We use cheap, thin materials such as paper, aluminium foil, and cling film all the time – but have you ever considered how these are made? Thin sheet products like these are remarkably high quality, considering how easy they are to rip, crease, or damage when handled!
This has been enabled by developing non-contact quality control methods using radioactivity, allowing manufacturers to inspect their product without ever needing an instrument to physically touch it. This article gives an outline of different measurement techniques available, the physics and equipment used in radioactive thickness monitoring, and a summary of industrial processes where this technique is used.
Thickness monitoring methods
When you need to measure something’s thickness, a tool such as a ruler can be used to a resolution of about 1 mm – however, trying to measure by eye with any more accuracy than this is unreliable.
For measuring smaller dimensions, the micrometre (similar to a vernier calliper) was invented. These use a precise screw thread to measure small distances with a resolution of about 0.01 mm, and digital or mechanical versions are relatively inexpensive.
Micrometres are still widely-used in low-volume manufacturing where processes are performed by hand. However, for mass production, taking repeated manual measurements using a micrometre is impractical as it would place a significant bottleneck on the production line and reduce its speed.
Micrometre used for measuring by hand.
For performing accurate measurements on a production line, a robotic Coordinate Measurement Machine (CMM) can be used. These use a probe to measure the 3D geometry of a manufactured part and can be automated to perform the inspection much faster and more accurately than a human operator. Robotic CMM inspection is used in high-volume production of parts with complex geometry, such as components in a car engine.
However, thin materials such as paper only have one important measurement to monitor – their thickness. Using a micrometre for this is too slow and could easily damage the material, while a CMM system is unnecessarily complex and could still slow the rate of production. To solve this problem, specialised radioactivity-based thickness monitoring instruments have been developed to provide a fast, non-contact method of measuring the thickness of thin sheet materials.
Thickness monitoring using radioactive isotopes
Alpha (α), Beta (β) and Gamma (γ) are the main types of radiation, with each having different penetration power through materials. All three types of radiation are radiation particles, with alpha being the heaviest and most ionising and gamma being the lightest and least ionising.
Alpha (α) radiation, a helium nucleus, is typically blocked by a sheet of paper and will only travel a few centimetres through the air.
Beta radiation, a fast-moving electron, can penetrate through a sheet of paper but will usually be blocked by a thin metal plate.
Gamma radiation, a high-energy photon, is the smallest particle and has the most penetrative power, only being blocked by a thick metal plate of a material like lead or iron.
Thickness Monitoring: Radiation attenuation
When nuclear radiation is 'blocked' by a material, it has been attenuated to a level where it is no longer detected. A single particle of radiation may be absorbed by a material or it may pass through, with a higher probability of it being absorbed as the particle size increases and the material becomes thicker.
The material can either scatter or absorb radiation particles, and a larger particle or thicker material increases the chance of this happening as the particle passes through the material. However, as a source of radiation emits many particles, some may be absorbed by the material while a portion may pass through.
If 25 percent of the initial amount of radiation passes through the material, it has been attenuated by 75 percent. For a given material and type of radiation, the amount of attenuation caused by the material is proportional to its thickness.
As the thickness of a material increases, the proportion of initial radiation intensity that penetrates through it decreases. This is due to the thicker material increasing the probability that a radiation particle is attenuated (absorbed/scattered) by the object - Vaia Originals
The relationship between material thickness and the amount of radiation attenuation can be used to provide a non-contact measurement. By knowing the initial radiation beam intensity and measuring the intensity of the beam after it has passed through the material, the amount of attenuation and corresponding material thickness can be calculated.
The amount a given thickness of the material will attenuate a beam of radiation depends on its attenuation coefficient. This is a material property that describes the proportion of radiation that is scattered or absorbed per unit thickness as it passes through the material. The attenuation coefficient μ is used in the following equation to find the transmitted beam intensity.
Where:
is the transmitted radiation beam intensity
is the initial beam intensity
is the linear attenuation coefficient
is the thickness of material or distance travelled
As the radiation beam intensity can be measured continuously, this technique can provide a way to inspect thin materials in real-time as they are manufactured on a non-stop production line.
Radioisotopes used in thickness monitoring
Beta radiation is most commonly used for thickness monitoring applications, as it has the most suitable penetration power; alpha radiation would be blocked by even a thin sheet of paper, while gamma radiation will pass through most thin materials - although gamma radiation is sometimes used for thicker sheet metal materials.
A radioactive source with a long half-life is ideal for this application, as this means the rate of activity will be constant for a long time. This is useful as it means the radioactive source will not have to be changed often, and the activity level will be almost constant each day.
The half-life of a radioactive isotope describes the duration of time it takes for half of the radioactive nuclei in a given sample to decay. The rate of decay is described in this way because it decreases over time - as the number of undecayed nuclei decreases, the rate of decay also decreases.
Thickness monitoring instrument
The instrument needed to perform radioactive thickness monitoring is sometimes called a radioactive gauge.
When implemented in the manufacturing process for a thin sheet material like paper or aluminium foil, the radioactive gauge is generally used to both monitor and control the product thickness. These products are usually produced using rollers which form the material into a sheet through compression and drawing.
Diagram of a radioactive thickness monitoring instrument (radioactive gauge) incorporated into a roller production system. - Vaia Originals
After the sheet material has passed through a set of rollers, it passes under a radioactive source with a known intensity.
A detector on the other side of the material measures the intensity of radiation that passes through the sheet and feeds this data to a computer processor.
The processor calculates the material thickness based on the known initial radiation intensity, the measured intensity of radiation that passed through the sheet, and the attenuation properties of the sheet material.
This calculated thickness is compared to the target thickness. If the material is too thick or thin, the computer adjusts the force on the rollers to correct the error.
Thickness monitoring examples
The most obvious application of radioactive gauges is in controlling the thickness of sheet materials during their manufacture. Depending on the material being produced, these use either beta or gamma emitters depending on the penetration power needed. However, the technology has also found several other uses in similar non-contact inspection applications. Here are some examples.
Thickness monitoring applications
Thickness Monitoring: Other industrial applications
Fluid level monitoring – detecting when a container has been filled to a certain level (Gamma)
Quantities/compositions of raw materials travelling on a conveyor belt (Gamma)
Analysing density distribution of materials within a closed container (Gamma)
Commonly-used radioisotopes for gauge applications
Krypton-85. Beta emitter with a half-life of 10.8 years.
Caesium-137. Beta emitter with a half-life of 30.17 years.
Americium-241. Primarily an Alpha emitter, with gamma radiation byproduct and half-life of 432.2 years. Used as a gamma emitter in radioactive gauge applications, as the alpha particle is blocked by a few cm of air.
Cobalt-60. Beta emitter with a half-life of 5.3 years, decaying into Nickel-60 which emits gamma rays.
Thickness Monitoring - Key takeaways
- Thin sheet materials require a non-contact inspection method to monitor their thickness during production.
- A radioactive gauge can be used to perform thickness monitoring of sheet materials in real-time as they are manufactured.
- The radioactive gauge works by measuring the proportion of initial radiation intensity that passes through the sheet material, and uses the amount of attenuation to calculate the material thickness.
- The calculated thickness is compared to the target thickness, and the force on the rollers is automatically adjusted to correct any error.
- Radioactive gauges are also used for other non-contact industrial applications such as fluid level monitoring, measuring quantities of material travelling on a conveyor belt, and analysing the density distribution inside closed containers.
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