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A neutron probe is not measuring water content directly. It is
measuring hydrogen atoms and these can be from any source,
including bound water and hydrocarbons. The use of a neutron
probe in environmental, or environmental remediation
applications usually requires frequent site recalibration due to
changes in the hydrogen provided by sources other than water. In
addition, a neutron probe is not accurate within the top 15
cm. of the soil surface due to neutron loss from the region of
influence into the atmosphere.

The user of a neutron probe usually requires special training,
and a government license for transport, ownership and use of a
radioactive source. The soil core from the probe borehole must
be gravimetrically analyzed to establish a calibration reference
curve to insure probe accuracy, and the site calibration curve
is specific to a particular probe. Additionally, neutron probes
"age" with use as the activity level of the source
degrades. This "aging" of the probe requires periodic site
recalibration to maintain accuracy.

Additional information available at

[Summary provided by ESI]