backscatter operations and permit
counting of both density and moisture.
of as being similar to light from an incandescent bulb.
The light rays diminish rapidly as we move away from the
Approximately 90% of the moisture count is obtained
bulb (by the inverse square of the distance from the
from returned thermal neutrons from the closest 6 inches
lamp), and they have the ability to penetrate various
materials to some degree, ranging from nearly complete
penetration (glass) to nearly complete blockage (metal
1-3. PRINCIPLES OF OPERATION
Radiation obeys the same rules, although its penetration
capabilities are generally much greater than light. The
farther we are from the source, the safer we are, and the
Various elements, both naturally occurring (Radium) and
more absorbing material (shielding) we place between
reactor produced (Cesium and Americium) are unstable
ourselves and the source, the safer we are. It is
and are slowly decaying to a more stable state. The act
theoretically impossible to shield any radioactive source
of decay produces emissions of energy upon
completely; however, careful tester design and
appropriate choice of shielding materials can reduce the
radiation to an acceptable level with negligible absorption
These emissions are either "rays" of electromagnetic
by the user under proper operating procedures.
radiation (Gamma Rays) or are actual particles of
material (neutrons, for example). Other emissions are
produced from various radioactive materials; however,
we are concerned with only the gamma and neutron
Gamma radiation is electromagnetic "photon" energy
radiation for purposes of nuclear soil testing.
capable of penetration of several inches of most
materials. It is essentially high energy "light ray" energy.
Gamma radiation is emitted in several energy levels by a
It is useful for the total mass measurement of heavy
sealed Radium source and in a single energy level by a
materials and is used to determine total density of soil.
Cesium source. The Cesium level is 0.66 MEV and
requires less shielding than the multi-level output of the
Radium source. The fixed spectrum emission is also
superior for soil density determination purposes.
Neutron radiation consists of small, noncharged particles
Cesium, a reactor produced isotope requires a license
emitted from the source at an average energy level of
for use anywhere in the U.S. and in foreign countries.
approximately 5.0 MEV. This is known as "fast" neutron
Neutron detectors "see" only slow, or
Neutron emission occurs when an alpha particle emitter
"'thermal" neutrons. Therefore, the fast neutrons must
(Americium, Plutonium, or Radium) is mixed with
slow down or they will be ignored by the detectors.
Beryllium powder in a tightly compressed pellet. The
Neutrons slow down by colliding with other objects much
alpha particles strike the Beryllium atoms to produce fast
like a rifle bullet ricocheting from rock to rock.
neutrons of an average energy of 5 million electron volts.
The suffix "Be" is attached to the alpha source name to
Collision of the fast neutrons with the nuclei of large
denote its use as a neutron source when it is mixed with
atoms results in rebounding of the neutrons with little
Beryllium (RaBe, AmBe, PuBe).
loss of energy. Collision with the orbiting electrons
(approximately 1/1840th the weight of a neutron)
These emissions are detected by appropriate detectors
produces little loss of energy. However, collision with an
(Geiger Mueller tubes) for gamma and (Boron Tri-
fluoride-BF3 or Helium-3
measurements. The resultant signals are displayed
electronically as an index of soil density and moisture.