Volume 20, Issue 1 (January 1992)
Experimental Evaluation of a Laboratory Twin-Probe Nuclear Gage for Specimen Density Measurement
This paper describes a series of tests conducted to establish the operating requirements of a laboratory twin-probe nuclear gage for density measurement of cylindrical specimens of homogeneous materials. The nuclear device is an adaptation of a direct transmission field gage for laboratory density measurement. It is shown in this evaluation program that three forms of analysis are required for the setting up of such an apparatus: calibration analysis, precision evaluation, and accuracy assessment. Three specimen diameters (75, 100, and 150 mm), three materials with densities from 0.8 to 2.7 g/cm3, and nuclear gage source-to-detector spacings ranging from 250 to 450 mm were included in the evaluation program.
It was found that density calibration based on radiation attenuation theory is valid, and that a diameter-count rate relationship can be established experimentally so that only a single set of density calibration using one specimen diameter is needed for routine testing. In the accuracy assessment, it was found that there exists a narrow range of source-to-detector spacings for unbiased density estimation. A spacing of 280 mm is recommended for the device studied. At this spacing, the density measurement precision at 95% confidence level, using four 1-min counts per test, is found to vary from 0.010 g/cm3 for 150-mm-diameter specimens to 0.017 g/cm3 for 75-mm-diameter specimens.