The Science Behind Gamma Ray Sensors and Their Applications
Nuclear gauging is used world-wide in almost every industry. From mining and ore-grading to finished product, the metallurgical industry is one of the biggest users of gamma ray sensors for inventory, process control, and quality assurance. Learn the basic theory of gamma transmission gauging techniques found useful in mining, milling, and metal production for the purposes of level and density measurement applications.
Properties of gamma rays
Gamma rays are part of the electromagnetic spectrum which includes infrared, microwaves, visible light, and other waves. Packets of gamma ray energy are called photons. Since we cannot see, smell, or feel gamma radiation, it is sometimes easier to understand its properties by comparing it to one of the more familiar forms of electro-magnetic radiation—visible light.
Figure 1 shows light shining through a thin piece of tinted glass onto a detector. Many factors affect the intensity of light sensed at the detector. Some light is reflected form the surface, some absorbed within, some scattered, and the remainder transmitted to the detector. If the glass thickness is increased, or if a darker piece is inserted, the intensity of light at the detector will be reduced. The same phenomena are observed when gamma rays are directed at a process vessel or pipe—some gamma rays are scattered, others absorbed, and some pass through. The degree to which a gamma ray beam is attenuated by any material is reflected to the material density, thickness, and a physical parameter called the mass absorption coefficient. If we measure the intensity of visible light at various distances from the source without any glass present, we find that the light intensity is inversely proportional to the square of the distance from the source. The same inverse square law applies to gamma radiation.
An important factor which must be considered in gamma radiation measurement is that the radiation measurement coming from a gamma source exhibits statistical variation, similar to a flickering light source. Hence, instantaneous readings are of little value; the measurement must be made over some period of time for acceptable repeatability. This fact will drive source sizing and time constant as a sensor is engineered for a specific application in level or density.
Another property of the gamma source is that the amount of radiation it emits slowly decreases with time. This property is called “source decay.” The rate of decay depends upon the radioisotope being used and is expressed as “half life”, the amount of time required for the source strength to be reduced to one-half of its original value. Since the half-life of a radioisotope is well known, it can be compensated easily via algorithms in the sensor microprocessor.
The penetrating power of a gamma ray is determined by its energy, commonly expressed in MeV (1 MeV = 1,000,000 electron volts). Selection of a radioisotope for a particular application is based on gamma ray energy, its half-life, and energy for some common radioisotopes used in industrial sensor gauging.
Any measurement made by an industrial gamma ray sensor is analogous to the measurement of visible light. What is actually being measured is the change in radiation from some reference condition (empty vessel or process pipe full of water for example), and the condition which exists when the measurement is being made (vessel full or solids added to the water.)
For example, consider a radiation source and detector positioned to shine through a process material as shown in Figure 2.
When material is interposed between the source and detector, a portion of the gamma radiation is scattered and/or absorbed, so that the radiation reaching the detector is reduced. For a well collimated beam, the amount of radiation reaching the detector can be calculated using the formula below:
An increase in either material thickness or density lowers the radiation intensity at the detector. The mass absorption coefficient is essentially constant for most industrial mining applications.
Advantage of gamma ray sensors
In a typical sensor installation, the source and detector are mounted externally to the pipe or vessel; hence, there is no physical contact with the process.
The process material may be corrosive or abrasive and at very high or low temperature. In many such applications the gamma sensor is often the only practical method of making the desired measurement. In all cases installation and maintenance is performed with no interruption to the process. This major long-term cost advantage should always be considered when equipment is specified.
Continuous level gamma transmission sensors
Continuous level sensors operate over spans from a few tenths of an inch to twelve feet or more and are installed external to process vessels. Many such sensors can be certified for use in hazardous environments. The most common configuration is for the source to be on one side of the vessel 180º from the active top of the detector. A fan beam, typically 45º, is transmitted across the vessel from the source to the detector. As the level of the process rises, radiation reaching the detector decreases. The transmitter interprets the decreasing radiation level as an increase in material level.
Continuous level applications
Typical examples of continuous level sensor application in mining and metallurgical processes are:
- Smelter furnace charge level control
- Control of continuous casting machines
- Uniform bed depth on sintering machines
- Bed depth control in sinter clinker coolers
Some of these gamma ray sensor applications are discussed in more detail in Appendix I.
Density gamma transmission sensors
A typical density sensor configuration is for the source to be mounted on one side of the pipe with the detector on the opposite side of the pipe. This configuration is accomplished via a pipe saddle or in some cases a chain mount. In referring to the formula in Figure 4 in the density configuration, the material thickness (t) does not vary since the source and detector are fixed to the pipe. The mass absorption coefficient (u) is assumed constant. Any change in the radiation intensity occurs only when the density (P) of the process varies.
To measure density inside a large vessel, it may not be practical to place both the source and detector outside the vessel due to an excessive large source being required in which to get radiation across the tank to the detector.
For this type application for gamma transmission, an insertion source placed in a thermo well inside the vessel would be used.
Nuclear density gauges are employed for gases, liquids, slurries, solutions, and emulsions. Since no contact is ever made with the process material, caustic, corrosive, and abrasive process materials can be measured easily.
Density sensor applications
- Thickener underflow density measurement and control
- Control of flotation process
- Control of heavy media separation process
- Solvent extraction process
- Coal bulk density control prior to coking
- Control of cyclone classifiers
- Acid concentration control in pickling baths
Gamma transmission applications are more numerous in mining and metallurgical applications than gamma transmission. These applications are discussed in more detail in Appendix I.
Gamma backscatter gauges
Gamma backscatter offers the benefits of being outside the pipe or vessel and using small sources, yet with very good precision offerings. Additionally, it offers the benefit of installation without affecting the process. Applications such paste tanks, slurry tanks, settling tanks, and tailings lines are well suited for gamma backscatter.
The basics of gamma backscatter come from Compton scattering. The scattering of photons from charged particles is called Compton scattering after Arthur Compton who was the first to measure photon-electron scattering in 1922.
The Compton Effect is the increase in wavelength which occurs when photons with energies of around 0.5MeV to 3.5MeV interact with electrons in a material. Compton's experiment became the ultimate observation that convinced all physicists that light can behave as a stream of particles whose energy is proportional to the frequency. When the incoming photon gives part of its energy to the electron, then the scattered photon has lower energy and according to the Planck relationship has lower frequency and longer wavelength. The wavelength change in such scattering depends only upon the angle of scattering for a given target particle.
When gamma rays are incident upon a target inside a vessel a portion of the energy gets scattered as per Compton Effect. The amount of energy that gets scattered is proportional to the density of the material. The key is to capture this energy, which is very small in nature, and amplify it to get meaningful information about the target. A patented gamma ray detector has been developed to capture this backscatter energy and derive the density of the target material. For this measurement the source head and the detector are both located on the same side of the vessel. A patented specialized source head has been developed to allow for the gamma energy to only hit the target and to prevent any direct energy to reach the detector.
As seen from Figure 5, the gamma backscatter technique provides information about the target material by taking information from only a small section of the vessel. Hence the amount of energy that is needed or the source size that is needed is much smaller than that required for the transmission method. As a result of which the Gamma Backscatter method can be used to measure the density of material stored in very large tanks that would have been practically impossible with the transmission method. The Gamma backscatter method also can now be used to measure very viscous material with very small source sizes, rendering it as a very safe environmental solution.
Experimental study has so far revealed that the gamma backscatter technique can be used to detect material densities from 0 to 3 g/cc for vessels with steel wall thicknesses of up to 2 inches. The measurement precision will be >=0.001 g/cc based on the target density and the wall thickness. Figure 6. shows a typical density curve for varying densities with a constant wall thickness.
Based on this new technique customers now can use it for the following applications with much lower source sizes:
a) Density at a point:
Densities of 0-3 g/cc can now be measured using source sizes of 100 mCi or less for vessel wall thicknesses up to 2 inches. Figure 7 shows some typical density applications where the gamma backscatter technique can be used.
b) Continuous level:
The gamma backscatter technology can also be used to measure continuous level of targets whose densities vary from 0-3 g/cc with vary low source sizes. With one source head a maximum of 4 ft of level can be determined and 3 source heads are needed to provide a continuous measurement of up to 12 ft.
c) Profile measurement:
Gamma backscatter technology can also be used to do density profiling provided the density change is larger than the detectors. Multiple detectors are used in such applications to provide density information at various sections of the tank.
Gamma transmission level measurement applications
Applications in which gamma transmission technology has been applied include the following:
1. Sinter plants operations
The sinter plant use fines, natural ores, and iron bearing particles recovered from the blast furnace as raw materials. These materials, along with coke, are premixed and spread on the moving bed of the sinter machine and then ignited. Air is drawn through the bed material to create a fused cake that is cooled during the final processing stages. There are several level measurements which must be maintained to ensure optimum operation of the sinter machine.
2. Uniform sinter bed depth
A level control on the roll feeder bin is used to provide uniform strand thickness on the sintering machine by adjusting strand speed. The continuous sensor source is located on one wall of the roll feed hopper with a detector on the opposite wall. Mounting the sensor components external to the hopper eliminates error in readings caused by build-up on probe type devices and protects the sensor from the abrasive material.
3. Clinker cooler bed depth
Optimum operation of the clinker cooler demands a constant clinker bed depth. Voids or excess depths result in extreme temperature variations of the screened clinker and can cause damage to the material handling equipment. A continuous level sensor can be used to maintain consistent bed depth of the cooler by adjusting its rotational speed. The primary advantage of the nuclear level sensor, in this application, is the ability to measure level across a broad expanse and to average the level of the lumpy bulk product. Frequently, water cooling of the detector is required to protect it from occasional excessive ambient heat conditions.
Gamma transmission density measurement applications
Thickener underflow density measurement and control
Thickeners are used extensively in mining and steel mills to separate particulate or suspended solids from effluent waste water streams. The underflow of a thickener is normally pumped to a filtering system where most of the remaining water is removed. The lower the solids content of this material, the greater filtering capacity is required to handle the material. The higher the solids content, the more likely that the pumps will bog down and subsequently the lines will plug. It is therefore necessary to control the solids content at some desirable intermediate level. The density sensor measures the underflow density and provides the signal to control the pumping rate so that constant solids content can be maintained. Operating in conjunction with a flow meter input, the density sensor can provide mass flow of the dry solids content of the thickener underflow.
Control of flotation processes
Efficient, economical operation of a flotation circuit requires maintaining the proper concentration of reagents and the optimum solids content in the feed slurry. Use of too much reagent increases costs. Use of too little reagent, or operation with slurry of non-optimum solids content, reduces yield, increases energy for pumping, and may increase water waste output. Reagent concentration may vary and /or need to be adjusted as the composition and/or feed rate or the raw material varies. Density sensors are mounted at critical points in the flotation circuit to monitor and control parameters such as reagent concentration and solids contents of slurries. A density sensor may be used in conjunction with a flow meter as a mass flow system to measure and control the flotation feed.
Control of heavy media separation
In HMS processes, crushed ore or coal is mixed with a fluid medium (either a liquid or a slurry) whose specific gravity is between that of the material to be recovered and that of the gangue (waste). The lighter material floats in the medium and is carried out of the separator with the overflow. Because the materials to be separated are heavier than water, the separation medium itself must be denser than water.
One of the most commonly used mediums is slurry of magnetic in water, but solutions of inorganic salts (e.g., ZnCl) and slurries of other materials are also used. For most efficient operation of such a process, the specific gravity of the separation medium itself must be held within narrow limits. However, the specific gravity of the medium tends to change continuously as slimes and/or fines accumulate; as material is lost through outflow; and, if the process material is a slurry, as water enters the separator with the ore or coal. A density sensor mounted on a recalculating loop monitors the density of the medium in the separator and produces a signal to control the feed of the medium so as to maintain the desired specific gravity.
Materials treated by HMS processes include, but are not limited to:
|Anthracite coal||Iron ores|
|Bituminous coal||Zinc ores|
|Dolomite||Slag (steel mill)|
Solvent-extraction is a simple and efficient process for recovering uranium from low-grade ores. In this process, the ground ore is leached out with sulfuric acid to get uranium into solution. The solids remaining after the leaching are separated from the pregnant acid in thickeners, washed, and discharged to the tailings pond as slurry. The pregnant acid is then stirred with an organic solvent which has a great affinity for uranium and non-cohesive with the aqueous sulfuric acid solution. The solvent strips the uranium from the acid liquor. In a typical process (there are several variations), the pregnant solvent is mixed with sodium carbonate solution to extract the uranium from the liquor. Uranium is extracted from the carbonate liquor by precipitation followed by filtration to recover the yellow cake.
Although the solvent extraction process is simple, there are several areas where monitoring and control of process variables is important. The solids content of the slurry fed to the leaching agitators’ should be held within prescribed limits for efficient operation; one manufacturer of recovery equipment recommends 55% to 60% solids for best results. A density sensor on the slurry-fed line to the leaching agitators’ monitors solids content and controls the addition of water to maintain the metallurgical desired percent solids in the feed.
It is important to also control the solids content of the pulp which passes through the cascade thickeners; in at least one process the pulp is diluted down to 10%–15% solids by pumping some of the floatable liquid back to the input of the thickener circuit. Multiple density sensors between the thickeners monitor the solids content of the pulp and control the feed of diluents to maintain the desired solids. It may be useful to control the draw-down from the precipitate storage tank and/or the last thickener with a density sensor. The sensor will monitor the solids content of the slurry during draw-down and shut off the pump when the solids content falls below a predetermined level, thus minimizing the quantity of too-dilute slurry and the waste water that is pumped.
Coal bulk density control
In the production of coke, crushed coal is filled into coking ovens. Usually a great number of coking cells (ovens) are arranged side by side. A moveable conveyor belt feeds the crushed coal from the top into cells. Coal bulk density will vary as a function of size, moisture, and compaction. Coal usually ranges in density from about 42 to 52 lbs/ft³.
The higher density coal has a larger coefficient of expansion. If a coking cell is filled with this type of coal, the expansion might damage the cell. Conversely, if low-density coal is used and the cell filled to a tolerable level for high-density coal at the end of the combustion cycle, the cell would not be completely filled. Constant bulk density results in more uniform coke size and strength, and maximizes cell output. Bulk density of coal can be controlled by the addition of oil or water in a blending drum prior to loading the coking ovens. A density sensor, mounted at a 45º angle to the conveyor axis to improve sensitivity, measures the coal bulk density as it is carried to the ovens. The density sensor output is used to adjust the oil/water addition to maintain constant bulk density.
Optimize cyclone separation
Wet cyclone classifiers to improve yield are used extensively in mining, metallurgical, and other material processes. The feed slurry enters into the top of a cylindrical section of the cyclone, where a spiral motion creates a centrifugal force that separates the heavy particles from the stream. The lighter particles move to the center of the cylindrical section and are carried away through the top overflow, while the heavier particles fall toward the outside and are carried away at the underflow. Typically, a density sensor is used to monitor the cyclone feed to insure constant volume and percent solids. The overflow density is also monitored. In heavy media cyclones, a density sensor is used to control medium density to insure a constant separation point.
Typical applications include, but are not limited to:
- Ore upgrading
- Ash and/or sulfur separation in coal
- Drill chip removal from oil drilling mud
- Sizing of ores, abrasives, pigments
- Cement manufacture
Pickling acid control
In a steel plant, molten steel is cast into product by continuous casters and sometimes finished further in rolling mills. To clean or pickle the steel after the rolling process, hot acid solutions are used. Efficient operation of the pickler is achieved when the acid concentration is maintained within the limits required for cleaning the product. A density sensor, mounted on a re-circulating line, measures the density of the pickling solution and controls the addition of acid to insure the optimum acid concentration is maintained.