Development of new methodologies for the measurement of radionuclides by gamma-ray spectrometry using detectors

Abstract

The environmental impact caused by pollutants, like radionuclides, released from different pollution sources, is currently of general concern, making controls for radioactivity levels be more exhaustive. Moreover, the potential radiological risk caused by the exposure to radiation is also an issue that has become more important in the last decades. This makes strict regulations have to be generally applied to different working places and building materials, and for that the analysis of radionuclides is essential to assess the radiological risks associated to the human activities. Therefore, this Thesis aims to develop new methodologies to determine both natural and artificial radionuclides by gamma-ray spectrometry for a wide variety of sample matrices and geometries. For this, a new and comprehensive calibration method to obtain the full-energy peak efficiency (FEPE) for volume samples in the case of cylindrical geometries, at each gamma emission energy (Eγ), as a function of the thickness (h), apparent density (ρ) and matrix composition has been developed for an extended range (XtRa) HPGe detector. The selected certified calibration standards contain only natural radionuclides belonging to the 238U series and 232Th series as well as 40K; codes RGU-1, RGTh-1 and RGK-1 from the International Atomic Energy Agency (IAEA). For well-type detectors, a novel methodology for the efficiency calibration has also been developed, based on obtaining a general FEPE function varying the apparent density and composition of the calibration standards, keeping h constant. For this, RGU-1, RGTh-1, KCl, PbS and activated carbon were used. To properly obtain the FEPE for problem samples whose chemical compositions and apparent densities were very different from those related to calibration samples, corrections due to self-absorption effects were needed. For this, the self-absorption correction factor (fa) was used, which made necessary to obtain a general function for the mass attenuation coefficient. To calculate fa, it was also necessary to use self-absorption correction models. In some cases, the corrections due to true coincidence summing (TCS) effects were found to be essential for the FEPE calculation. For this, a novel method to correct the TCS effects has been addressed, varying the sample-detector distance (d), finding a d for which the obtained FEPE function provided the best fit for the corrected experimental FEPEs. All the methodologies developed to calculate the FEPE were subjected to both internal and external validations by using several certified and non-certified materials, respectively. Moreover, the validity of the self-absorption effect corrections provided by the Cutshall, and Appleby models were analyzed in depth. A new and general methodology for efficiency calibration of coaxial Ge detectors has been accomplished for the measurement of radionuclides using different geometries of atmospheric aerosol filters. For this, RGU-1, RGTh-1 and RGK-1 were chosen, allowing us to reproduce the same particulate matter deposition geometry. Furthermore, for filters whose surfaces were relatively large, they were cut in several pieces and placed one on top of the other. This makes easier to achieve the same geometry as that resulting from folding a problem filter to be measured. Then, the general FEPE function depending on Eγ, found for each filter type, was validated. In addition, a new and precise methodology to measure the 222Rn and 220Rn short-lived daughters (214Pb and 214Bi, and 212Pb and 212Bi, respectively) has been developed, where a very high-volume sampler was used. This methodology was applied to estimate aerosol residence times using the 214Pb/222Rn and 212Bi/212Pb activity ratios, and to obtain equilibrium factors, achieving consistent results. The consistency and reproducibility of the methodology were tested for different sampling and counting times. In the case of the 212Bi and 214Bi, the total times for their precise measurement were found.