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Figure 16 represents the geometry for the direct transmission
and backscatter modes of the 3400 series. An 8 mCi (0.3 GBq)
Cs-137source is located in the tip of the rod. Gamma photons
emitted by the source radiate to the detectors. Some of the
photons emitted by the source collide with electrons in the soil,
reducing the amount of gamma photons reaching the detectors.
For the backscatter mode, photons must be "scattered" at least
once to reach the detectors. The electron density is directly
related to mass density. Counts over a time such as 1 minute
can be directly related to density as shown in the section on
calibration.
In direct transmission, photons reaching the detector must pass
through the full distance from the source to the detectors. The
density measurement is an average of the material between the
source depth and the gauge base.
In backscatter, photons must be scattered at least once to reach
the detectors. This geometry yields a reading which is biased
toward the surface. Figure 17 shows the portion of the reading
taking place in the top layer. For example, approximately 94% of
the reading takes place in the 80 mm (3 in.) of material
measured.
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