Nuclear density gauge
Encyclopedia
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. Those particles are effective in analyzing the top 10 inches (25 centimeters) with high accuracy. 226Radium
is used for depths of 328 yards (300 meters). 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 dependant 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, ( for Cs137, 30 years) 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 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 Separator (oil production)
in the same way. Gas, oil, water and sand all have different concentrations of hydrogen atoms which reflect different amounts of slow neutons. Using a head which contains an Am241Be 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 (scattered). 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 (scattered). 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 creating suitable soil environments to support structures, streets, highways, and airport runways.
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
, as well as for mining and archaeology purposes. It consists of a radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
source that emits a directed beam of particles and a sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
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
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
source emitter such as 137Cesium is used to produce gamma radiation. Those particles are effective in analyzing the top 10 inches (25 centimeters) with high accuracy. 226Radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
is used for depths of 328 yards (300 meters). 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 dependant upon the shielding provided by the liquid in the pipe. Tracerco
Tracerco
Tracerco is the oil and gas services subsidiary of British chemical company and conglomerate Johnson Matthey.- History :In 1958 ICI established a division to research the use of ionizing radiation within chemical processes...
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, ( for Cs137, 30 years) 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 to produce Neutron radiation
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
and then measure the energy of returning neutron scattering
Neutron scattering
Neutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...
. As hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
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 Separator (oil production)
Separator (oil production)
The term separator in oilfield terminology designates a pressure vessel used for separating well fluids produced from oil and gas wells into gaseous and liquid components. A separator for petroleum production is a large vessel designed to separate production fluids into their constituent components...
in the same way. Gas, oil, water and sand all have different concentrations of hydrogen atoms which reflect different amounts of slow neutons. Using a head which contains an Am241Be 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 (scattered). 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 (scattered). 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 creating suitable soil environments to support structures, streets, highways, and airport runways.