AMS for M > 36 with a gas-filled magnetic spectrograph

doi:10.1016/0168-583X(94)95995-1

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 92, Issues 1–4, 3 June 1994, Pages 146-152

https://doi.org/10.1016/0168-583X(94)95995-1 Get rights and content

Abstract

Small concentrations of long-lived radioisotopes such as ⁵⁹Ni are not necessarily easy to determine by AMS because of the background of stable isobars, in this case ⁵⁹Co. The gas-filled Q3D magnetic spectrograph has been successfully used for isobaric separation. Additional suppression of non-isobaric background is accomplished by means of a Wien filter directly after the tandem and a highly sensitive time-of-flight measurement just before the spectrograph. Until now we have used these techniques for the radioisotopes ⁴¹Ca, ⁵⁹Ni, ⁹⁰Sr and ¹⁰⁷Pd. The limits in sensitivity are at present between ~ 3 × 10⁻¹⁵ for ⁴¹Ca/Ca and ~ 1 × 10⁻⁸ for ¹⁰⁷Pd/¹⁰⁶Pd. Measurements have been performed on environmental, extraterrestrial and nuclear waste samples. Because of the insufficient sensitivity for measurements of very low concentrations with the existing Q3D, a dedicated gas-filled magnet with a deflection angle of 135° is now under construction.

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W. Rühm, B. Schneck, K. Knie, G. Korschinek, E. Nolte, H. Vonach, D. Weselka and L. Zerle, to be published in Planetary...

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First experiments had entirely a nuclear physics motivation, but soon first AMS experiments were performed [3,4]. Due to the high voltage of the tandem (14 MV MP), the facility is predestined for isobaric suppression techniques which was demonstrated with a gas-filled magnetic spectrograph [5]. By the installation of the dedicated Gas-filled Magnet System (GAMS) in 1997, the tandem laboratory houses a permanent AMS experimental setup since then [6].
The AMS setup at the Maier-Leibnitz-Laboratory (MLL) in Garching features a 14 MV tandem accelerator and two dedicated beam lines for high-sensitivity measurements of isotopes in all mass ranges. Especially in the medium mass range (70  $<$  A  $<$  120), AMS sensitivity is often limited by isobaric and isotopic background with only small relative difference in Z and/or A. Here, we present recent developments and upgrades at the MLL with special focus on improved sensitivity and new techniques for challenging AMS isotopes such as $^{93}$ Zr and $^{99}$ Tc. Upgrades of the measurement setup as well as radiopurity studies for AMS samples, which could be altered by atmospheric neutrons during transportation by aircraft, are discussed.
Developments in accelerator mass spectrometry
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In combination with differential energy loss measurements, this can significantly improve suppression, since the parasitic ions will not degrade the detector performance [16–19]. However, this technique is most effective only at high beam energies [20,21]. In addition to active energy loss measurements, a passive degrader foil can be used to introduce selective energy loss of different elements and separate related ion beams after an energy dispersive spectrometer [22,23].
This report attempts to summarize the technical evolution of accelerator mass spectrometry (AMS) instrumentation over the last thirty-five years. The related impact of the AMS measurement technology to the wide variety of applications of long-lived radionuclides in modern research is not covered. The accompanying article by W. Kutschera overviews these applications. Here, I am providing an introduction to the basics principles of AMS measurement technology, describe the set-up of a typical AMS instrument, and discuss in general specific requirements to reach sensitivity as it is required to measure long-lived radionuclides at their natural levels in the environment. A retrospective view is given on the major development steps of AMS instruments and measurement technique. Special attention is paid to the simplification of AMS systems by reducing their size and complexity. These developments have launched the wide spread use of AMS in modern research fields. Today, commercially available high-performance instruments are standard for more than 100 AMS facilities around the World. There are a number of primary important radionuclides in focus of AMS measurement procedures but radiocarbon is still of paramount importance. Consequently, I will summarize the latest developments in radiocarbon AMS.
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Despite its ability to deal with molecular interferences, the measurement of 90Sr provided major challenges for AMS. Korschinek et al. [5] used an MP tandem with terminal voltage VT = 14 MV, a gas-filled Q3D magnetic spectrograph and a time-of-flight system to reach a detection limit of 2.6 pg/g. Three years later, Paul et al. [6] using SrH2 targets, a 12UD Pelletron with VT = 12 MV, time-of-flight and a four element gas ionization detector (3 anodes + a Si residual energy detector) reported a limit of 7.5 fg in a laboratory blank samples. Recently, Tumey et al. [7,8] using SrF2 targets, an FN tandem at VT = 9.25 MV, foil stripping to charge state 11+ and a gas ionization detector with a silicon nitride window and three anodes designed to optimize the measured energy loss difference between 90Sr and 90Zr.
The analysis of ⁹⁰Sr by AMS has so far required the use of very large tandem accelerators in order to separate the isobar ⁹⁰Zr by the rate-of-energy-loss method. The analysis of ^135,137Cs by AMS has never been attempted as the separation of the isobars ^135,137Ba by the traditional method requires even higher energies, so that this approach would become prohibitively expensive for routine analysis. Following the successful demonstration of Cl⁻–S⁻ separation by the Isobar Separator, the same apparatus was used to test the separation of other pairs of isobars. Surprisingly effective results were obtained with NO₂ gas in the cases of SrF₃⁻–ZrF₃⁻ and CsF₂⁻–BaF₂⁻ separations. Reduction factors of ∼4 × 10⁻⁶ for ZrF₃⁻/SrF₃⁻ and ∼2 × 10⁻⁵ for BaF₂⁻/CsF₂⁻ were measured. SrF₃⁻ and CsF₂⁻ are both super-halogen anions and are preferentially produced in the ion source rather than ZrF₃⁻ and BaF₂⁻ when using the PbF₂ matrix-assisted method. Reduction factors for ion source production with such targets of ∼3 × 10⁻⁵ for ZrF₃⁻–SrF₃⁻ and ∼5 × 10⁻⁴ for BaF₂⁻–CsF₂⁻ were found. The combined methods would suggest a theoretical detection sensitivity for ⁹⁰Sr/Sr ∼6 × 10⁻¹⁶, ¹³⁵Cs/Cs ∼7 × 10⁻¹⁵ and ¹³⁷Cs/Cs ∼1 × 10⁻¹⁴, assuming 10 ppm Zr and Ba contamination in the AMS targets. In addition to the earlier Cl⁻–S⁻ separation work, these measurements further illustrate the potential of on-line ion chemical methods for broadening the analytical scope of small AMS systems.
AMS measurement technique after 30 years: Possibilities and limitations of low energy systems
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A brief summary of the development of AMS over the last 30 years is given and major development steps for the measurement technique are described. With the appearance of compact low energy AMS systems 10 years ago, a new category of AMS instruments has been introduced. This has resulted in a boom of new AMS facilities with more than 20 installations over the last 5 years. But low energy AMS is not limited to radiocarbon only and there is a great potential for ¹⁰Be, ²⁶Al, ¹²⁹I and actinides measurements at compact AMS systems. The latest developments towards the low energy limit of AMS resulted in two new types of systems, the NEC Single Stage AMS and ETH MICADAS operating with terminal voltages of about 200 kV only. These systems have enormous impact, not only on the use of AMS in biomedical research but also on radiocarbon dating. Precision limits of radiocarbon dating will be discussed for the MICADAS type instruments.
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Citation Excerpt :
The enhanced sensitivity of AMS would allow 90Sr to be used a biokinetic tracer to elucidate the functionality of strontium ranelate which would have great impact on the development of future bone related disease treatments. Based upon the encouraging experience of other AMS labs [12–14], we have begun development of 90Sr measurement capabilities at the Lawrence Livermore National Laboratory (LLNL) Center for AMS (CAMS). This development has been performed using the CAMS heavy-isotope system, which is described elsewhere [15].
Strontium-90 is one of the most hazardous materials managed by agencies charged with protecting the public from radiation. Traditional radiometric methods have been limited by low sample throughput and slow turnaround times. Mass spectrometry offers the advantage of shorter analysis times and the ability to measure samples immediately after processing, however conventional mass spectrometric techniques are susceptible to molecular isobaric interferences that limit their overall sensitivity. In contrast, accelerator mass spectrometry is insensitive to molecular interferences and we have therefore begun developing a method for determination of ⁹⁰Sr by accelerator mass spectrometry. Despite a pervasive interference from ⁹⁰Zr, our initial development has yielded an instrumental background of ∼10⁸ atoms (75 mBq) per sample. Further refinement of our system (e.g. redesign of our detector, use of alternative target materials) is expected to push the background below 10⁶ atoms, close to the theoretical limit for AMS. Once we have refined our system and developed suitable sample preparation protocols, we will utilize our capability in applications to homeland security, environmental monitoring and human health.
Computer simulation of ion-beam optics in a gas-filled magnetic spectrometer
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Citation Excerpt :
The first application of a GFM for AMS was done at the Argonne National Laboratory [8] for detection of 41Ca. Since then several other installations have been reported [9–14]. The first instruments were old reoriented Enge split-pole spectrographs, Q3D, and dipole magnets, while later presented GFMs were specially designed ones (two 180° dipole magnets [10,14] and one 135° dipole magnet [13]).
A Monte-Carlo simulation of ion-beam optics in a gas-filled magnetic spectrometer has been conducted for ions at ∼1 MeV/amu corresponding to the energies achievable at the new Uppsala tandem accelerator (NEC 15SDH-2, 5 MV tandem Pelletron). Comprehensive calculations are performed for ³⁶Cl at 33–45 MeV kinetic energy with the following parameters: 1–5 Torr N₂ and 60–100 cm bending radii in a dipole magnetic field. It is shown that the separation between the ³⁶Cl and ³⁶S beams has a broad maximum in the interval 150–180° at optimal gas pressures. A 150° dipole magnet is preferable due to smaller beam extension in the non-deflection plane and larger residual energies. A dipole magnet with 180° bending angle, 60 cm bending radius and 36 mm vacuum chamber height will be installed at the Uppsala Tandem Laboratory in November 2002.

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AMS for M > 36 with a gas-filled magnetic spectrograph

Abstract

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

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Nucl. Instr. and Meth.

Geochim. Cosmochim. Acta

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.