Speaker
Description
$^{129}$I is one of the AMS radionuclides which benefits most from a compact and low-energy AMS system. The interfering stable isobar $^{129}$Xe is completely suppressed as it does not form negative ions, thus high energies are not required for isobar separation in the detector. Mass separation needs to be high enough to discriminate against the stable isotope $^{129}$I. The HE side of a compact low-energy AMS system can be equipped with two magnets and an electrostatic analyzer (ESA), providing excellent suppression of neighboring masses, in particular of the stable isotope $^{127}$I and of a potential interference from $^{128}$Te. As one of the heavier AMS nuclides $^{129}$I requires high magnetic fields in combination with large bending radii, even more so when the measurements are performed at high ion energies. This is often compensated with the selection of a high charge state with a low yield at the high energy (HE) side to bring the magnetic rigidity down for reasonably sized magnets. In contrast at lower energies (around a few 100 keV) the most probable charge state 2+ can be selected and with He stripping transmissions of >50% are achieved. Furthermore, the use of the 2+ charge state avoids mass/charge ambiguities at other charge states (e.g. 3+).
All these benefits are implemented at the 300 kV multi-isotope AMS system MILEA, which was built from more than 10 years of experience of $^{129}$I at the first compact 500 kV multi-isotope AMS system Tandy. Here we describe the MILEA system, review the key aspects of the performance and discuss challenges that still arise: Destruction of molecules of same mass are key for all AMS measurements; at the low charge state 2+ we control the rate of surviving molecules with the stripper gas pressure and thus by the number of collisions. Attention must be paid to molecules other than $^{127}$I$^1$H$_2^-$ that do not scale with the $^{127}$I current and can occur in natural samples. Another critical component is the ion source. Due to the volatility of iodine, cross-talk between samples can limit the performance for samples with low $^{129}$I content.
We show that with careful selection of the source parameters we can control the cross-talk and are able to measure Woodward Iodine down to $1\times10^{-14}$ while having high transmission of >50% and measure the nominal ratio with >90%. The optimized performance combined with the reduced complexity of the AMS measurement at MILEA are beneficial particularly for high throughput applications such as $^{129}$I as an ocean tracer.
| Student Submission | No |
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