DENSITY
The Troxler Model 3440-L Surface Moisture-Density Gauge uses two
modes of operation: direct transmission mode (with the source rod
extended) and backscatter mode (with the source rod placed in the first
notch below the SAFE position).
In direct transmission mode (Figure A–1), the rod containing the
cesium-137 source is lowered to the desired depth, resulting in gamma
radiation emission. The detectors in the gauge base measure this radiation.
Gamma photons reaching the detectors must pass through the full distance
of soil, resulting in a large number of photons colliding with electrons
present in the soil. These collisions reduce the number of photons that
reach the detectors and thus allow the density of the material to be
calculated. In short, the lower
the number of photons reaching the
detectors, the higher
the density of the material. Since all of the photons
arriving at the detectors have passed through the full distance from the
source depth, the measured density is a true average for the material
between the surface and the source.
Figure A–1. Direct Transmission Geometry
A–2
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