Model 3440-L A–5
. THEORY OF OPERATION
MOISTURE
The 3440-L gauge uses an americium 241:beryllium source with a yield of
33,000 neutrons per second. The neutrons are used to measure the
hydrogen (water) content of the material because hydrogen in the material
will thermalize the neutrons.
Thermalization is the process whereby fast (or epithermal) neutrons
emitted by the source are slowed to velocities where additional collisions
with hydrogen or other molecules will not further slow the neutrons.
Neutron thermalization requires an average of nineteen collisions with
hydrogen nuclei. Other elements require a greater number of collisions for
thermalization (Table A–1).
Due to their high sensitivity, the He-3 detectors in the 3440-L gauge are
very suitable for detection of thermalized neutrons. The He-3 tube is
insensitive to fast neutrons and the counts obtained are directly
proportional to the hydrogen/moisture content of the material.
Additionally, there are elements that absorb, or capture, neutrons into their
nuclei. The extent to which neutrons interact with nuclei is described in
terms of quantities known as cross-sections. The larger the cross-section,
the greater the possibility of neutron capture or absorption. Boron,
cadmium, and chlorine are elements that have large cross-sections and may
cause problems when attempting to measure the moisture of materials
containing these elements. An offset may be needed for the gauge if
moisture measurements are required in these materials.
Depth of Measurement, or the depth below which no more than 2% of the
counted neutrons pass before reaching the detector, is generally a function
of the moisture content. The normalized curve set shown in Figure A–4
illustrates the effects of moisture content on the radius of measurement.
This function may be expressed by:
Depth (in.) = 11 – (0.17
×
M), where M = Moisture in pcf
or
Depth (mm) = 280 – (0.27
×
M), where M = Moisture in kg/m
3
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