H.R. von Gunten

10Be: Half-life and AMS-standards☆

Absolute AMS-measurements of 10Be require reliable standards for calibration. Among the existing standards, rather large differences have been observed. These differences were found partially to be due to the different half-life values which were assumed. Also for comparison of AMS-data with activity measurements, it is necessary to know the 10Be half-life as precisely as possible. Starting with 5 ml of the standardized ORNL-MASTER solution, a working solution with a well-defined 10Be content was prepared. Its specific activity was determined by liquid scintillation counting. This measurement yielded a new value of (1.51 ± 0.06) Ma for the 10 Be half-life, which is in good agreement with the previously reported values but is almost three times more accurate. Two independent dilution series produced new AMS-standards with 10Be/9Be-ratios of the order of 10−10 and 10−11. These standards were measured at the ETH/SIN-AMS-facility with high accuracy and are compared with other available 10 Be-standards.

Speciation of Radionuclides in the Environment

Methods for the determination of the speciation of radionuclides in aerosols, in aquatic solutions, in sediments, soils and rocks are reviewed. At present, most of the results about speciation are deduced from model calculations, model experiments, and separation of species (forms) of radionuclides, e.g., by sequential extraction procedures. Methods of direct determination of speciation of radionuclides (e.g., by laser induced spectroscopy) are in general not yet sensitive enough for a measurement of the very low concentrations of radionuclides in the environment. The methodological part of this paper is followed by a review of the very abundant literature about speciation of important radionuclides in the environment, i.e., in the atmosphere, hydrosphere and lithosphère. The review does not include the biosphere. Literature up to spring 1993 is included (with a few more recent additions).

Redox chemistry of iron and manganese minerals in river-recharged aquifers: A model interpretation of a column experiment

Abstract The transport of Fe and Mn observed in a laboratory column filled with sections of riverbed sediments and fine grained aquifer materials is modeled for steady-state conditions. The simulated processes include oxidation of organic matter by O 2 and NO - 3 in the riverbed sediment section, reductive dissolution of Fe and Mn (hydr) oxides, and precipitation of dissolved Fe and Mn, mainly as amorphous FeS. Local equilibrium for aqueous species is assumed. Kinetic reactions are used in the calculations for the oxidation of organic matter and for dissolution and precipitation of minerals. The observed concentration profiles of H + , NO - 3 , SO 2- 4 , and dissolved Fe and Mn in the laboratory column are reasonably explained by the model. The most sensitive model parameters are the rate constant for decomposition of organic matter and those for the dissolution of Fe and Mn (hydr) oxides. The results of a sensitivity analysis suggest that the decomposition rate of organic matter is responsible for the observed seasonal variations in concentrations of dissolved Fe and Mn in river-recharged aquifers, such as at Glattfelden in Switzerland. The observed reactions are mediated by bacteria.

Light scattering characterization of laporite sols

Abstract The structure and dynamics of dilute Laponite RD sols were studied by static and dynamic light scattering. From the scattered light intensities the particle molecular weight was derived to be 3000 ± 500 kg/mol. The static structure factor S depended very slightly on the scattering angle, but decreased rapidly with Laponite concentration in dilute sols, indicating long-range particle interactions. Collective diffusion coefficients D c were determined by slope analysis of the autocorrelation functions. A combination of light scattering data with membrane filtration and electron microscopy suggest the existence of tactoids containing between two and four Laponite platelets with an average diameter of 30 ± 10nm. Variations of the electrolyte content (0 ⩽ [KCl] ⩽ 0.0025 M ) and of the Laponite concentration (0.025 ⩽ c ⩽ 5.0 g/liter) of the sols allowed a systematic evaluation of the collective friction. Hard-body and electrostatic contributions to friction were determined and showed that electrostatic friction retards collective diffusion significantly.

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

Abstract210Pb- and137Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with210Pb/210Po and revealed sediment accumulation rates of (0.055±0.015) g·cm−2·y−1 and (0.09±0.03) g·cm−2·y−1, respectively. A comparable rate of (0.07±0.01) g·cm−2·y−1 has been obtained from137Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm−2·y−1. Constant210Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct137Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of7Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant210Pb activities. Remobilization may also be the reason for an incomplete210Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the210Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.ZusammenfassungSedimente aus dem Zürichsee wurden mit Hilfe von drei unabhängigen Methoden datiert. Für210Pb ergaben sich Sedimentationsraten von 0.055 g·cm−2y−1bzw. 0.09 g·cm−2y−1. Die137Cs-Datierung und die Auszählung von Jahreslagen (Varven) ergab 0.07 g·cm−2y−1. Ähnlich wie in anderen Seen wurden auch im Zürichsee in den obersten 6 cm aller untersuchten Kerne konstante210Pb-Aktivitäten festgestellt. Dies wird in der Literatur in der Regel auf Bioturbation oder auf sekundäre Durchmischung der stark wassergesättigten Sedimente bei der Probennahme zurückgeführt. Im Zürichsee widersprechen die Erhaltung von millimetermächtigen Jahreslagen, das Auftreten eines ausgeprägten137Cs-Maximums und ganzjährig anoxische Bedingungen an der Sediment/Wassergrenzfläche solchen Deutungsversuchen. Auch das Vorhandensein des kurzlebigen Isotopes7Be (HWZ 53 Tage) in den obersten Sedimentschichten beweist eine komplette und ungestörte Probennahme. Für die beobachtete konstante210Pb-Aktivität werden daher Remobilisationsprozesse nach der Ablagerung der Sedimente verantwortlich gemacht. Nachträgliche Remobilisierung könnte auch die Ursache für das beobachtete unvollständige-210Pb-Inventar in den Sedimenten sien, das nur ca. 50% des atmosphärischen Ausfalls beträgt. Die Untersuchungen zeigen, dass die Ergebnisse der210Pb-Datierung in diesem eutrophen See mit Vorsicht interprtiert werden müssen und dass die Auszählung von Jahreslagen—falls vorhanden-die am wenigsten aufwendige und genaueste Methode zur Datierung von Seesedimenten darstellt.

A new determination of the half-life of 32Si

Abstract The half-life of Si has been redetermined. A value of 133±9 yr was obtained from different Si-samples produced by proton activation of 37 Cl (as LiCl) and neutron activation of 31 P (as Mg 2 P 2 O 7 ). After chemical separation, purification and conversion to K 2 SiF 6 , the specific activity of 32 Si was measured by liquid scintillation counting. Additional chemical treatments were used to test the radiochemical purity of the samples. The 32 Si/Si tot ratio was measured by accelerator mass spectrometry (AMS). To determine and correct for the mass fractionation in the tandem accelerator system, the four silicon isotopes 28 Si, 29 Si, 30 Si and 32 Si were measured quasi-simultaneously under as identical conditions as possible. Special care was taken to estimate the contributing uncertainties for the determinations of both the specific activity and the isotopic ratio.

Sedimentation rates in the central lake constance determined with210Pb and137Cs

Sediment cores from central Lake Constance were dated with210Pb and137Cs. A sedimentation rate of (0.11±0.02) g·cm−2·y−1 was determined with the210Pb method.137Cs measurements revealed sedimentation rates of (0.11±0.01) g·cm−2·y−1 and (0.08±0.01) g·cm−2·y−1 respectively for two different cores sampled at the same location. The lower Cs-dated value indicates incomplete core recovery and demonstrates the sensitivity of this simple dating method to small losses of material at the water/sediment interface. An unambiguous application of the137Cs method is, therefore, only possible if complete core recovery is ensured. Sedimentation rates based on particulate matter, collected in sediment traps at various water depths, agree with the results of the radioisotope methods. Estimates of 30–125 days residence times for suspended particulate matter were calculated from7Be measurements.

Determination of chlorine in stony meteorites by neutron activation analysis

Abstract The chlorine content of stony meteorites was measured by 38 Cl produced by neutron activation. Nine hypersthene chondrites showed chlorine abundances from 27 to 266 ppm with a mean of 114 ppm; two enstatite chondrites contained 222 and 500 ppm, a Soko-Banja type chondrite 57 ppm, and a bronzite chondrite 97 ppm Cl; 2 and 5 ppm Cl were measured in two samples of the enstatite chondrite Norton County. Hot water leaches applied to the irradiated samples, showed for most of the samples that the leachable chlorine was in the order of a few per cent of the chlorine content of the meteorite. An exception of this result was found in the case of the hypersthene chondrite Dhurmsala where about 80 per cent of the chlorine was leachable.

High intensity isobar separation at the Zürich AMS facility

Abstract A dedicated gas-filled magnet has been installed on the Zurich AMS beam line. It was designed with special emphasis on isobar suppression for 32Si and 36Cl AMS. Calculations and Monte Carlo simulations have been performed to estimate separation, profile and suppression of the isobaric beams through the gas-filled magnet. The calculations are in good agreement with measurement on the new separator. For 32Si a suppression of 32S of 11 orders of magnitude was obtained. This should make it possible to measure 32Si in natural samples.

Determination of Trace Elements in the River Aare (Switzerland) by Neutron Activation Techniques

Abstract More than 20 trace and minor elements were determined in water samples and in the suspended load of the river Aare (Switzerland) using a simple sample preparation technique and subsequent instrumental neutron activation analysis. The water samples were irradiated without preconcentration. The interfering activity of 24Na was eliminated with hydrated antimony pentoxide. The trace elements were co-precipitated as sulfides and hydroxides and were analyzed on Ge(Li)gamma-ray spectrometers. Particulate matter was filtered off in the field, irradiated and assayed by γ-ray spectroscopy. Main results: (a) the concentrations of all the elements investigated are much lower than WHO-limits for drinking water; (b) human activities are responsible for an increase of some elements along the river: (c) concentrations of dissolved trace elements are anticorrelated to the amount of water; (d) concentrations of trace elements in particulates (μg/l) are correlated to the amount of water: (e) significant correlation...