A nuclear density gauge measures in-place HMA density using gamma radiation. Gauges usually contain a small gamma source (about 10 mCi) such as Cesium-137 on the end of a retractable rod.
Gamma rays are emitted from the source and interact with electrons in the pavement through absorption, Compton scattering, and the photoelectric effect. A Geiger-Mueller detector (situated in the gauge opposite from the handle) counts gamma rays that reach it from the source. Pavement density is then correlated to the number of gamma rays received by the detector.
Nuclear density gauges are typically operated in one of two modes, each of which uses a different correlation to determine pavement density (Figure 2):
- Direct transmission. The retractable rod is lowered into the mat through a pre-drilled hole (this hole can be formed by pounding a steel rod with a similar diameter to that of the gauge’s retractable rod). The source emits gamma rays, which then interact with electrons in the material and lose energy and/or are redirected (scattered). Gamma rays that lose sufficient energy or are scattered away from the detector are not counted. The more dense the pavement, the higher the probability of interaction and the lower the detector count. Therefore, the detector count is inversely proportional to pavement density. A calibration factor is used to relate gamma count to actual pavement density.
- Backscatter. The retractable rod is lowered so that it is even with the detector but still within the instrument. The source emits gamma rays, which then interact with electrons in the material and lose energy and/or are redirected (scattered). Gamma rays that are scattered towards the detector are counted. The more dense the pavement, the higher the probability that a gamma ray will be redirected towards the detector. Therefore, the detector count is proportional to pavement density. A calibration factor is used to relate gamma count to actual pavement density.
When operated in backscatter on relatively thin mats (less than 75 to 100 mm (3 to 4 inches)) gamma rays from the source will not only interact with electrons from the newly paved mat, they will also interact with electrons from material below the paved mat. Nuclear density gauges advertised as “thin lift” gauges account for this by using two Geiger-Mueller detectors, one closer to the source than the other. The detector further from the source is more likely to detect gamma rays scattered by the material below the paved mat. Therefore, the difference in the depth of material that influences each detector and mathematical modeling allow the gauge to determine the density of only the top, newly placed lift (Troxler, 2002).
A nuclear density gauge offers the following key advantages over destructive density measurement (cores):
- Portability. One person can easily transport a typical nuclear density gauge.
- Quick results. Most nuclear gauges allow both one and four minute readings. These are much quicker than typical densities obtained from cores which could take from several days to several weeks.
- Virtually non-destructive. Used in the backscatter mode, the nuclear density gauge is entirely non-destructive. Used in the direct mode, the gauge only requires a small penetration into the finished mat approximately 20 mm (less than 1 inch) in diameter and about 50 mm (2 inches) deep.
Nuclear Moisture Gauge
A nuclear moisture gauge (not pictured) uses a neutron source, such as Americium-241:Beryllium, placed inside the gauge. The source emits high energy, “fast” neutrons, which then collide with various nuclei in the pavement. Due to momentum conservation, those neutrons that collide with hydrogen nuclei slow down much quicker than those that collide with other, larger nuclei. The gauge detector counts only thermal (low energy) or “slow” neutrons thereby making the detector count proportional to the number of hydrogen atoms in the pavement. Since water contains many hydrogen atoms (H2O), the detector count is proportional to moisture content. A calibration factor is used to relate thermal neutron count to actual moisture content.