Geoscience Reference
In-Depth Information
we speak about the thermalization of fast neutrons and about thermalized or thermal
neutrons. In order to measure the concentration of these thermalized neutrons, a
special detector of slow neutrons has to be placed immediately next to the source of
fast neutrons. Pulses of slow neutrons are preamplifi ed and sent to the counting
system. Generally, the higher is the content of thermalized neutrons, the higher is
the content of soil water. A calibration between counted pulses and volumetric soil
water content is developed. Electronic drifts, temperature migrations, and counting
time variations are avoided by dividing each count rate by that of a reference mate-
rial placed inside of the protective shield. A typical standardized calibration curve
manifesting a straight line between count ratio and volumetric soil water content is
illustrated in Fig.
8.2
.
Because various amounts of hydrogen also occur in different clay minerals and
in different amounts and kinds of humus, each soil has therefore its own specifi c
calibration requirement. A typical average linear calibration can be adjusted for
each specifi c soil or fi eld location by measuring paired values of count ratio and
volumetric water content when the soil is relatively dry and wet during rainless and
rainy periods, respectively. Dealing with soils in arid regions, we can wet the soil by
ponding water on a small surface area of a few square meters bounded by low dikes.
The advantage of the neutron method follows from one simple example. We
placed two access tubes in two neighboring small fi elds with the same soil. Field
number one was plowed, fi eld number two was not plowed, and both were without
vegetation. The fi elds are on a large plain having groundwater level at a depth of
Fig. 8.2
Example of calibration curve of soil water content
ʸ
(cm
3.
cm
−3
) versus count ratio
CR
of
slow neutrons
