Thursday, 8 September 2005 - 9:30 AM

This presentation is part of: New Innovations and Revolutionary Concepts

Radiocarbon detection by ion charge exchange mass spectrometry

In a paper at the first AMS symposium, Middleton [1] raised the possibility of measuring radiocarbon with a low energy mass spectrometer based on charge exchange. In this method, the same two principles that enable AMS to work are used, but in reverse order. Molecular interferences are eliminated in the first stage, by producing high charge state ions directly in the ion source (charge state ³ 2). ¹⁴N interference is eliminated by converting the beam to negative ions in a charge exchange cell. The beam is mass-analysed at each stage, and additional energy analysers can be used to reduce backgrounds. Here we refer to this method as ion charge exchange mass spectrometry (ICE-MS).

At the time of Middleton's proposal in 1978, the idea was not practically achievable, because there existed no efficient means of producing multiply-charged ion beams. This situation has now changed, and the idea was recently revived in a paper by Schubank [2]. Electron cyclotron resonance ion sources (ECRIS) can produce multiply-charged ions with high efficiency [3].

We have built a test apparatus to measure key parameters relevant to the use of ICE-MS for measuring radiocarbon. Using an ECRIS and a pair of analysing magnets with a charge exchange cell in between, we have measured charge exchange probabilities for (Cⁿ⁺ ® C- ) from 5 to 40 keV (n = 1 to 3). Our apparatus also includes an electrostatic analyser after the second magnet to suppress scattered ions. We have used this system to detect ¹⁴C in enriched samples of CO₂ gas.

We have demonstrated that our current system is capable of measuring samples with ¹⁴C/¹²C isotopic ratio down to the 10- ⁹ level. This is several orders of magnitude better than is possible with any other method except AMS. Most importantly, this can be achieved with very high efficiency. This ratio, combined with a measured sample consumption rate of 4ng/s, corresponds to a capability to detect transient signals containing only a few m Bq of ¹⁴C activity. This level of sensitivity is well suited to tracer studies in biomedical research and drug development.

1. R. Middleton, in Proc. First Conf. on Radiocarbon Dating with Accelerators, H.E. Gove (ed.), University of Rochester, USA, 1978.
2. R.B. Schubank, Nucl. Inst. & Meth. B 172 (2000) 288.
3. R. Geller, Electron Cyclotron Resonance Ion Sources and ECR Plasmas, IOP, Bristol, 1996.

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