Neutron-capture cross-section measurements by activation and subsequent detection using AMS

The technique of accelerator mass spectrometry (AMS) was described in detail as based on a 3 MV tandem accelerator at the Vienna Environmental Research Accelerator (VERA). AMS is used over a wide range of different applications, whereby an accelerator is used and molecular isobaric interferences are almost completely ruled out. The potential of this technique was exemplified for measurements in the actinide mass range that are actually underway at the VERA laboratory. Measurements are available for the various Pu isotopes in environmental samples, the search for minute quantities of possible extraterrestrial Pu or Cm in terrestrial archives, and cross-section measurements in the mass range above that where one faces isobaric interferences, i.e. for masses larger than 209. As a proof-of-principle experiment the 235U(n,γ)236U neutron-capture reaction is being measured on the VERA facility in cooperation with FZ Karlsruhe. Neutrons with energies around 25 keV and 500 keV are produced to activate natural U samples, and the resulting U will be measured by AMS. Limitations in AMS occur as consequence of background signals or the overall efficiency in the measurement. Isotope ratios for U are measured with expected background ratios of 236U/238U in the range of a few 10-12; typical overall efficiencies are around 10-5 to 10-4 which defines the counting statistics. One of the main issues in mass spectrometric methods is the sample material consumed during the measurement. Precautions have to be taken to avoid contamination of the facility by such samples and to exclude memory effects which might influence future experiments. Therefore, elaborate and efficient sample chemistry is required prior to a potential AMS measurement. The latter can also be applied to other mass spectrometric techniques, which sputter the sample material during the measurement process. In addition, the amount of radionuclides of interest in the sample prior to irradiation has to be determined and, if possible, separated from the bulk material. Several reactions in the MA region were shown which are possibly accessible for AMS measurements