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

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.Element identification in six soil samples. Atoms are the basic unit of all matter, and they are constituted of three types of particles, such as: protons, neutrons and electrons. The protons and the neutrons are inside the nuclei and the electrons are orbiting around the nuclei. All the chemical elements of the periodic table are composed of atoms. However, chemical elements also have unstable atoms, which means that the number of neutrons in the nuclei are low or excessive, so the atoms reach the stable configuration by releasing energy in form of radiation. These emissions of energy can occur in different ways, but one of the most important is gamma radiation, where a nucleus releases the excess energy by emitting a gamma ray. There are many devices capable in detecting the gamma radiation, in order to identify the element that is emitting it. One of the most used to detect the gamma radiation, is the scintillator detector. The scintillator detectors detect the energy and transform it, into an electrical signal, which can be processed by electronic devices, and later analyzed by the experimenter. Despite that the scintillator detectors perform well detecting gamma radiation, the full energy of high energy gammas are not always detected but sometimes only a portion of its energy is detected. Unfortunately this portion of the energy might shadow the signal from the chemical elements of interest. One way to minimize this problem is to use two or more detectors, detecting simultaneously several gamma rays from the same nucleus. This procedure is the coincidence technique, and it improves the measurement. In this project six soil samples were analyzed using the coincidence technique with fourteen scintillator detectors organized around the samples. Before the analysis the samples were sent to a thermal neutron reactor where they were bombarded with neutrons after which the samples become radioactive. The six samples belong to different sedimentary layers of the Earth. They were analyzed earlier with the purpose to find small concentrations of iridium (Ir). The purpose of this experiment is to identify which other elements can be detected in the samples and to measure the concentration, by using the combination of the coincidence technique and the neutron activation analysis