Troxler EGauge 4590 - Mode of Operation

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The Troxler Model 4590 Surface Density Gauge uses direct
transmission 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.