868 Nuclear Instruments and Methods in Physics Research B56/57 (1991) 868-872 North-Holland

Recent developments in AMS at the IsoTrace Laboratory

W.E. Kieser IsoTrace Laboraiov, University of Toronto, Toronto, Canada M5S IA7

The equipment required for the accelerator mass spectrometry (AMS) analysis of heavy elements using small tandem accelerators is reviewed, and examples of the analytical capability for ‘29I and precious metals, including the platinum group elements, are given.

1. Introduction

Accelerator mass spectrometry (AMS) using low en- ergy (I 3 MV) electrostatic accelerators has, for the most part, concentrated on the analysis of the lighter cosmogenic radionuclides such as “Be, 14C and 26A1 [l]. These do not require a high degree of electron stripping to obtain a charge state which can be analysed in the small magnets usually associated with such accelerators, and can be readily identified from their energy loss in a simple gas ionization detector. After almost a decade of experience, such measurements make up the bulk of analytical activities of most AMS laboratories [2] and still provide the ground for research into improved techniques [3,4]. However, shortly after the establish- ment of the light ion analysis capability at IsoTrace, attempts were made to extend the range of masses which could be analysed: to impurities in semiconduc- tor materials [5], to stable elements of geochemical interest [6] and to heavier rare radioisotopes such as 12’1 [7]. The analysis of heavier elements on a routine basis became practical with the installation in 1987 of a large, high resolution magnet on a separate analysis line at the high energy end of the accelerator. This line is now providing commercially available analyses for environ- mental scientists and economic geologists, as well as being a useful workbench for further research into AMS.

This paper will describe the additions and modifica- tions required for heavier mass analysis which have been made to the conventional low energy AMS equipment at IsoTrace, and then give a short descrip- tion of the capabilities available for the analysis of several different species. For a schematic diagram of the entire AMS system, reference is made to fig. 1 in ref. [l]. An updated version of the heavy element portion of this drawing is presented in fig. 1 of this paper.

2. Equipment requirements

2.1. Pre-acceleration analysis

The energy distribution of the atomic ions from a sputter source, while highly concentrated about one maximum value, nevertheless has tails on both the high and low energy side of the maximum. When only mag- netic analysis is used before injection into the accelera- tor, the selection of a rare species for injection will include ions from the tails of more abundant nearby masses. The topology and the formation mechanisms of these tails are discussed in detail in ref. [S]. As the mass of the element analysed increases, the level of this interference becomes more critical. For example, the tail on the higher energy side, until it approaches the kine- matic limit for collisions between the sputtering and sputtered ion in the source, has an intensity which varies as l/(E - Eo)2 where E, is the energy imparted to the ion by the source acceleration potential. The intensity of an ion of mass m’ which is n amu less than the mass m of the atom being selected, relative to the maximum intensity of ion m’, varies as m’*/n2. Hence the interference of ‘*‘I in the analysed 129I is 112 times greater than the interference of “C in 14C. While the use of an electric analyser for 14C analysis becomes important only for older samples, for the analysis of 1291, it reduces the injected ions from the ‘*‘I sputter tail by a factor of > 1000 [9]. The analyser in use at IsoTrace (E2) has a bending angle 9 = 45 O, a radius p = 40.6 cm and an energy resolution of E/AE = 400.

Mass selection is accomplished using a 90’ magnet with p = 36 cm and a mass resolution M/AM = 300.

While this is excellent for 14C and adequate for ‘291, better resolution is required for platinum, thorium and yet heavier elements. An alternate input to the inflec-

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

WE. Kieser / Recent developments in A MS 869

tion line is planned, incorporating the 180” double focussing magnet obtained from Oxford University. This will provide a mass resolution of M/AM = 750 [lo].

Further background reductions would be obtained by

using an electric analyser both before and after the magnet. With this configuration, the hydride molecular ions MH-, which are formed from the decay of di-hy-

dride molecular ions MH; occurring between the first electric analyser and the magnet and which are virtually unresolvable from the atomic ion M- in the magnet, can be eliminated by the second electric analyser [ll].

2.2. Stripper gas control and terminal pumping

Heavier atoms require lower stripper gas pressure, for optimum transmission, than atoms such as carbon, even at higher charge states. Without differential pump- ing at each end of the stripper canal, residual gas from the canal in the acceleration tubes will reduce both efficiency and sensitivity: in the low energy tube, more negative ions will be neutralized and thus lost to the counting process; in the high energy tube more scatter- ing and charge changing collisions will occur, increasing the level of the background continuum at the final detector. Some method of monitoring stripper gas pres- sure is necessary so that yields for a particular charge state can be reproducibly peaked, ideally a readout from a capacitance manometer located in the terminal, transmitted to ground through a light link. At IsoTrace, the partial pressure of the stripper gas is measured at the high energy end of the accelerator with a residual gas analyser. Finally, if recirculating terminal pumping is used as at IsoTrace (where the output from the first stage of differential pumping, a 150 l/s turbopump running at half speed, is returned to the canal) then all components of the recirculation system should be rigor- ously leak tested. Even slight admixtures of the SF, insulating gas, which would be concentrated in the stripper canal by the turbopump, could raise the stripper gas pressure above the optimum for charge state selec- tion and may also cause increased X-ray yields from the accelerator tubes as shown in figs. 1 and 2 of ref. [12].

2.3. Post-accelerator analysis

The purpose of the post-accelerator analysis is to remove molecular fragments and other charge-changed and scattered ions which will mimic the ion of interest in the final detector. Since any analytical element can itself generate additional background by providing surfaces and a volume containing residual gas from which ions can be further charge-changed and scattered, the lowest overall background is achieved by locating different types of analysers adjacent to each other so that unwanted ions which are passed by one analyser are eliminated by the next [13]. Large dispersion for the

Fig. 1. Schematic of the heavy element analysis line. A3, 7: apertures; DA: the data acquisition electronics and computer; El. 3: electric analysers; F2-F9: Faraday cups; ID2: a gas

ionization detector; L4, 6: electric quadrupole lenses; M2, 4:

magnetic analysers; and S4: a set of steerer plates.

electric and magnetic analysers is also important to reduce the transmitted portion of the background con- tinuum.

The post-accelerator analysis system at IsoTrace is shown schematically in fig. 1. The first electric analyser E2 (p = 2.5 m, $J = 15 o and E/AE - 200) is also part of the original 14C system, but at least provides charge state selection and some fragment discrimination for heavy elements as well. This is followed by the high resolution magnet M4 (p = 1.32 m, $I = 90 o and M/AM = 2600). (The field of magnet M2, the first mass analyser for the i4C system, is nulled for heavy element analysis.) The final dispersive element is the electric analyser E3 (p = 2.5 m, 9 = 45 o and E/AE =

900). The electric quadrupole lens L6 is required so that the analysis system fits into the available space. The heavy element line is pumped by five 1000 l/s cryo- pumps which maintain the vacuum lines at a pressure of 5 X lo-’ mbar except at the end of the accelerator where the pressure is - 1.5 X 10e7 mbar largely as a result of the stripper gas which escapes through the second differential pumping apertures. The gas ioniza- tion detector ID2, built by Zhao, has a resolution A E/E for platinum of = 6% [14].

2.4. Accelerator terminal voltage stability

The requirement of high energy and momentum resolution for the post-accelerator analysers places severe constraints on the stability of the accelerator voltage. For example, a fluctuation of 750 V on a terminal voltage of 2.0 MV will cause the beam centroid

X. MASS SPECTROMETRY

870 W.E. Kieser / Recent developments in AMS

at the image plane of the magnet M4 to move 1 mm. In AMS, there is usually no beam of an abundant species which, after magnetic or electric analysis, can be used to provide an error signal for terminal voltage stabiliza- tion; this is normally provided by a generating voltme- ter (GVM). The adjustments to the GVM and power supply feedback circuitry required to bring the terminal stability to k 125 V are described in ref. [l].

A recent measurement, made as part of the search for group II negative ions, provides a good illustration of the overall system stability. The mass spectrum shown in fig. 2 was obtained from a metallic Yb target by scanning the magnet M4 and counting the transmitted ions in the gas ionization detector ID2 [14]. The higher energy peak results from the injection of Yb- ions, whereas the lower energy one results from the injection of YbH- ions from the decaying YbH; ions through the process described in section 2.1. The energy dif- ference between these peaks, corresponding to E/AE = 1225, is well resolved by the system.

2.5. Sample viewing optics

For the analysis of bulk, homogeneous samples, the ability to view the sample directly is not critical, as setup samples can be used to adjust the properties of the cesium sputtering beam to optimize the yield from the target. For i4C analysis at IsoTrace, up to 16 positions on each sample target are analysed in order to reduce cratering effects [15,16] and so the initial setup routines require a view of the target with moderate resolution. However, for the in situ analysis of geologi- cal specimens (section 4) distinct mineral grains must be re-located when the sample is in the ion source; for this purpose, good resolution (between 1 and 10 pm) and good illumination of the target is required. The

?-g 300

-c e

From YbH- --, r From Yb-

0.9054 0.9059 0 9064 0.9069

Magnetic Field (T)

Fig. 2. Ion counts per 10 s interval as a function of the field of

magnet M4 with inflection magnet Ml set to accept mass 174

amu, for ions from a Yb metal target. The reason for the

presence of the lower peak is discussed in the text.

Press Pin

Clamp

Sample Holder

Anvil Liner

Anvil

Fig. 3. Apparatus for loading and pressing AgI and Nb mix-

ture into sample holders for AMS analysis. The anvil liner,

sample holder and press pin are used only once. The diameter

of the hole which is filled with the sample material is 1.5 mm.

system currently in place at IsoTrace, designed by Kilius and shown schematically in fig. 1 of ref. [l], makes use of a microscope illumination light, insertable mirrors in the vacuum system and an alignment telescope (Tl). This is marginally adequate for mineral identification especially when the mirror surfaces become degraded and so we are planning to replace this system with an in-vacuum light source and properly shuttered viewing optics.

3. Analysis capability for ‘=I

Of the heavy elements now routinely analysed, ‘291 follows the procedures for 14C most closely. The iodine must be chemically extracted from the raw samples and converted to the form of AgI [17,18]. The AgI is mixed with Nb in a ratio 1: 2 by weight, and loaded into stainless steel target holders, similar to those used at Gif-sur-Yvette [19] and Arizona [20]. In this process, the holders are held in a loading press as shown in fig. 3, clamped against a disposable anvil liner and the target material is loaded through a disposable plastic pipette tip from above. The material is then pressed using a disposable stainless steel rod which, if the sample is very small, can be swaged into position to provide support [21]. The target holder is removed and then baked in a vacuum oven for over 6 h at 175 o C [18]. Typical ‘*‘I negative ion currents are over 3 PA [17].

Samples are loaded into the ion source in batches of 7: 1 standard, 1 background and 5 unknowns. The standards are made from a dilution of the NBS 4994B standard, and background samples are made from KI obtained from Banco Anderson Laboratories Inc., Fort Worth, TX [18]. The mass 127 current is measured in the off-axis Faraday cup F6 in front of the image slits of magnet M4 and the mass 129 ions are counted in

W. E. Kieser / Recent developments in Ah4S 871

= cr%J’c -2

“I” f 2;

1

t IIUI. 1 * 1 b 1 * I. 1, I I

0.858 0.860 0.862 0.864 0.866 0.868 0.870

Magnetic Field (Telsa)

Fig. 4. Ion counts per 100 s interval as a function of the field of magnet M4 with the inflection magnet Ml set to accept mass 129. The origin of the counts at each end of the scan is

explained in the text and in ref. [II].

detector ID2. A scan of the magnet M4 with the inflec- tion magnet set for mass 129 is shown in fig. 4 for a sample with 1291/‘271 = lo-‘* [lo]. The 12915+ peak is well resolved, with no evidence of a background con- tinuum. The peaks at lower momentum are, in order of $+c~~g momentum, ‘*‘I’+ from the sputter tail,

I from the decay of IH$- into IH- (both de- scribed in section 2.1), and ‘*‘15+ from the breaking up of ‘*‘IH; in the accelerator terminal. The background at higher momentum is caused by the fragments of complex molecules with negative ion mass of 129. While the analysis of ‘291 in charge state 4 + would result in increased yield, the breakup of another complex mole- cule, 97Md60;, results in a 97M03+ beam which cannot be resolved from 12914+ [ll].

4. Capabilities for in situ precious metal analysis

The procedure for the analysis of platinum group elements (PGE) and other precious metals differ signifi- cantly from other AMS work. For sample preparation, no chemical procedures are required, only the mechani- cal cutting and polishing of sections of the ore sample involved [23]. Mineral grains of interest are selected using a microscope, and 4 mm diameter cores contain- ing the grains to be analysed are removed from the section for direct insertion into the ion source. The production of standard for the normalization of yields is somewhat more difficult, as the properties of the sulphide matrix from which the atoms of interest are sputtered must be taken into account. Several intema- tionally recognized reference materials for the PGE exist, notably SARM-7 which was obtained by homoge- nizing 7 t of rock from the Merensky Reef, Bushveld

igneous complex in South Africa [22]. Targets from this and other more locally available reference materials are made by casting the material in the form of NiS fire assay beads [23] which are sectioned, polished and cored as in the case of the unknown samples.

Setup and tuning procedures for the spectrometer system vary with the element to be detected, as for each element there exist different interferences such as mass to charge ratio ambiguities and different sources of background. For example, in the analysis of platinum from a copper rich ore, the injection of %t; which breaks up in the terminal into three “Cu*+ interferes with the measurement of 195Pt6+ and so 196Pt6+ must be used [23]. However, the presence of both 63Cu and 65Cu provides valuable calibration information and pilot beams for tuning the spectrometer.

At present, in situ analyses can be made for Au, Pt. Ir, OS, Ag, Pd, Rh and Ru, as well as for S, Te and Se. Detection limits for the precious metal elements are given in table 1 [23,11]. As the analysis of more than on of these elements in a single sample is frequently re- quired, computer control fo many of the elements of the spectrometer is needed and, in particular, high resolu- tion control of both the pre- and post-accelerator (Ml and M4). Magnet M4 is controlled through a Metrolab model 4025 NMR teslameter with a resolution of 10e6 T; however, the Hall probe control on the magnet Ml has inadequate resolution (10e4 T) and must be re- placed as soon as possible.

5. Conclusions

The incorporation of some of the basic principles of AMS as outlined in section 2.1 and 2.4 and in more detail in ref. [13] has resulted in the establishment of a system for the AMS analysis of the heavier elements using a small accelerator. With the lower costs for operating a small accelerator, the price of such analysis becomes affordable for both academic and industrial research [24]. This addition to the IsoTrace system is

Table 1 In-situ detection limits (at the 10% counting statistics level) [2%111

Element Limit [parts per billion]

Gold 0.1 Silver 0.1 Platinum 1.5 Rhodium 3.0 Palladium 20.0 Iridium 30.0 Ruthenium 120.0 Osmium 600.0

X. MASS SPECTROMETRY

872 W.E. Kieser / Recent developments in AMS

not only providing data in environmental and other Earth science applications, but is also a useful collection of tools with which further developments in AMS can be explored, for example, new methods of isobar sep- aration [25] and the search for new negative ions [14,26].

Acknowledgements

The author is grateful to Prof. A.E. Litherland for helpful discussions and for his detailed reading of the manuscript. He also wishes to thank all the IsoTrace staff, especially those who have contributed results prior to publication. The operation of the IsoTrace Labora- tory is supported by Infrastructure funds from the Natural Sciences and Engineering Research Council (NSERC) of Canada and by the University of Toronto. The development of the heavy element line has been supported by NSERC equipment grants, the University of Toronto and by the long term loan of magnet M4 from Atomic Energy of Canada Ltd.

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