T.R Niklaus

MICROBEAM AMS: PROSPECTS OF NEW GEOLOGICAL APPLICATIONS

Abstract AMS applications in geology have hitherto concentrated on the use of cosmogenic isotopes and rate in-situ produced isotopes for geomorphological and geophysical studies. Special features of AMS lend themselves to more general applications to other isotopes and geochronological systems. In-situ measurements in geochronology carried out with ion-microprobes are restricted by isobaric and molecular mass interferences to special systems where the problem is minimal. AMS can be used to alleviate this mass interference problem, and opens up the prospect of a less restrictive in-situ microanalysis for geochronology. At CSIRO, a microbeam AMS system designed to achieve this capability is under construction. With this system, several interesting applications such as the ReOs system became accessible more conveniently. The UPb system becomes accessible for hydrous minerals, and RbSr systems for Rb-rich samples. In addition, microbeam AMS allows determination of trace elements at lower levels than those accessible with the proton microprobe. This paper discusses these prospects and describes the AUSTRALIS system (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) being developed at CSIRO.

The AUSTRALIS microbeam ion source

Abstract The AUSTRALIS (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) system being developed at the HIAF laboratory is a microbeam AMS system designed for in situ microanalysis of geological samples. The microbeam source was implemented by modifying a HICONEX source, resulting in a versatile source for microbeam as well as high intensity macrobeam operations. The source features a high magnification sample viewing system, enabling live observation of the sputtering process and visual tuning of the primary beam. Microbeam of Cs + as small as 30 μm in diameter has been obtained in the tests. The secondary ion extraction system features a “screen” electrode that is used to correct the primary beam trajectory affected by the extraction field, in order to return it to the geometric center of the sample. The paper will describe the source, and results of the test of the source and the injector system.

A fast bouncing system for the high-energy end of AMS

Abstract Drifts in the accelerator stability as well as those in beam transport components are the main limitations in achieving high precision in accelerator mass spectrometry (AMS). In order to achieve the high precision required for isotopic geology and geochronology applications, the sequence of isotopes of interest should be counted for as short an interval as possible. While fast isotope switching can be achieved readily at the low-energy end, it has hitherto not been possible to do so at the high-energy end. For a narrow range of transmission typical for most analysing magnets, fast switching (

Precision Pb and S isotopic ratio measurements by microbeam AMS

Abstract Measurements of Pb and S isotopes with precision of better than 1 permil have been achieved using AUSTRALIS (A.M.S. for Ultra Sensitive TRAce eLement and Isotopic Studies), a microbeam AMS system based on a 3 MV Tandetron at the HIAF laboratory. The high precision is made possible by a fast bouncing system both at the low and the high energy ends, to counter the effect of instabilities in the ion source and beam transport system. The system is aimed at enabling in situ microanalysis of geological samples for radiogenic and stable isotope data free from molecular and isobaric interferences. The microbeam source, developed from a HICONEX source, produces a 30 μm diameter Cs+ beam routinely, and includes a high magnification sample viewing system in the reflected geometry, facilitating sample positioning and tuning of the primary beam. Measurements were conducted at 1.5–2 MV terminal voltage. Lead isotopes are measured as Pb4+ ions, from PbS− injected ions. Sulfur isotopes are measured as S2+ or S3+ ions from injected S− ions. Fractionation effects still vary ∼2% over longer periods, attributable to the slight variation in some manual controls of the accelerator. This is expected to be improved when all controls are brought under computer control.

A MICROBEAM CS ION SOURCE FOR ACCELERATOR MASS SPECTROMETRY

A negative ion sputter source incorporating a microbeam Cs+ source intended for application in accelerator mass spectrometry (AMS) has been developed from a General Ionex HICONEX 834™ source. (HICONEX is a trade mark of High Voltage Engineering Europa.) The source is part of a microbeam AMS system, designed for in situ microanalysis of geological samples known as AUSTRALIS (AMS for UltraSensitive TRAce eLement and Isotopic Studies) recently developed at the Heavy Ion Analytical Facility laboratory. With requirements of precise beam positioning on mineralogical samples, a high magnification sample viewing system is a vital part of the source, enabling live observation of the sputtering process and visual tuning of the primary beam. Microbeam of Cs+ of 30 μm in diameter can be produced routinely with adequate intensity for a number of applications. The “screen” electrode in the secondary ion extraction system facilitates steering of the primary beam affected by the extraction field, back into the geometric ...

Prospects of new geological applications with a microbeam-AMS

Microbeam AMS enables measurements of trace elements at the sub-ppb level and of isotopic data from single mineral grains, the basic constituents of geological samples, offering possibilities of new applications. With elimination of molecular ions, and the flexibility of choice of injected ions to suppress isobaric interference, micro-AMS eliminates the problems encountered by ion microprobes, allowing more general access to a number of important geochronological systems such as U/Pb, Rb/Sr and Re/Os. The strict demands of applications in geochronology pose a great challenge in the development of a microbeam AMS, testing the limits of the analytical method itself and in the design of the instrument.

Microbeam AMS for Trace Element and Isotopic Studies

A microbeam Accelerator Mass Spectrometry (AMS) system has been developed at the CSIRO Heavy Ion Analytical Facility (HIAF) in Sydney, known as AUSTRALIS: A.M.S. for Ultra Sensitive TRAce eLement and Isotopic Studies. Based on a 3MV Tandetron, the system is designed for in-situ micro analysis of geological samples by mass spectrometric method virtually unrestricted by molecular and isobaric interferences. The microbeam source was developed from a HICONEX source, producing a 30 μm diameter Cs+ beam routinely, but designed to operate ultimately at resolution as high as 1 μm. The source features a high magnification sample viewing system in the reflected geometry, facilitating sample positioning and tuning of the primary beam. Isotope systems measured routinely at present include Pb and S isotopes, conducted at 1.5 to 2 MV terminal voltage. Lead isotopes are measured as Pb4+ ions, from either Pb−, PbO− or PbS− injected ions. Sulfur isotopes are measured as S2+ or S3+ ions from injected S− ions. A fast bounci...

AUSTRALIS: A microbeam AMS beamline

The AUSTRALIS (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) system being developed at the HIAF laboratory is a microbeam AMS system designed for in-situ micro-analysis of geological samples for ultra-trace and isotopic data. This enables analysis of microscopic geological samples or micro-features of minerals. AMS has only minor mass interference problems and together with the high sensitivity of AMS, AUSTRALIS opens the way for many interesting applications. AUSTRALIS is based on a standard AMS design and enhanced by some unique design features. The primary ion beam gun (modified HICONEX source) produces a microbeam of Cs+ as small as 30 microns in diameter. The sample chamber features an efficient sample viewing system with viewing angle normal to the sample, that enables live observation of the sputtering process and visual tuning of the microbeam. In order to achieve the high precision required for isotopic ratios in geochronological applications of AMS, a fast isotope switching system ...