TM 5-6635-386-12&P
backscatter
operations
and
permit
simultaneous
counting of both density and moisture.
Approximately 90% of the moisture count is obtained
from returned thermal neutrons from the closest 6 inches
of soil.
1-3. PRINCIPLES OF OPERATION
General
Various elements, both naturally occurring (Radium) and
reactor produced (Cesium and Americium) are unstable
and are slowly decaying to a more stable state. The act
of
decay
produces
emissions
of
energy
upon
disintegration of the atoms.
These emissions are either "rays" of electromagnetic
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
radiation for purposes of nuclear soil testing.
Gamma radiation is emitted in several energy levels by a
sealed Radium source and in a single energy level by a
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.
Cesium, a reactor produced isotope requires a license
for use anywhere in the U.S. and in foreign countries.
Neutron emission occurs when an alpha particle emitter
(Americium, Plutonium, or Radium) is mixed with
Beryllium powder in a tightly compressed pellet. The
alpha particles strike the Beryllium atoms to produce fast
neutrons of an average energy of 5 million electron volts.
The suffix "Be" is attached to the alpha source name to
denote its use as a neutron source when it is mixed with
Beryllium (RaBe, AmBe, PuBe).
These emissions are detected by appropriate detectors
(Geiger Mueller tubes) for gamma and (Boron Tri-
fluoride-BF3
or
Helium-3
H3
tubes)
neutron
measurements. The resultant signals are displayed
electronically as an index of soil density and moisture.
Radioactivity, both gamma and neutron, may be thought
of as being similar to light from an incandescent bulb.
The light rays diminish rapidly as we move away from the
bulb (by the inverse square of the distance from the
lamp), and they have the ability to penetrate various
materials to some degree, ranging from nearly complete
penetration (glass) to nearly complete blockage (metal
shield).
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
more absorbing material (shielding) we place between
ourselves and the source, the safer we are. It is
theoretically impossible to shield any radioactive source
completely;
however,
careful
tester
design
and
appropriate choice of shielding materials can reduce the
radiation to an acceptable level with negligible absorption
by the user under proper operating procedures.
Gamma Radiation
Gamma radiation is electromagnetic "photon" energy
capable of penetration of several inches of most
materials. It is essentially high energy "light ray" energy.
It is useful for the total mass measurement of heavy
materials and is used to determine total density of soil.
Neutron Radiation
Neutron radiation consists of small, noncharged particles
emitted from the source at an average energy level of
approximately 5.0 MEV. This is known as "fast" neutron
emission. Neutron detectors "see" only slow, or
"’thermal" neutrons. Therefore, the fast neutrons must
slow down or they will be ignored by the detectors.
Neutrons slow down by colliding with other objects much
like a rifle bullet ricocheting from rock to rock.
Collision of the fast neutrons with the nuclei of large
atoms results in rebounding of the neutrons with little
loss of energy. Collision with the orbiting electrons
(approximately 1/1840th the weight of a neutron)
produces little loss of energy. However, collision with an
1-3
