Adaptation of a field nuclear gauge for laboratory density measurement

Abstract

Nondestructive density measurement of ground and fill materials by means of field nuclear gauges is widely known in civil engineering but its laboratory counterpart is given less attention. This paper describes the adaptation of a two-probe field nuclear gauge into a laboratory device for density measurement of cylindrical specimens. Considerations for its adaptation for laboratory use include the influence of specimen geometry comparing cylindrical to slab type specimens where the basic attenuation law applies directly, specimen position between source and detector, source to detector spacing, and specimen diameter. Density is obtained by calibration to a basic radiation attenuation law based on parallel rays passing through a slab specimen. It is found that the curvature and obliquity effects of a diverging beam passing through a cylindrical specimen can be accounted for by a 'c' factor which is approximately the volume ratio of the irradiated portion of the cylinder to the slab of the same external dimension. From the modified attenuation law a diameter ratio to a count ratio relationship is established. This diameter relationship can be used to determine the density of specimens of other diameters from the calibration curve of a single diameter. The validity of the diameter correction rule is found to be applicable for a practical working source-detector spacing of 280 mm to 450 mm. © 1991.