Troxler 3241-C - Chapter 2: Theory of Operation; Theory

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The Model 3241 Series Asphalt Content Gauges measure the
amount of asphalt (bitumen) in a bituminous mixture by
determining the hydrogen content of the material. The amount
of asphalt and the amount of moisture can be related to
hydrogen content since both contain hydrogen.
In all 3241 configurations, the fast neutrons emitted are
thermalized (slowed) by the hydrogen in the asphalt. The
Helium-3 detectors located in the gauge detect the slowed
neutrons, since helium-3 detectors are insensitive to fast
neutrons. The thermalized neutrons are counted over time and
this “count” is proportional to the asphalt content of the sample.