these two counts will cancel the 1.1% effect on the counts and
will in effect normalize the counts, regardless of the date of
testing after calibration. Not using a count ratio will result in
erroneous readings, if source decay is not accounted for.
Gauge software uses the gamma count taken at the test site
and the standard count to automatically calculate and display
wet density (WD) for the material.
Moisture Measurement
Moisture measurement in the gauge is accomplished by using
an Americium-241: Beryllium (Am-241:Be) source, which emits
fast neutrons, and a single Helium 3 (He-3) tube. Neutron
measurements are always accomplished in the backscatter
mode. Both the Am-241:Be and the He-3 tube are fixed inside
the gauge base.
The average energy released by the Am-241:Be source is 4.5
Mev, with a spectrum energy ranging from 0 to 10 Mev. In the
moisture measurement process, fast neutrons from the source
interact with the hydrogen nucleus present in water and
thermalize (slow down). The thermal or slow neutrons are then
counted by the He-3 tube. Increase in water content results in
a proportional increase in thermal neutron counts detected by
the He-3 tube.
There are two assumptions made in the measurement of
moisture by neutron method. First is that any interaction
between the source and material is due to interaction with
hydrogen in the form of water. Reviewing the composition of
normal soils in the literature, the probability is very strong that
any thermalization in soils is due to interaction with hydrogen in
water. Second, that there are no elements that absorb
neutrons. However, in construction type soils, boron in some
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