J. Beer

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.

Influence of Aspirin on Platelet Count and Volume in Humans

Platelet volume and total platelet mass are parameters of platelet activity and have become a prognostic factor in coronary heart disease. We have studied the influence of aspirin on platelet count, volume and total platelet mass in vitro and in a randomized double-blind, placebo-controlled study in 20 healthy young male volunteers in vivo. Aspirin had no influence in vitro on either platelet count, volume or mass. Repeated blood sampling during a 7-day treatment with 250 mg aspirin daily showed an increased platelet count (7.3% on day 1, 3.0% on day 2, 6.8% on day 4 and 9.3% on day 7; p < 0.01) and total platelet mass (7.2, 5.0, 8.6 and 11.5% on days 1, 2, 4 and 7, respectively, p < 0.01). Aspirin treatment without repeated blood withdrawal had no effect. These data indicate that aspirin may affect the circulating platelet mass under certain conditions.

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.

36Cl studies at the ETH/SIN-AMS facility

Abstract 36 Cl has been measured in polar ice from Dye 3 (65° 11 N, 43° 50 W) and Camp Century (77° 11 N, 82° 08W) in Greenland. In an exploratory study 4 series consisting of about 20 samples each have been selected covering approximately the following time periods: 12000-10000 BP (transition Wisconsin-Holocene), 1530–1730 a.d. (Maunder minimum), 1942–1977 a.d. (nuclear bomb pulse) and 1978 (seasonal variations). The results are compared with 10 Be data of the same cores which were measured previously. The variations of the 10 Be 36 Cl ratio are generally larger than expected from the experimental errors indicating that this ratio is not suitable for dating of old ice.

Detection of 32Si and 36Cl with the ETH/SIN EN-Tandem

Abstract Improvements made at the Zurich AMS-facility in the detecting technique of heavier cosmogenic radioisotopes are described. It is possible to perform now 36Cl measurements of natural samples with isotope ratios of greater than 10−14. The reproducibility of 36Cl measurements with a beam energy of 48 MeV is discussed. In the case of 32Si successful efforts were made to determine absolute isotope ratios of artificially enriched silicon samples.

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.

Precision Measurements of Rare Radioisotopes with a Tandem Van-de-Graaff Accelerator

The existing accelerator facility has been improved for accurate measurements of rare radioisotopes. So far the system has been applied for 14C dating and studies of 10Be in various geophysical samples. The properties of the accelerator based mass spectrometer have been studied extensively.

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.

10Be measurements on polar ice: Comparison of Arctic and Antarctic records

Abstract A new semicontinuous 10 Be record from Byrd station, Antarctica, is presented and compared with the Camp Century record from Greenland. Although the comparison of the Holocene parts of the two profiles is seriously hampered by lack of a reliable time scale for the Byrd core, there is evidence for common features. The establishment of the existence of global features in 10 Be records is of great importance for the interpretation of 10 Be concentrations in terms of production rate changes due to helio- and geomagnetic modulation. In addition 10 Be time marks could be used to synchronize the climatic records of the Northern and Southern hemispheres.