What is a Nuclear Density Gauge?
A nuclear density gauge is a portable instrument that measures the density and moisture of soil and asphalt with radioactive sources.
How it works
The gauge holds two sealed sources. A gamma source (caesium-137) measures density. The radiation level at the detector shows how compact the material is. A neutron source (americium-241/beryllium) measures moisture. Hydrogen in the material slows the fast neutrons, and the gauge reads moisture from this effect.
Direct transmission and backscatter
In direct transmission, the source rod goes into a hole in the material, and the radiation travels to a detector in the gauge base. ASTM D6938 allows a source depth of up to 300 mm (12 in.). This method is the more precise one.
In backscatter, the source stays on the surface and the detector reads the radiation that the material scatters back. The method does not detect changes in density deeper than about 5 to 7.5 cm (2 to 3 inches). It suits uniform material such as asphalt paving. ASTM D6938 measures moisture in backscatter mode whichever method measures the density.
Use in airport construction
The gauge controls the compaction of new work. In FAA AC 150/5370-10H, Item P-152 sets the field density of subgrade and embankment by ASTM D1556 or by ASTM D6938 direct transmission. The text requires the machine to be calibrated to D6938.
In Item P-401, the contractor keeps a density gauge on site to find the best rolling pattern. The specification allows a nuclear gauge to monitor density under ASTM D2950. Acceptance is different. It uses one core from each sublot, with a surface course mat density limit of 92.8 percent.
Safety and control
The sources are radioactive, so a regulator licenses each user. In the United States, 10 CFR 30.34(i) requires two independent physical controls against removal when no one is watching the gauge. In Canada, a gauge needs a leak test every 12 months. The requirements differ by country and by license.
This page covers construction quality control. It does not describe a way to inspect an in-service airfield.
Frequently Asked Questions
- What is the difference between direct transmission and backscatter?
- In direct transmission, the gamma source rod goes into a hole in the material and the detector is in the gauge base. In backscatter, the source stays on the surface. Direct transmission is the more precise method. Backscatter does not detect density changes deeper than about 5 to 7.5 cm (2 to 3 inches).
- Does the FAA use a nuclear gauge to accept asphalt density?
- No. In FAA Item P-401, acceptance of mat density uses one core from each sublot, tested under ASTM D2726. The specification allows a nuclear gauge only to monitor the density during paving, under ASTM D2950.
- Which standards cover nuclear density gauges?
- ASTM D6938 covers soil and soil-aggregate. ASTM D2950 covers bituminous concrete. FAA AC 150/5370-10H names both in its standard construction specifications.
References
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports FAA
- cnsc-portable-gauges
- Working Safely With Portable Gauges Canadian Nuclear Safety Commission
- astm-d6938-23
- ASTM D6938-23, Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth) ASTM International
- cfr-10-30-34
- 10 CFR 30.34, Terms and conditions of licenses US Nuclear Regulatory Commission (eCFR)
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.