36 has become a routine AMS-isotope at VERA. Recently we also explored novel techniques for additional sulfur suppression already in the ion source. While results with a small gas reaction cell in front of the sputter target were discouraging, a decrease in the sulfur/chlorine ratio by one order of magnitude was achieved by directing 300 mW continuous wave laser beam at 445 nm towards the cathode in the ion source. (C) 2012 Elsevier B.V. All rights reserved.

Applications of accelerator mass spectrometry (AMS) evolved into multi-disciplinary research covering virtually every domain of our environment at large. The possibilities of applications are clearly coupled to technical developments of AMS, which will be covered by the accompanying article of H.-A. Synal. The present review therefore concentrates on describing AMS applications to the largest extent possible. Since the knowledge of the author on the many fields where AMS measurements are performed is, of course, limited, the selection of examples discussed in this review is somewhat biased. In order to compensate for this, a rather long list of references is presented, which should be consulted for a deeper understanding of the respective fields. The seven domains of our environment at large (atmosphere, biosphere, hydrosphere, cryosphere, lithosphere, cosmosphere, and technosphere) are being used as a guideline for the present review. (C) 2013 Elsevier B.V. All rights reserved.

Continuous turnover of neurons in the olfactory bulb is implicated in several key aspects of olfaction. There is a dramatic decline postnatally in the number of migratory neuroblasts en route to the olfactory bulb in humans, and it has been unclear to what extent the small number of neuroblasts at later stages contributes new neurons to the olfactory bulb. We have assessed the age of olfactory bulb neurons in humans by measuring the levels of nuclear bomb test-derived C-14 in genomic DNA. We report that C-14 concentrations correspond to the atmospheric levels at the time of birth of the individuals, establishing that there is very limited, if any, postnatal neurogenesis in the human olfactory bulb. This identifies a fundamental difference in the plasticity of the human brain compared to other mammals.

Accelerator mass spectrometry (AMS) of Cl-36 (t(1/2) =3D 0.30 Ma) at natural isotopic concentrations requires high particle energies for the separation from the stable isobar S-36 and was so far the exclusive domain of tandem accelerators with at least 5 MV terminal voltage. Using terminal foil stripping and a detection setup consisting of a split-anode ionization chamber and an additional energy signal from a silicon strip detector, a S-36 suppression of >10(4) at 3 MV terminal voltage was achieved. To further increase the S-36 suppression energy loss straggling in various counter gases (C4H10, Ar-CH4 and C4H10-Ar) and the effect of "energy focusing" below the maximum of the Bragg curve was investigated. The comparison of experimental data with simulations and published data yielded interesting insights into the physics underlying the detectors. Energy loss, energy straggling and angular scattering determine the S-36 suppression. In addition, we improved ion source conditions, target backing materials and the cathode design with respect to sulfur output and cross contamination. These changes allow higher currents during measurement (Cl-35(-) current approximate to 5 mu A) and also increased the reproducibility. An injector to detector efficiency for Cl-36 ions of 8% (16% stripping yield for the 7+ charge state in the accelerator, 50% Cl-36 detection efficiency) was achieved, which can favorably be compared to other facilities. The memory effect in our ion source was also thoroughly investigated. Currently our measured blank value is Cl-36/Cl approximate to 3 x 10(-15) when samples with a ratio of 10(-11) are used in the same sample wheel and Cl-36/Cl approximate to 5 x 10(-16) if measured together with samples with a ratio of 10(-12) or below. This is in good agreement with the lowest so far published isotope ratios around 5 x 10(-16) and demonstrates that 3 MV tandems can achieve the same sensitivity for 360 as larger machines. (C) 201 1 Elsevier B.V. All rights reserved.

A growing number of AMS laboratories are pursuing applications of actinides. We discuss the basic requirements of the AMS technique of heavy (i.e., above similar to 150 amu) isotopes, present the setup at the Vienna Environmental Research Accelerator (VERA) which is especially well suited for the isotope U-236, and give a comparison with other AMS facilities. Special emphasis will be put on elaborating the effective detection limits for environmental samples with respect to other mass spectrometric methods. At VERA, we have carried out measurements for radiation protection and environmental monitoring (U-236, Pu-239,Pu-240,Pu-241,Pu-242,Pu-244), astrophysics (Hf-182, U-236, Pu-244, Cm-247), nuclear physics, and a search for long-lived super-heavy elements (Z > 100). We are pursuing the environmental distribution of U-236, as a basis for geological applications of natural U-236. (C) 2009 Elsevier B.V. All rights reserved.

Cl-36 AMS measurements at natural isotopic concentrations have yet been performed only at tandem accelerators with 5 MV terminal voltage or beyond. We have developed a method to detect Cl-36 at natural terrestrial isotopic concentrations with a 3-MV system, operated above specifications at 3.5 MV. An effective separation was obtained with an optimized split-anode ionization chamber design (adopted from the ETH/PSI Zurich AMS group), providing a suppression factor of up to 30,000 for the interfering isobar S-36. Despite the good separation, a relatively high sulfur output from the ion source (S-36(-)/Cl-35(-) approximate to 4 x 10(-10) for samples prepared from chemically pure reagents), and a possibly cross contamination resulted in a background corresponding to Cl-36/Cl approximate to 3 x 10(-14). The method was applied to samples containing between 10(5) and 10(6) atoms Cl-36/g rock from sites in Italy and Iran, which were already investigated by other laboratories for surface exposure dating. The Cl-36/Cl ratios in the range from 2 x 10(-13) to 5 x 10(-12) show a generally good agreement with the previous results. These first measurements demonstrate that also 3-MV tandems, constituting the majority of dedicated AMS facilities, are capable of Cl-36 exposure dating, which is presently the domain of larger facilities. (C) 2009 Elsevier B.V. All rights reserved.