Isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry

Accelerator mass spectrometry for measurement of long-lived radioisotopes

Particle accelerators, such as those built for research in nuclear physics, can also be used together with magnetic and electrostatic mass analyzers to measure rare isotopes at very low abundance ratios. All molecular ions can be eliminated when accelerated to energies of millions of electron volts. Some atomic isobars can be eliminated with the use of negative ions; others can be separated at high energies by measuring their rate of energy loss in a detector. The long-lived radioisotopes (10)Be, (14)C,(26)A1, 36Cl, and (129)1 can now be measured in small natural samples having isotopic abundances in the range 10(-12) to 10(- 5) and as few as 10(5) atoms. In the past few years, research applications of accelerator mass spectrometry have been concentrated in the earth sciences (climatology, cosmochemistry, environmental chemistry, geochronology, glaciology, hydrology, igneous petrogenesis, minerals exploration, sedimentology, and volcanology), in anthropology and archeology (radiocarbon dating), and in physics (searches for exotic particles and measurement of halflives). In addition, accelerator mass spectrometry may become an important tool for the materials and biological sciences.

Tandem-accelerator mass-spectrometry measurements of 36 C1, 129 I and osmium isotopes in diverse natural samples

Tandem AMS measurements at Rochester in the past few years have mainly involved the radioisotopes 36 Cl and 129 I, and some work on 10 Be, in a variety of terrestrial and extraterrestrial samples. Some m easurem ents have also been m ade on certain stable isotopes of osmium in meteorites and geological samples from impact craters. M easurem ents of 36 Cl have been made in groundwater and surface rocks for dating purposes, in ice and soil samples containing nuclear-weapon testing fallout for tracing water movement, and in meteorites and Antarctic ice for terrestrial and extraterrestrial meteoritic age determination. Also, 10 Be has been measured in a lake sediment, and 36 Cl in Greenland ice, through the period of the Maunder minimum ; 129 I has been measured in hydrological systems, in petroleum and in hydrothermal convection cells in the oceanic crust. Other applications involving measurements of these two radioisotopes include hydrotherm al fluids associated with gold mineralization and the determination of the integrity of possible sites for deep nuclear-waste disposal. Previously, the Rochester tandem was employed to measure 14 C and isotopes of platinum and iridium in natural samples.

Detection of a Meteoritic Component in Ivory Coast Tektites with Rhenium-Osmium Isotopes

Measurement of rhenium (Re) and osmium (Os) concentrations and Os isotopic compositions in Ivory Coast tektites (natural glasses with upper crustal compositions that are ejected great distances during meteorite impact) and rocks from the inferred source crater, Lake Bosumtwi, Ghana, show that these tektites incorporate about 0.6 percent of a meteoritic component. Analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals. The Re-Os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons. (i) The isotopic compositions of the target rocks and the meteoritic impactor are significantly different. (ii) Closed-system mixing of target rocks and meteorites is linear on Re-Os isochron diagrams, which thus permits identification of the loss of Re or Os. (iii) Osmium isotopic compositions are not likely to be altered during meteorite impact even if Re and Os are lost.

Ultrasensitive radioisotope, stable-isotope, and trace-element analysis in the biological sciences using tandem accelerator mass spectrometry

The new technique of tandem accelerator mass spectrometry (TAMS) has improved the sensitivity for measurement of several long-lived radioisotopes and certain stable isotopes by many orders of magnitude. Nuclear physics tandems and new small dedicated accelerators are now able to measure(14)C,(10)Be,(26)Al,(32)Si,(36)Cl,(41)Ca, and(129)I in natural materials. Sensitivities down to 10(5) atoms per sample can be achieved in favorable cases. By accelerating ions to MeV energies, one can eliminate molecules and uniquely identify the atomic numbers below 20. Although most applications to date have been in the earth sciences, the opportunity now exists for important new applications in biology and toxicology. Trace elements can be measured at the parts per billion (10(9)) level using a secondary ion mass spectrometry (SIMS) ion source. Radioactive tracer measurements can be made for elements, such as aluminum, for which there are no isotopes with suitable half-lives for conventional decay counting methods. For(14)C, counting times become much shorter and dose levels can be reduced.