For decades, nuclear density gauges have been used to measure the density and moisture content of soil during compaction testing in road and embankment construction. Worldwide regulatory agencies consider nuclear density gauges as controlled devices because of their inclusion of radioisotope sources. Regulatory requirements, such as the need for licensing, special storage, special transportation procedures, gauge operator training, and personal dosimetry, have become burdens for gauge users. Recently, a new nuclear density gauge for measuring soil wet density was declared exempt from licensing and other nuclear regulatory requirements in the United States. The new gauge uses an extremely low-activity radioisotope source. It further incorporates a separate probe operating on electromagnetic principles for soil moisture measurement or any other moisture measurement method the user desires. This paper presents the design features and measurement properties of this low-activity nuclear density gauge and its associated moisture probe. The measurement properties were determined from laboratory and field studies that were conducted in North Carolina, United States. These studies showed that the sensitivity and precision of wet density measurements made by the low-activity nuclear density gauge were similar to those made by a conventional nuclear density gauge. The wet density measurements made by the low-activity gauge and conventional gauge showed a strong correlation and agreement. For the two field sites studied, a silty sand subbase and a cement-treated aggregate base course, the dry density agreement between the low-activity nuclear density gauge/moisture probe and the conventional nuclear density gauge was within 3 lb/ft3 (48 kg/m3).

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