Grade assignments for models used for calibration of gross-count gamma-ray logging systems

This report presents the results of work conducted to make grade assignments for 45 calibration models at the US Department of Energy (DOE) calibration facility in Grand Junction, Colorado, and at six secondary field sites in the United States. The assignments which were made carry three restrictions. First, these grade assignments are intended to be used when calibrating gross-count, or total-count, gamma-ray logging systems. Second, the assignments are made only for models which contain only uranium, that is, negligible thorium and potassium. Third, these grade assignments are based solely on gamma-ray measurements performed both in the laboratory and on the models themselves; therefore, they are assignments made from measurements of radium-226 daughters, but are stated in units of equivalent uranium (eU), where equivalent uranium is defined as the concentration of uranium (in naturally occurring isotopic abundance) which would be present if the entire uranium decay chain was in secular equilibrium. In theory, grade assignments can be made by analyzing, in the laboratory, physical samples from the calibration models. However, in practice, the physical sampling process contains uncertainties and leads to laboratory results which are inconsistent with gamma-ray measurements in models. Therefore, the gamma-ray measurements from the models are compared with the results from the laboratory analysis, and the laboratory values are subsequently adjusted to produce a best estimate of the grade in the model. The advantage of this method is that the laboratory data produce assignments which are traceable to standards, while the gamma-ray data from the models produce assignments which are consistent from one model to another. 13 references, 10 figures, 14 tables