Mode of Operation
The Troxler Model 4540 Soil & Asphalt Density Gauge uses
direct transmission mode and Backscatter mode. Source rod
positions are described in Figure 4 on page 3–5.
In the direct transmission mode, the source rod extends through
the base of the gauge into a pre-drilled hole to a desired depth.
Photons from the cesium-137 (Cs-137) source in the source rod
pass through the test material, the photons collide with
electrons and lose their energy. A high material density
increases the probability of these photon collisions. This
decreases the number of photons that reach the highly efficient
gamma ray detector in the base of the gauge.
Thus, the number of photons reaching the detectors is inversely
related to the density of the material; the higher the density of
the material, the fewer the photons that reach the detectors.
Using the gauge calibration, the gauge software converts the
detector counts to a density value.
In the backscatter mode, the source rod is lowered to the first
notch below the SAFE (shielded) position.
Gamma particles are emitted from the source and collide with the
material beneath. The collisions can alter the trajectory of the
gamma radiation and some amount will be directed towards the
detector in the gauge. The denser the material, the more
collisions take place, which results in more gamma radiation
being detected.
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