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 ¹³⁷Cs and ¹³³Ba sources with a 37 mm (diameter) by 75 mm NaI (TI) scintillation detector. Ten source–detector configurations were analyzed using ¹³⁷Cs, and another nine source–detector configurations are analyzed, using ¹³³Ba source, both for four geophysical models, having a borehole diameter of 90 mm. 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 ¹³⁷Cs source was determined to be 80 mm source to detector spacing, and 80 mm 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 ¹³³Ba 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%.