Applications also include thickness measurements of aluminium foil paper…

The idea

Absorption of radiation can monitor the thickness of moving sheets in manufacture. A source and detector are placed on opposite sides of the material, and the detected count rate depends on how much material the radiation passes through. Alpha is unsuitable for most industrial foils because it is stopped by air and thin layers, so beta from a suitable source is used for thin sheets such as paper and aluminium foil, while gamma is used for thicker or denser steel. If the sheet becomes thicker, more radiation is absorbed and the count rate falls; if it becomes thinner, the count rate rises. A control system compares the count rate with a set value and adjusts rollers to keep thickness constant.

What earns the marks

• A radioactive source and detector are placed on opposite sides of the moving sheet, and the count rate indicates the thickness of material between them.
• Beta radiation is suitable for measuring thin materials such as paper and aluminium foil because beta is partly absorbed by small thicknesses.
• Gamma radiation is suitable for measuring thicker or denser materials such as steel because gamma is highly penetrating and is not completely stopped by thin sheets.
• A fall in count rate indicates increased thickness, while a rise indicates reduced thickness, because more material absorbs more radiation.
• The count rate is compared with a reference value and a feedback system adjusts the rollers to maintain the required thickness.
• The method is non-contact and can operate continuously on a moving production line.

Common mistakes and how to put them right

• Choosing alpha for thickness measurement.

  Put it right: Alpha is stopped by air and by very thin layers, so it cannot penetrate the sheet reliably.

• Saying that a higher count rate means a thicker sheet.

  Put it right: Greater thickness absorbs more radiation, so the count rate falls.

• Ignoring background count: background must be subtracted before comparing the measured count rate with the reference value.

Exam tips

• Name the radiation and the material together, for example beta for paper and gamma for steel, and justify the choice by penetration.
• Describe the sequence: source, material, detector, count rate, comparison, adjustment.
• State the relationship between thickness and count rate clearly, using falls and rises correctly.