THEORY OF OPERATION
The 4640-B Thin Layer Density Gauge obtains density
measurements using backscattered gamma radiation (photons). The
photons from an 8 mCi cesium-137 source located in the base of the
gauge penetrate the asphalt overlay (see Figure 2–1). The photons
collide with the electrons in the material and are either scattered or
absorbed. As the density increases, the number of photons scattered
back to the detectors decreases.
The 4640-B uses two sets (referred to as System 1 and System 2) of
Geiger-Mueller (G-M) tubes for photon detection. When the source
rod is in the “measure” position, the source and detectors are in the
same horizontal plane. Ideally, no photons should reach the
detectors in a direct path from the source. In the “backscatter”
mode, the photons must be scattered at least once before reaching
the detectors.
Figure 2–1. Gauge in Measurement Position
on Thin Layer Material
2–2
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