Photons reaching the G-M detectors in the 4640-B are counted for
selected intervals of time (the number of photons counted is directly
related to the density of the material). The gauge program uses the
counts to determine the density of the material top layer and the
results are automatically displayed on the screen.
The gauge is calibrated using a set of metallic blocks of different
densities and thicknesses. Readings are taken on these blocks and
the counts obtained are used during the density calibration. The
calibration range is 100 to 170 pcf (1602 to 2723 kg/m
3
). The
thickness range is 1.0 to 4.0 in. (2.5 to 10 cm) with over 95% of the
influence from the top 2.5 in. (6.4 cm).
2. OPERATION THEORY
NOTE
The source rod of a 4640-B gauge has two positions:
SAFE and measurement. With the source rod all the
way up, the gauge is referred to as being in the SAFE
position (page 3–8). In this configuration, the source is
surrounded by shielding. This position is used for
transporting and storing the gauge. In the measurement
position, the source rod is pushed down against the stop
pin (page 3–8). This removes the shielding under the
source, allowing the photons to penetrate the material
directly under the gauge. The shielding around the
source rod remains in place at all times.
Model 4640-B 2–3
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