Geoscience Reference
In-Depth Information
moving neutrons, and the more intensive is the interaction, the greater is the number
and the fi nal concentration of slow neutrons that form a sort of a spherical cloud in
the soil. See Fig.
8.1
. The radius of the slow neutron cloud usually ranges between
10 and 15 cm, but in extremely dry soils it could be up to 30 cm. Smaller radii pre-
vail in wet loams and clays, while larger radii develop in dry soils of all textures.
The concentration of slow neutrons is measured by a special counter.
A fast neutron is slowed down the most when it collides with the proton of a
hydrogen nucleus. The principle of the strong conversion of fast neutrons into ther-
mal neutrons depends upon the similarity between a neutron and hydrogen - their
sizes and masses are virtually identical. When a tennis ball hits the hard surface of
a tennis court, it bounces and continues to move with a just slightly reduced veloc-
ity. However, if the ball hits another ball lying quietly on the court surface, the fl ying
ball loses its original velocity giving roughly half of its energy to the ball initially at
rest. Frequently, both balls fi nally roll on the surface approximately in the direction
of the fl ying ball before the collision. Or during tennis practice, after a tennis ball
hits the wall, it comes back to the practicing tennis player with only a slight reduc-
tion of its velocity. The change of the velocity of the ball after its collision with
another object depends upon the ratio of the mass of the ball to the mass of the
object. Our observation of the collision of fl ying ball with another initially at rest is
a rough model of what happens if a fast neutron hits a hydrogen nucleus. In reality
a very fast neutron must collide many times with many hydrogen nuclei in order to
slow down. After many such collisions, the neutron approaches the same velocity of
other neutrons that is consistent with the prevailing temperature of the soil. Hence,
Fig. 8.1
Schemes of neutron probe for measurement of soil water content in the soil profi le (
left
side
of the fi gure) and on the soil surface (
right side
). The source of fast neutrons is denoted by S
and the detector of slow neutrons by D
