Current dependency of isotope ratio measurements at the Trondheim 1 MV AMS
Dr Martin Seiler1, Prof. Pieter Meiert Grootes1, Dr. Helene Svarva1, Dr. Marie-Josée Nadeau1
1The National Laboratory for Age Determination, NTNU, Trondheim, Norway
A current dependency of isotope ratio measurements is often seen in accelerator mass spectrometry. It presents an obstacle when increasing currents to yield higher precision and throughput or measuring samples with different current outputs. It can be observed when measuring carbon, as there are two abundant isotopes of which the abundance ratio can be measured with great accuracy in Faraday cups. For other elements, a current dependency is often difficult to determine due to the limited precision in counting rare isotopes. Nevertheless, such an effect can lead to imprecise normalization of unknown samples, especially when the currents of the standards do not cover the full current range of the unknowns.
Based on data of the carbon measurements, we estimated the fractionation for beryllium by correcting for effects caused by different masses and energies of the nuclides, as well as different intensity of the ion beam. For comparison, we prepared a set of ¹⁰Be samples with different BeO-Nb mixing ratios to yield different Be ion currents under similar ion source conditions. In addition, we varied the Cs flux in the source to test the influence of ion source conditions and increase the current range covered by this test.
Our ¹⁰Be/⁹Be data, especially for increasing currents, is affected by the intense ¹⁰B beam that partially hits the detector. While the separation of the ¹⁰Be and ¹⁰B counts in a 2-dimensional ΔE-Eres spectrum works well, the dead time caused by the boron reduces the efficiency of the ¹⁰Be detection causing an additional component of the current dependency of the ¹⁰Be/⁹Be ratio. We will present the data both for carbon and beryllium showing the change in measured ratio for varying currents as well as our calculations.
I studied Physics at ETH Zürich where I also did my PhD on small accelerators for Radiocarbon measurements at the Laboratory for Ion Beam Physics. Afterwards I have been a Postdoc/Researcher at the National Laboratory for Age Determination at NTNU in Trondheim where I’m working with AMS and Radiocarbon dating.