M. di Tada

Accelerator mass spectrometry of plutonium isotopes

The feasibility of measuring plutonium isotope ratios by accelerator mass spectrometry has been demonstrated. Measurements on a test sample of known composition and on a blank showed that isotope ratios could be determined quantitatively, and that the present limit of detection by AMS is ∼ 106 atoms of plutonium. For 239Pu, this limit is at least two orders of magnitude lower than that practicable by alpha-spectrometry. In addition, 240Pu239Pu ratios were measured for four samples for which the combined activity of the two isotopes had been determined previously by alpha-counting. All measurements of plutonium isotope ratios entailed injection of PuO− into the 14UD accelerator operating at 3.5 MV, gas stripping, and analysis of the 7+ charge state after acceleration. Plutonium ions at 28 MeV were detected in a longitudinal-field ionisation chamber with an energy resolution of 3%. Using uranium oxide as a surrogate for plutonium oxide, it was shown that UO− was the predominant negative ion and that the probability for its formation and extraction was 0.3%.

Kinetics of uptake and elimination of silicic acid by a human subject: A novel application of 32Si and accelerator mass spectrometry

Silicon is possibly important in human physiology in protecting against the toxic effects of aluminium, but the kinetics of uptake and excretion of silicic acid, the bioavailable form, are not well characterised. We have used 32Si as a tracer in a human uptake experiment to determine a gastrointestinal uptake factor for silicic acid, and to elucidate the kinetics of renal elimination. Urine collections were made for extending intervals from 2 to 12 h over 2 days following ingestion by a single human subject of a neutral silicic acid solution containing tracer levels of 32Si (t1/2 approximately 150 y). Silicon was isolated as SiO2 and the 32Si content determined by accelerator mass spectrometry (AMS), using a gas-filled magnet technique to eliminate a prolific isobaric interference from 32S. Silicon uptake appears to have been essentially complete within 2 h of ingestion. Elimination occurred by two simultaneous first-order processes with half-lives of 2.7 and 11.3 h, representing around 90% and 10%, respectively, of the total output. The rapidly eliminated 32Si was probably retained in the extracellular fluid volume, whilst the slower component may represent intracellular uptake and release. Elimination of absorbed 32Si was essentially complete after 48 h and was equivalent to 36% of the ingested dose. This establishes only a lower limit for gastrointestinal absorption as, although there was no evidence for longer term retention of additional 32Si, the possibility could not be excluded by these results.

Radiocarbon dating of the human occupation of Australia prior to 40 ka BP; successes and pitfalls.

Charcoal samples from ancient human occupation sites in Australia have been subjected to a rigorous pretreatment and stepped combustion regime in order to explore the possibility that these sites may be older than previous radiocarbon dating had suggested. In one case, the Devils Lair site in southwest Australia, the methodology has clearly removed vestiges of contamination by more modern carbon and has led to a revised radiocarbon chronology that provides evidence for human occupation of southwest Australia by at least 44 ka BP and probably by 46-47 ka BP. In contrast, charcoal from the Nauwalabila site has been so severely altered that insufficient of the original carbon remains for reliable (super 14) C dating. Finally, where the charcoal is well preserved, such as at the Carpenters Gap site, the new results provide reassurance that earlier (super 14) C results of approximately 40 ka BP are indeed true ages and are not simply at the limit of the (super 14) C technique.

14C AMS dating of fires in the central Amazon rain forest

Abstract Soil samples were collected in tierra firme upland and lowland areas of the Km 41 reserve near Manaus (20°30′S and 60°W), in Central Brazilian Amazon, within a 1700 m transect, at eight different depth ranges, from surface to 100 cm. The highest charcoal concentrations were found at the depth range of 20–50 cm. AMS radiocarbon dating of 31 samples were performed at the ANU. The ages of the charcoals were found to vary within the 130 to 2400 years BP range, mostly between 1200 and 1400 years BP, one of the known Holocene dry periods of the Amazon region. The results show that the fires have regional dimensions and are associated with climate regional changes.

Accelerator mass spectrometry of 99Tc

Abstract The measurement of low levels of 99Tc (T 1/2 =213 ka ) by accelerator mass spectrometry (AMS) is reported for the first time. Because there are no stable isotopes of technetium, the 99Tc content of a sample is determined from the ratio of 99Tc to a known trace amount of 103Rh. High-purity aluminium oxide constitutes the bulk of each sample. The contribution from the 99Ru isobar may be quantified and subtracted using 101Ru. In contrast to conventional AMS in which at least one isotope is measured as a beam current, here all three isotopes, 99Tc, 101Ru and 103Rh, are counted ion-by-ion in a propane-filled ionization chamber. This ionization chamber permits partial, but not complete, separation of 99Tc from 99Ru ions. The technique has been validated using a series of standards, as well as a set of five seaweed samples from an inter-laboratory comparison exercise. In addition, the technique has been used to measure the relative intensities of the Tc− and TcOm− (m=1–4) negative ions from a Cs sputter ion source.

The AMS system and research program at the TANDAR laboratory

Abstract Several aspects of the AMS program at the TANDAR laboratory are discussed. The current activities are focused on environmental and cosmological applications. One of the projects has been the study of the global fallout of the radioisotope 36Cl and its latitudinal distribution in the southern hemisphere. We have also undertaken some of the preparatory work to measure the concentration of several radioisotopes in extraterrestrial samples obtained from a meteorite fall in northern Argentina. An important fraction of our effort in pursuing these studies has been the optimization of the technique in our 20 UD tandem. We discuss the results of recent tests carried out to evaluate the current capabilities of the accelerator regarding general stability, predictability and transmission efficiency, following the upgrading and adaptation activities.

The Brazilian Bragg curve detector built for AMS studies

A Bragg curve detector (BCD) has been designed and built to be used for AMS purposes. Its design is described and its performance, obtained in tests at the Australian National University (ANU), is reported. At a 36 Cl energy of 154 MeV, the energy resolution was found to be 0.38%, and the charge resolution 1.3%. The 36 Cl and 36 S are completely discriminated. Good separation was also achieved at 64 MeV, and a measurement at this energy of the 36 Cl/Cl ratio of a sample measured previously at 154 MeV with the standard ANU ionization detector agreed well with the earlier value. This very simple detector has been shown to be suitable for use in AMS programs. ” 2000 Elsevier Science B.V. All rights reserved.

The AMS program at the TANDAR accelerator

Abstract The accelerator mass spectrometry (AMS) program that is under development using the 20UD electrostatic accelerator TANDAR at Buenos Aires is presented. Tests and measurements in order to evaluate and to improve the accelerator performance are described. Preliminary measurements have been done by tuning 14C beams using as detection system a ΔE-ER telescope. For the detection of heavier isotopes some new developments are being currently implemented in a quadrupole-dipole-dipole (QDD) magnetic spectrometer. Furthermore, work is in progress to construct a time-of-flight system using two micro-channel plates prior to the entrance to the QDD spectrometer.

DEVELOPMENTS OF AMS AT THE TANDAR ACCELERATOR

Abstract Man-made long-lived radioisotopes have been produced as a result of different nuclear technologies. The study of accidental spillages and the determination of radioisotope concentrations in nuclear waste prior to final storage in a repository are subjects of great interest in connection with this activity. The accelerator mass spectrometry (AMS) technique is a powerful tool to measure long-lived isotopes at abundance ratios as low as 10−12–10−15 in small samples. Applications to the Argentine nuclear program like those mentioned above, as well as applications to archaeology, hydrology and biomedical research, are considered in an AMS program using the TANDAR 20 UD electrostatic accelerator at Buenos Aires. In this work we present the status of the program and a description of the facility.

Initial tests of the magnetic spectrometer at the TANDAR accelerator

Abstract A new magnetic spectrometer has started operating at the 20 UD tandem accelerator of the TANDAR laboratory. A large acceptance angle and the ability to handle a broad range of energies and masses are its main characteristics. The associated detection system consists of a multiwire vertical drift chamber, an ionization chamber and a plastic scintillator. The complete device was tested using the scattering of 12 C, 16 O, and 32 S on 197 Au, and 16 O beams on 120 Sn. A summary of the most important features of the system and the results of the tests are presented.