Study of the natural radioactivity background of the Douala University Campuses and surrounding by Nuclear Techniques: Validation by GEANT4 Monte Carlo simulations

Abstract

The present thesis focused on an ionizing radiation monitoring project of the soils of the University of Douala campuses (campus 1 and 2) and surrounding, the Littoral region of Cameroon. The purpose was to provide a baseline to document the conditions present at the time of sampling. The methodology used γ-ray spectrometry-based High Purity Germanium detector (HPGe), both Energy Dispersive X-Ray Fluorescence (EDXRF) and Wavelength Dispersive X-Ray Fluorescence (WDXRF) spectrometry for sample’s elemental characterization, and the Monte Carlo simulation-based Geant4 toolkit for detector efficiency calibration. The Geant4 toolkit also provides the opportunity to optimize the detection systems using computer simulations and greatly reduces the need for expensive (radiation exposure to calibration sources) testing in the laboratory. The assessment of 238, 235U, 232Th, 137Cs, and 40K concentration was done by measuring soil and sand samples by γ spectrometry-based High Purity Germanium detectors (HPGe). Both laboratories of the National Radiation Protection Agency (NRPA) of Cameroon and the Atomic and Nuclear Spectroscopy, Archeometry Laboratory of the University of Liege were used for experiments. Geochemical characterization of soil samples, origin determination, and provenience were accessed by X-ray spectroscopy. By comparing the results of two detectors and the technics used according to the detector type, improvements on the γ spectrometry methodology were made. The relative uncertainty activity concentration was calculated for 226Ra, 232Th, and 40K. The average report between the GC0818-7600SL model and the BEGe-6530 model was the main outcome that suggested real attention that should be paid when selecting the radionuclide to be investigated on a specific type of detector. The BEGe detector was found to be more suitable for low γ energy emitters measurement, compared to the GC0818-7600SL model, found more efficient for high energy γ emitters. The potential radiological hazards parameters were assessed by calculating successively the following parameters from using those sands in the construction of dwellings and large buildings: Ra-dium Equivalent activity (Raeq), Outdoor absorbed γ dose rate (Dout), Annual Effective Dose rate (AED), Internal hazard (Hin) and external hazard (Hex) indexes, and and γ indexes for sand samples used as building materials. Results obtained show that Annual Effective Dose absorbed by in-habitants due to the use of the investigated sand as construction materials was found to be below 1.0 mSv y-1. Therefore, sand used as building materials from the investigated quarries appears to be radio-logically safe for building construction and for the environment (beaches, built houses, …) where people could safely spend time. Soil characterization using EDXRF in the present study provided an overview of the geological origin or provenience of the investigated area. As a result, the analyzed soil samples could be classified chemically as Fe-soil and are illustrative dregs from the Continental margin because of the high concentration of Fe in all the investigated samples. These data record the elemental composition and the natural radionuclide’s radioactivity levels of the studied area and could be set as reference database information in the region, in Cameroon as well as for the Gulf of Guinea’s data. Monte Carlo validation based on the GEANT4 toolkit has been used to validate the efficiency calibration of the system, and it has been noticed that the combination of γ-ray spectrometry, the development of related Monte Carlo methods, and the GEANT4 (GEometry ANd Tracking) toolkit developed for γ spectrometry simulation are compelling and useful for detector characterization nowadays. It can then be concluded that the Monte Carlo simulation gives more prominent adaptability, greater flexibility, gained time, precision, and accuracy when determining the detector response and efficiency in the routine of environmental radioactivity monitoring