A calibration factor is used to relate gamma count to actual pavement density. Therefore, the detector count is inversely proportional to pavement density.
The more dense the pavement, the higher the probability of interaction and the lower the detector count. Gamma rays that lose sufficient energy or are scattered away from the detector are not counted. The source emits gamma rays, which then interact with electrons in the material and lose energy and/or are redirected (scattered). 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). Nuclear density gauges are typically operated in one of two modes, each of which uses a different correlation to determine pavement density (Figure 2): Pavement density is then correlated to the number of gamma rays received by the detector. A Geiger-Mueller detector (situated in the gauge opposite from the handle) counts gamma rays that reach it from the source.
Gamma rays are emitted from the source and interact with electrons in the pavement through absorption, Compton scattering, and the photoelectric effect. Figure 1: Thin lift nuclear density gauge. Gauges usually contain a small gamma source (about 10 mCi) such as Cesium-137 on the end of a retractable rod. A nuclear density gauge measures in-place HMA density using gamma radiation.
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