In-field determination of ²²⁶Ra and ²²⁸Ra in the oil exploration sector



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The presence of radioactivity in the production of oil is now a well-known phenomenon. Specifically ²²⁶Ra, ²²²Rn, ²²⁸Ra and ²¹⁰Pb and their various decay products are of environmental concern especially to the workers in the field. The Nuclear Engineering Teaching Lab at the University of Texas has been in collaboration with Enviroklean Product Development Inc. (EPDI) in Houston, an environmental restoration and chemical product company, in the clean-up of technologically enhanced naturally occurring radioactive material in the oil exploration sector. In particular radioactive scale build-up in the equipment has been an on-going concern. Unlike typical naturally occurring radioactive material (NORM) samples, ²²⁶Ra and ²³²Th, and their daughters dominate the radioactivity. As such, they are not in secular equilibrium with any of the radionuclides above the decay chain that emanate from ²³⁸U or ²³²Th. Respectively, this thesis sought to test a Cesium Iodide (CsI) detector for in-field analysis of NORM samples. Additionally, quality control procedures and an in-house reference material were created to facilitate this testing. This in-house reference material was used to thoroughly test the CsI detector against the radiation detection stalwarts of High Purity Germanium (HPGe) and Sodium Iodide (NaI). Neither the NaI nor CsI were able to detect the low energy ²¹⁰Pb photons, but ²²⁶̕²²⁸Ra were no issue for either detector. The CsI was found to be 33% more efficient than an HPGe and have twice the full-width half maximum resolution of the NaI (16 keV vs 31 keV at the 186.5 keV peak). The CsI was also able to reach detection limits of 361 +/- 20 pCi/g for ²²⁶Ra and 25 +/- 2 for pCi/g ²²⁸Ra in scale from west Texas. Additionally, the self-attenuation of the sample was evaluated and, as expected, the lower energy photons, e.g. ²¹⁰Pb, are significantly more attenuated than photons at higher energies. We found approximately a 79 % reduction in counts at 47 keV, 7% at 186 keV, and 5% at 911 keV. Self-attenuation must be factored in or there will be an underestimation of the radioactivity, especially for ²¹⁰Pb. The last issue faced while testing the CsI detector was its inability to discriminate the ²²⁸Ra peaks around 911 keV from other peaks in the vicinity. This problem was solved using an HPGe detector to find the ratio of wanted peaks to unwanted peaks around 911 keV photon. This ratio can be applied to the convoluted CsI peak around 911 keV to find the actual net counts coming from ²²⁸Ra. The detection limits, self-attenuation, and deconvolution ratio can all be used to develop software capable of accurately determining radioactivity concentrations in NORM in a field environment.