THIN LAYER
The Model 3450 contains two sets of G-M tubes for photon
detection. The set closest to the source rod is referred to as System 1,
while the set farthest from the source rod is System 2. When the
source rod is in the backscatter position, the source and the detectors
are on approximately the same plane.
Both System 1 and System 2 tube sets will primarily detect photons
that have been scattered by the material as described in the Density
theory of operation (see page 2–2). However, due to its greater
distance from the source, System 2 is more likely to detect scattered
photons from deeper in the material than is System 1. The
difference in the depth of material measured by each system, the
factory calibration, and mathematical modeling allow the 3450
gauge to determine the density of the top layer of material.
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