A nuclear density gauge is a tool used in civil construction and the petroleum
industry, as well as for mining and archaeology purposes. It consists of a
radiation source that emits a directed beam of particles and a sensor that
counts the received particles that are either reflected by the test material or
pass through it. By calculating the percentage of particles that return to
the sensor, the gauge can be calibrated to measure the density and inner
structure of the test material. Different variants are used for
different purposes. For density analysis of very shallow objects such as roads or
walls, a gamma source emitter such as 137Cesium is used to produce gamma
radiation. These isotopes are effective in analyzing the top 10 inches with high
accuracy. 226Radium is used for depths of 328 yards. Such instruments can help
find underground caves or identify locations with lower density that would
make tunnel construction hazardous. Nuclear density gauges can also be used
to measure the density of a liquid in a pipe. If a source is mounted on one side
of a pipe and a detector on the other, the amount of radiation seen at the
detector is dependent upon the shielding provided by the liquid in the pipe.
Tracerco pioneered the use of radiation to measure density in the 1950s and
determined that the Beer–Lambert law also applied to radiation as well as
optics. Gauges are normally calibrated using gas and a liquid of known density
to find the unknowns in the equation. Once it has been calibrated and as long
as the source detector alignment remains constant, it is possible to calculate
the density of the liquid in the pipe. One factor is the half life of the
radioactive source, which means that the system needs to be recalibrated at
regular intervals. Modern systems incorporate correction for source decay.
Another variant is to use a strong neutron source like
241Americium/Beryllium to produce Neutron radiation and then measure the
energy of returning neutron scattering. As hydrogen characteristically slows
down neutrons, the sensor can calculate the density of hydrogen - and find
pockets of underground water, humidity up to a depth of several meters,
moisture content, or asphalt content. Neutron sources can also be used to
assess the performance of a Separator in the same way. Gas, oil, water and sand
all have different concentrations of hydrogen atoms which reflect different
amounts of slow neutrons. Using a head which contains an 241AmBe neutron source
and a slow neutron detector, by scanning it up and down a separator it is
possible to determine the interface levels within the separator.
Nuclear density gauges are typically operated in one of two modes:
Direct transmission: The retractable rod is lowered into the mat through a
pre-drilled hole. The source emits radiation, which then interact with
electrons in the material and lose energy and/or are redirected. Radiation
that loses sufficient energy or is scattered away from the detector is not
counted. The denser the material, the higher the probability of interaction
and the lower the detector count. Therefore, the detector count is
inversely proportional to material density. A calibration factor is used to
relate the count to the actual density. Backscatter: The retractable rod is
lowered so that it is even with the detector but still within the
instrument. The source emits radiation, which then interact with electrons in
the material and lose energy and/or are redirected. Radiation that is scattered
towards the detector is counted. The denser the material, the higher the
probability that radiation will be redirected towards the detector.
Therefore, the detector count is proportional to the density. A
calibration factor is used to correlate the count to the actual density.
Many devices are built to measure both the density and moisture content of
material. This is important to the Civil construction industry specifically as
both are essential to verifying suitable soil conditions to support structures,
streets, highways, and airport runways. References
External links Nuclear Density Gauge
e.washington.eduModulesnuclear_gauge.htm Nuclear Density Gauge
http:www.troxlerlabs.comindex.php