Troxler Enhanced RoadReader Plus 3451, Manual Of Operation And Instruction

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2–2
DENSITY
The Troxler Model 3451 Enhanced RoadReader Plus uses two
modes of operation:
direct transmission mode (with the source rod
extended into the material to be measured) and backscatter mode
(with the source rod in the backscatter position). Source rod
positions are described in
Chapter 3 .
Direct transmission mode is used on materials with layers four
inches thick or more. This mode is most commonly used on soils,
but can also be used to measure asphalt and concrete. In the direct
transmission position, 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. While passing through the test material, the photons
collide with electrons and lose energy. A high material density
increases the probability of these photon collisions. This decreases
the number of photons that reach the Geiger-Mueller (G-M)
detectors 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 G-M detector counts to a density value.
Backscatter mode is used on concrete, asphalt, and soil layers
approximately four inches thick. In the backscatter position, the
source rod is lowered to the first notch below the
SAFE (shielded)
position. This places the source and the detectors in the same plane.
Shielding between the source and detectors greatly reduces the
number of photons reaching the detectors in a direct path from the
source. Thus, the photons from the Cs-137 source must travel into
the test material and scatter (or reflect) at least once to reach the
G-M detectors. The detectors in the gauge base count these scattered
photons. For
thin layer readings, the gauge uses two sets of G-M
detectors. Refer to the Thin Layer section on page 2–5.