Optimization of low activity spectrometric gamma-gamma probes for ash determination in coal stockpiles

A stockpile probe based on backscattered gamma-gamma-ray technique was tested for the determination of coal ash. A centralized tool employing a gamma-ray source of very low radioactivity (1.85 MBq) was used. This logging tool was tested using Cs-137 and Ba-133 sources with a 37 mm (diameter) by 75 mm NaI (TI) scintillation detector. Ten source-detector configurations were analyzed using Cs-137, and another nine source-detector configurations are analyzed, using Ba-133 source, both for four geophysical models, having a borehole diameter of 90mm. Regression analysis on the laboratory logging data for each studied configuration was carried out in order to establish calibration equations for ash prediction. The optimum configuration for the logging stockpile probe using Cs-137 source was determined to be 80mm source to detector spacing, and 80mm iron thickness shielding. This configuration gives the best results for ash prediction. The root mean square (rms) deviation for ash is 1.52%. The optimum configuration for the logging stockpile probe using Ba-133 source is determined to be 85 mm source to detector spacing, and 50 mm iron thickness shielding. This configuration gives the best results for ash prediction, where the rms deviation for ash is 1.47%. (C) 2003 Elsevier Science Ltd. All rights reserved.