Performance of the rebuilt SUERC single-stage accelerator mass spectrometer

The SUERC bipolar single-stage accelerator mass spectrometer (SSAMS) has been dismantled and rebuilt to accommodate an additional rotatable pre-accelerator electrostatic spherical analyser (ESA) and a second ion source injector. This is for the attachment of an experimental positive-ion electron cyclotron resonance (ECR) ion source in addition to a Cs-sputter source. The ESA significantly suppresses oxygen interference to radiocarbon detection, and remaining measurement interference is now thought to be from 13C injected as 13CH molecule scattering off the plates of a second original pre-detector ESA

A gas ion source for radiocarbon measurement at SUERC

Development of radiocarbon AMS measurement of CO2 samples using the 39-cathode NEC MC-SNICS gas/solid hybrid ion source in the SUERC AMS system is in progress. Stable and reproducible C− currents up to 12 μA can be achieved for an optimal CO2 gas flow rate of 1.5 ± 0.5 μl min−1, resulting in typical efficiency of 8% from CO2 to C− and that of 5% from CO2 to C4+. The precision is limited by the lifetime of the gas cathode, but measurements with modern materials have shown that a precision of better than 0.5% and 1% on >200 μg C and 10–200 μg C respectively is obtainable on a routine basis. The normalized 14C values for TIRI intercomparison sample A (Barley mash) agree well with the consensus value within the margin of uncertainty. The background comes from the gas cathode and probably cross contamination, which is estimated to be ca. 1 pMC. Large-sized samples (usually >200 μg C) can readily be run as pure CO2 while samples as small as 40 μg C can be measured at an optimal flow rate using helium carrier gas. Even smaller samples (>5 μg C) have been analysed at a reduced current. At present, source poisoning effects at helium flow rates above 130 μl min−1 limit the further reduction in measurable sample size. A manifold redesign is expected to address this problem. PACS 27.20.+n; 29.25.Ni; 82.80.M

Initial measurements with the SUERC accelerator mass spectrometer

Be-10, C-14, Cl-36 and I-129 test measurements have been made with a new Pelletron-based accelerator mass spectrometer operating at up to 5.2 MV. All ion detection was with a versatile gas ionization detector. Low-background radiocarbon measurements with 2% scatter of identical samples was performed with both spectrometer ion sources. Be-10/Be backgrounds of 3 x 10(-15) were achieved using a gas cell adjoining the detector for B-10 suppression. High sample-throughput Cl AMS with Cl- 36/Cl backgrounds of 4 x 10(-15) was accomplished

129I/127I ratios in Scottish coastal surface sea water: geographical and temporal responses to changing emissions

This work constitutes the first survey of I isotope ratios for Scottish sea water including the first data for the west of Scotland. These data are of importance because of the proximity to the world’s second largest emission source of 129I to the sea, the Sellafield nuclear reprocessing plant, because of the increasing importance of the sea to land transfer of 129I and also as input data for dose estimates based on this pathway of 129I. 129I/127I ratios in SW Scotland reached 3 × 10−6 in 2004. No strong variation of I isotope ratios was found from 2003 to 2005 in Scottish sea waters. Iodine isotope ratios increased by about a factor of 6 from 1992 to 2003 in NE Scotland, in agreement with the increase of liquid 129I emissions from Sellafield over that time period. It is demonstrated that 129I/127I ratios agree better than 129I concentrations for samples from similar locations taken in very close temporal proximity, indicating that this ratio is more appropriate to interpret than the radionuclide concentration