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The 3400 Series utilizes a 40 mCi (1.48 GBq) Am-241:Be source
with a yield of 70,000 neutrons per second. The neutrons are
used to measure hydrogen concentration in a material which
will be related to moisture (water) content. Hydrogen can be
measured with neutrons because hydrogen thermalizes (slows)
neutrons.
This means that fast neutrons emitted by the source are slowed
to a velocity where additional collisions with hydrogen or other
molecules will not slow the neutrons any further. About 19
collisions with hydrogen are required to thermalize a neutron.
For elements other than hydrogen to thermalize a neutron, far
more collisions are necessary (see Table 4). Generally, the
larger the element's atomic weight, the more collisions are
required to slow the neutron.
Since soils will contain varying amounts of compounds
containing hydrogen other than water, the gauge may require
calibration for individual soils. Also, some elements absorb
neutrons into their nucleus. The absorption cross section
indicates the probability the neutrons will be absorbed. Table 4
lists absorption cross sections of several elements. Boron and
chlorine are typical problem elements, since they have high
absorption cross sections.
Gauges are calibrated at the factory to measure moisture in an
ideal soil without neutron absorbing elements or chemically
bound hydrogen. In the field, adjustments to calibration are
generally done by comparison with conventional type tests such
as oven dry. Calibration is described in Appendix A.
A helium-3 (He
3
) tube is used to detect thermalized (slowed)
neutrons. The tube is insensitive to fast neutrons, so no
shielding is required between the source and the detector.
Generally, the counts obtained from a He
3
tube are directly
proportional to hydrogen, or moisture, content.
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