M. Nessi

Precision measurements of 14C in AMS — some results and prospects

Abstract Some of the factors affecting the precision in AMS measurements will be discussed and the specific developments undertaken to reduce the errors at the ETH facility are described. Based on a large number of 14C measurements we show the present limitations of our system and consider what improvements might be possible. Further, a comparison is made between high precision measurements of 14C and current achievements relating to Be, Al and Cl.

The camp century 10Be record: Implications for long-term variations of the geomagnetic dipole moment

Abstract 10 Be concentrations measured in ice samples from Camp Century, Greenland, show short term variations which in general correspond to the 100–200 year “wiggles” in the 14 C tree ring record. There is, however, no evidence for a long term variation over the last 5000 years. This constancy is in contrast to the approximately sinusoidal variation of the atmospheric 14 C concentration which has generally been attributed to a changing geomagnetic dipole moment. This discrepancy implies that the 14 C trend might stem from other causes such as changes of oceanic circulation processes or from higher production rates during the Wisconsin rather than from variation in the geomagnetic field.

14C/12C-ratios in organic matter and hydrocarbons extracted from dated Lake sediments

Abstract Extractable organic matter (lipids) and hydrocarbons isolated from lake sediments were investigated by AMS by directly evaporating the extracts onto targets. Carbon-14 ages obtained for the postglacial sediments of Lake Baldegg (Switzerland) are in good agreement with results from pollen stratigraphy. The hydrocarbons contained in modern sediments of Lake Constance were found to be largely of fossil origin.

10Be annual fallout in rains in India

The 10Be concentrations of annual rainfall collections during 1979–1981, at eight stations in India, ranged from 0.43 × 107 to 8.48 × 107 atoms/l and the corresponding 10Be fallouts are in the range of 0.31 × 10 6 to 2.73 × 106 atoms cm−2 a−1. The estimated 10Be global fallout based on the presently available data is 1.55 × 107 atoms−2 a−1 or 5 × 10−2 atoms cm−2 s−1. Most of the measured rates of fallout and deep sea deposition of 10Be are a factor of 2–3 lower than the present estimate.

14C dating of polar ice

Abstract We have dated ice core samples by 14C AMS to show that this method extends other ice dating methods and enables direct comparison with 14C related climatic events found elsewhere. For the measurement we use the CO2 that was occluded together with the air at the time of ice formation. To extract the CO2 we crush the samples with a milling cutter. For a 14C AMS measurement the CO2 content of about 10 kg of cleaned ice (0.25 cm3) has to be converted to amorphous carbon. First 14C results on the Dye 3 (Greenland) ice core are in good agreement with other age determinations.

14C measurements on foraminifera of deep sea core V28-238 and their preliminary interpretation

Abstract In this paper first results obtained by AMS dating foraminifera are presented. The amount of material used for these studies was 7 to 10 mg calcium carbonate. A preliminary interpretation is given.

Spallogenic nuclides in meteorites by conventional and accelerator mass spectrometry

Light noble gases and 10Be were determined in 15 ordinary chondrites by conventional mass spectrometry and accelerator mass spectrometry, respectively. Aliquots of these samples had previously been studied for 26Al and 53Mn. Be was extracted from 10–20 mg of meteorite and 10Be was determined by means of the 6 MV tandem Van de Graaff accelerator at ETH-Zurich with a reproducibility better than 3%. A detection limit of 0.7 dpm/kg meteorite was achieved. Based on the noble gas data a 10Be production rate of 17.8 ±0.4 dpm/kg was established for chondrites with H-group chemistry. Short exposure ages of meteorites were determined via the 10 Be concentrations. A slight correlation of 10Be production rates with shielding could be observed. During the last 4 million years the long-term averaged cosmic ray intensity did not vary by more than 30%.

Optimizing the geometry of a negative ion sputter source

Abstract In order to avoid isotope fractionating effects due to slight fluctuations in the position, direction or divergence of the emerging beam, the emittance of an AMS ion source has to be lower than the acceptance of the following beam handling system. The emittance of a negative ion sputter source is determined by the energy spread of the sputtered ions and by its extraction geometry. For various configurations of the latter, we have calculated particle trajectories. Greatly reduced aberrations are attained if the electric field close to the emitting surface is nearly uniform and only weakly focussing and if the acceleration takes place in at least two lens sections. The single acceleration gap of our Hortig-type ion source [1] has therefore been replaced by a two-stage accelerating lens. Further, the curvature of the field at the surface of the sputter target is controlled by an additional electrode.

Target preparation for milligram sized 14C samples and data evaluation for AMS measurements

Abstract Our preparation technique produces in a glow-discharge an amorphous carbon deposit on a copper substrate. The process starts with 1.6 cm3 CO2 STP (900 μg carbon) which is reduced over hot zinc to CO and subsequently cracked in the discharge. The yield of the process is typically 80%. With these targets in the Zurich ion source ion currents up to 20 μA are obtained. The background of samples prepared with this technique is presently around 30 ka (2.5% MODERN). The precision after half an hour measuring time for a modern sample is 0.7% and 2.7% for a three half-lives old sample, including the errors of the background and the NBS oxalic acid measurement. The method we use to correct for the background of the preparation and the accelerator as well as for the fractionation in the accelerator is presented.

Charge state distributions and resulting isotopic fractionation effects of carbon and chlorine in the 1–7 MeV energy range

Abstract Equilibrium charge state distributions have been measured for 2–7 MeV 35Cl ions stripped in ca. 10 μg/cm2 carbon foils and for 2–7 MeV 12C ions stripped in ca. 880 ng/cm2 oxygen gas. In addition, for 4.95 MeV gas stripped 12C ions, the non-equilibrium distribution has been recorded for stripper thicknesses of 50–2500 ng/cm2. The results are compared with previous measurements. Isotopic fractionation versus energy is shown for some charge states of carbon, chlorine and beryllium.