Trace element analysis based on neutron activation with coincident gamma ray detection

Sammanfattning: In this work, six soil samples have been analyzed. The samples come from different sedimentary layers on Earth. They were previously studied with the objective to find the trace concentration of iridium, to correlate the increased concentrations of iridium with meteorite impacts that have occurred on Earth. In this work the samples are reanalyzed with the purpose to investigate which other trace elements can be detected. Before the measurement with the gamma spectrometer, the samples are activated in a thermal neutron reactor. The radiation from the samples is detected with a setup that consists of 14 LaBr3:Ce detectors. The gamma rays from the different elements in the sample should be well separated, but this is not always possible. There is a background coming from the Compton scattering effect, which causes problems in the spectra and causes difficulties in the quantification of trace elements. To improve the sensitivity of the spectrometer and reduce background, a coincidence condition on two or more gamma rays can be applied. The condition is characteristic for each element. Using this technique, the identification of 7 different isotopes in the previous measured samples (46Sc, 134Cs, 60Co, 58Co, 124Sb, 181Hf and 152Eu) was possible. Additionally, the amounts of each element in the samples were estimated by comparing with the previous data. The amounts obtained were in the order of ppm. The minimum concentration needed in order to assure the presence of the element in the sample was calculated for each isotope and a limit below 1 ppm was obtained for all the elements. This means that by using the coincidence gamma ray spectrometer it was possible to identify low concentration of elements in the samples.