InstroTek
15
4. Calibration
The neutron probe is a source of fast or high energy neutrons
and a detector of slow or thermal neutrons.
The fast neutrons are
slowed down by collision
with the nucleus of matter
in the soil, and then
absorbed by the soil
matter. Since the mass of
the nucleus of hydrogen is
the same as that of a free
neutron, the presence of
hydrogen will result in a
high
field
of
thermal
neutrons. Heavier elements
will also slow down the
neutrons, but not nearly so effectively. While it takes, on the
average, only 18 collisions with hydrogen, it takes 200 with the
next element normally found in agricultural soil.
The thermal neutrons are continually being absorbed by the
matter in the soil. Boron, for example, has a high affinity for
thermal neutrons. The resulting thermal neutron flux will depend
upon a number of factors, both creating and absorbing
thermal neutrons, but most importantly will be how much
hydrogen is present. The neutron probe may thus be used as a
measuring device for moisture in the soil, but it may require
calibration for local soil conditions.
Field Calibration
A field calibration requires the probe, a volume sampler, a
scale and a drying oven. Install the access tube in a
representative point in the soil. Take probe readings in the tube
and volume samples in pairs around the tube. Take them at the
same depth and within a foot or two of the tube.
Seal the volume samples in a sample can or plastic seal bag
immediately after removing from the soil. Be careful not to
