Graham C. Wilson

AMS of heavy ions with small accelerators

Abstract Recent advances in the detection and the routine measurements of heavy elements by accelerator mass spectrometry (AMS) are reviewed. Particular emphasis will be given to the measurement of low energy (⩽ 15 MeV) and high-Z ions using small (⩽ 3 MV) accelerators.

Charge ratio mass spectrometry of heavy elements

Abstract A simple method of stable element assay at the parts per billion level using a sequence of charge changes at MeV ion energies has been demonstrated. Isotopic ratios and abundances of platinum have been studied using this technique in nickel sulphide and copper samples. Silver isotopic distributions have also been investigated in a number of terrestrial and meteoritic materials. A similar charge changing sequence between electrostatic analyzers at keV ion energies was shown to reduce backgrounds for a mass independent search of fractionally charged particles. The methods described form the bases of a broad band mass spectroscopic system under development at IsoTrace.

In situ trace element determination by AMS

Abstract Because of its negligible background, AMS can be used for the direct measurement of trace element concentrations in materials at the lowest possible levels. The primary beam can be made small enough to be positioned within the boundaries of individual mineral grains in a polished rock section. Standards can be prepared which relate well to the complex natural matrices found in minerals, so that concentrations down to the ppb level can be measured. Enrichments and depletions of platinum group elements (PGE) and Au have been found in minerals such as millerite, arsenopyrite, pyrite, etc. Sensitivities vary widely because of the different ion yields for different elements. Sample surfaces should be electrically conducting to avoid charging and beam instability.

In situ analysis of precious metals in polished mineral samples and sulphido “standards” by Accelerator Mass Spectrometry at concentrations of parts-per-billion

Abstract This article addresses the analysis of eight precious metals (Au and Ag, plus the six Platinum Group Elements (PGE): Pt, Ir, Os, Ru, Rh, and Pd). The metals are measured in Fe-Ni-(Cu) sulphidos by Accelerator Mass Spectrometry (AMS). Samples are analysed in the form of small polished cores, derived from sulphido- and oxide-rich rocks, and from Ni-S-dominated fire assay beads. The chosen analytical strategies are shown to have essentially zero background for Au, Ag, and the PGE, with the notable result that detection limits, like precision, are limited only by the time available for analysis. Sensitivities for monatomic negative ions are in the order Au ≈ Ag > Pt ≈ Rh > Pd ≈ Ir > Ru > Os, with nominal detection limits (quoted on the basis of equal counting times of 100 sec) ranging from 0.1 ppb for Au to

Development of an ion microprobe stage for accelerator mass spectrometry

Abstract This report summarises the design and construction of an ion microprobe attachment under development for the accelerator mass spectrometry (AMS) laboratory at Toronto. The sample chamber and the stage assembly, which is movable in three dimensions, are described, and a synopsis of relevant ion-optical and light-optical (viewing) design work is appended.

Applications of PIXE to mineral characterization

Abstract This article illustrates the application of the proton-induced X-ray emission (PIXE) technique to detailed documentation of mineral assemblages, with emphasis on base–metal ores. Some of the investigations aided by the PIXE laboratory at Guelph since 1993 include determinations of the distribution of minor and trace elements in magmatic Ni–Cu ores, volcanogenic massive sulphide Cu–Pb–Zn–(Ag–Au) ores and lode Au–(Ag) deposits. Minor elements of importance include possible by-products or co-products of metal refining, as well as deleterious impurities in mill-feed, e.g. Cd, In, Sn, As, Se, Te, Tl and Hg. Weathering products of primary sulphide mineralization, including tropical laterites and other oxidized assemblages, have been analysed successfully and can contain a wide range of minor elements which reflect the bedrock style of mineralization. The iron oxyhydroxide goethite, α-FeO(OH), contains trace levels of many elements, and in some cases 1 wt.% or more of base metals and arsenic, elements which are invisible in reflected-light microscopy. Other metals such as Ag are of sporadic occurrence in oxidized ores: they may be found as discrete mineral species, not incorporated into the dominant oxyhydroxides. A summary of findings from three base–metal deposits in Canada, the Philippines and Portugal serves to illustrate the manner in which PIXE data benefit our knowledge of metal distributions in metallic ores. PIXE can contribute to several facets of mineral-deposit research, such as: (1) the development of ore textures, and specifically the distribution of elements within zoned crystals, or between multiple generations of a particular mineral; (2) the location of precious metals, Ag being in general the simplest case; and (3) pinpointing elements that may have implications for ore genesis, environmental quality or metal refining, such as Cr, As and Se.

Precious metal abundances in selected iron meteorites: in-situ AMS measurements of the six platinum-group elements plus gold

Abstract We have measured the abundances of seven precious metals in NiFe phases (kamacite and plessite) in six iron meteorites. These in-situ analyses, obtained by accelerator mass spectrometry (AMS) on small polished samples previously characterized by electron microprobe techniques, constrain the distribution of the rare siderophile elements. Within our set of irons, a small but varied suite, concentrations measured by AMS vary by factors of 9–13 for Au, Pd, Pt, Rh and Ru, and by factors of 90 and 250 for Ir and Os respectively. Data are presented for all six platinum group elements (PGE) plus gold. The AMS data suggest a variation in overall precious-metal abundances of a factor of 16 between the most-enriched (Negrillos, Σ PGE + Au = 270 ppm) and the least-enriched (Welland, 16–19 ppm). A clear illustration of the use of AMS data for provenance studies of meteoritic iron is presented for the Welland IIIA iron, an 1888 find from Ontario. Few published data are available for Welland: comparison of a type sample with a smaller piece of unknown metal, with respect to chondrite-normalized PGE patterns, major-element chemistry and textures of the metals, strongly support a suggestion that the latter is a fragment of the same iron.

Trace-element analysis of mineral grains using accelerator mass spectrometry — from sampling to interpretation

Abstract A brief overview is provided of the uses of AMS in mineral analysis, emphasizing the selection of appropriate samples. Simple guidelines are given for judging the suitability of a set of samples (and the type of problem that they pose) for AMS, as opposed to other methods of in-situ analysis. Optimal interpretation of the AMS data requires that the method be employed in conjunction with a range of other types of information. These include textural and mineralogical observations obtained with petrographic or scanning electron microscopes, plus in-situ chemical data for areas of the target typically 1–250 μm in diameter, obtained by some combination of complementary techniques, such as electron, proton or ion microprobe analysis (EPM, PIXE and SIMS, respectively).

Progress in AMS research at Iso Trace

Some of the continuing research into AMS at IsoTrace: neutral injection AMS, heavy element backgrounds, the analysis of non-conducting samples and low level 14C detection is described.

AMS advances in the geosciences and heavy-element analysis

Abstract Until recently small accelerators, such as the 2 MV Tandetron, have been used only for the lighter elements such as 10Be, 14C and 26Al, while the heavier isotopes such as 36Cl, 41Ca and 129I have been perceived to need the higher energies only available on large machines. We describe the use of a Tandetron for measurement of 129 I 127 I at natural levels, and the in situ assay of some heavy elements of interest in economic geology at detection levels superior to those of more widely used techniques. A few recently described novel applications of AMS are also given.