Have you ever wondered how paper, aluminium foil, or cling film is made? We use these products every day, but they're so thin that they can easily rip or crease. It's amazing how high-quality they are, considering their fragility.

Manufacturers have developed innovative ways to ensure the quality of these thin materials using non-contact quality control methods. With the help of radioactivity, they can inspect their products without even touching them! This technique is called thickness monitoring.

In this article, we'll discuss different ways to measure thin products, the physics and equipment used in radioactive thickness monitoring, and the industrial processes where this technique is applied. So, let's dive in and learn more about thickness monitoring!

Thickness monitoring methods

When you need to measure the thickness of something, a ruler can be used, but it's only accurate to about 1 mm. So, if you need more precision, you need a micrometre. This tool is like a vernier calliper and uses a precise screw thread to measure small distances with an accuracy of about 0.01 mm. You can get digital or mechanical versions of micrometres, and they're relatively cheap.

Micrometres are still used in low-volume manufacturing where processes are done by hand. But, for mass production, measuring with a micrometre isn't practical. It would take too much time and slow down the production line.

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In the picture above, the micrometre reading is 1.640, with a tolerance of +-0.005 mm. This means the actual dimension could be 0.005 mm larger (1.645 mm) or smaller (1.635 mm) than the measured dimension.

For accurate measurements on a production line, a robotic Coordinate Measurement Machine (CMM) can be used. It measures the 3D geometry of a manufactured part using a probe and is much faster and more accurate than a human operator. Robotic CMM inspection is useful for high-volume production of parts with complex geometry, like car engine components.

However, thin materials like paper only need their thickness monitored. A micrometre is too slow and could damage the material, and using a CMM system is too complex and could slow down production. To solve this problem, radioactivity-based thickness monitoring instruments have been developed. These instruments provide a fast, non-contact way of measuring the thickness of thin sheet materials.

Thickness monitoring using radioactive isotopes

In the picture above, the micrometre reading is 1.640, with a tolerance of +-0.005 mm. This means the actual dimension could be 0.005 mm larger (1.645 mm) or smaller (1.635 mm) than the measured dimension.

For accurate measurements on a production line, a robotic Coordinate Measurement Machine (CMM) can be used. It measures the 3D geometry of a manufactured part using a probe and is much faster and more accurate than a human operator. Robotic CMM inspection is useful for high-volume production of parts with complex geometry, like car engine components.

However, thin materials like paper only need their thickness monitored. A micrometre is too slow and could damage the material, and using a CMM system is too complex and could slow down production. To solve this problem, radioactivity-based thickness monitoring instruments have been developed. These instruments provide a fast, non-contact way of measuring the thickness of thin sheet materials.

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The relationship between material thickness and the amount of radiation attenuation can be used to measure thickness without contact. By using the initial radiation beam intensity and measuring the intensity of the beam after it passes through the material, the amount of attenuation and corresponding material thickness can be calculated.

The amount of attenuation that a given thickness of material causes depends on its attenuation coefficient. This coefficient is a material property that describes how much radiation is scattered or absorbed per unit thickness as it passes through the material. The attenuation coefficient μ is used in an equation to find the transmitted beam intensity. This equation is:

Transmitted Radiation Beam Intensity = Initial Beam Intensity x e^(- μ x d)

where:

• Transmitted Radiation Beam Intensity is the intensity of the radiation beam after passing through the material.
• Initial Beam Intensity is the intensity of the radiation beam before it passes through the material.
• μ is the linear attenuation coefficient.
• d is the thickness of the material or the distance it travels.

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 the most commonly used type of radiation for thickness monitoring applications. This is because it has the most suitable penetration power - alpha radiation would be blocked by even a thin sheet of paper, while gamma radiation is typically used for thicker sheet metal materials.

A radioactive source with a long half-life is ideal for this application as it means the rate of activity will be constant for a long time. This is useful because 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 how long 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.

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In a typical thickness monitoring setup, the sheet material passes through a set of rollers and then under a radioactive source with a known intensity. A detector on the side of the material measures the intensity of radiation that passes through the sheet and feeds this data to a computer processor.

Using the information from the detector, 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 then compared to the target thickness, and if the material is too thick or too thin, the computer adjusts the force on the rollers to correct the error.

This process can be repeated continuously, providing a real-time measurement of the thickness of the material as it is being produced. Any variations in thickness can be quickly detected and corrected, ensuring that the final product meets the required specifications. Additionally, the use of radiation for thickness monitoring allows for non-contact and non-destructive measurement, making it a safe and efficient method for quality control in manufacturing.

Thickness monitoring examples

The use of radioactive gauges is not limited to just controlling the thickness of sheet materials during manufacturing. There are several other non-contact inspection applications where the technology has found use. Here are a few examples:

• Paper production: Beta radiation is used to monitor the thickness of paper during production.
• Aluminium foil production: Beta radiation is used to measure the thickness of aluminium foil during production.
• Metal sheet and plate production: Gamma radiation is used to measure the thickness of metal sheets and plates during production.
• Fluid level monitoring: Gamma radiation is used to detect when a container has been filled to a certain level.
• Quantities/compositions of raw materials travelling on a conveyor belt: Gamma radiation is used to measure the quantities and compositions of raw materials on a moving conveyor belt.
• Analysing density distribution of materials within a closed container: Gamma radiation is used to analyze the density distribution of materials within a closed container.

There are several radioactive isotopes used in radioactive gauge applications, including:

• Krypton-85: A beta emitter with a half-life of 10.8 years.
• Caesium-137: A beta emitter with a half-life of 30.17 years.
• Americium-241: Primarily an alpha emitter, with gamma radiation as a byproduct and a half-life of 432.2 years. Used as a gamma emitter in radioactive gauge applications as the alpha particle is blocked by a few centimeters of air.
• Cobalt-60: A 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.

Thickness Monitoring

Why is beta source 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.

What is thickness monitoring?

Thickness monitoring is an industrial inspection process used to monitor the thickness of thin sheet materials as they are produced. A key difference between inspection and monitoring is that monitoring is performed in real-time, with live data used to control the manufacturing process.

Which isotope is used in thickness monitoring?

Different radioisotopes are used for different thickness monitoring applications depending on their exact requirements - however, typically a beta emitter with a long half-life is used. Some common isotopes used are:Krypton-85. Beta emitter with a half-life of 10.8 years.Caesium-137. Beta emitter with a half-life of 30.17 years.Cobalt-60. Beta emitter with a half-life of 5.3 years, decaying into Nickle-60 which emits gamma rays.

What are the thickness monitoring methods?

For thickness inspection, measurement tools such as a micrometre or coordinate measuring machine (CMM) can be used. However, for real-time monitoring a tool known as a radioactive gauge is used to measure the thickness of thin materials. This measures the amount of radiation attenuation a material causes, and calculates the thickness based on this.

How is radiation used in thickness monitoring?

A radioactive isotope is used to produce a beam of radiation particles. This beam is then directed at the sheet material, and a sensor behind the material measures the intensity of the beam that penetrates through the material. The amount the beam is blocked (attenuated) by the material corresponds to its thickness, and a computer can calculate the measured thickness in real-time.

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