Radioactive Gauges

Instruments for monitoring industrial processes

Editorial review 2026

 

Modern industry, in order to remain efficient, has set very high standards for instruments that continuously monitor the stages of an industrial process. The aim is to measure and, when necessary, rapidly adjust the quality and specifications of products during their manufacturing process. Many process controls use radioactive gauges.

For example, controls are carried out on production lines to continuously monitor liquid levels in drums, the density of materials in containers, the thickness of metal sheets, foils and coatings, as well as the quantities and compositions of raw materials on conveyor belts.

Because of the penetrating nature of radiations such as gamma rays or neutrons, direct contact between the object being monitored and the source or detection device is not necessary. This makes it possible to carry out real-time, non-destructive measurements while the materials undergo the stages of an industrial process.

The operating principle of these various gauges is the same: the attenuation of the received signal compared with the emitted signal makes it possible to evaluate the quantity being measured.

 

In the case of a level gauge, the source is placed on one side of a container being filled and the detector on the opposite wall. When the liquid rises and intercepts the line between the two instruments, the amount of radiation received by the detector drops sharply. This technique is used, for example, to fill soda cans or lighters. The attenuation of the signal on the detector makes it possible to determine the filling level of the container and to automatically trigger the stop, continuation of filling or an alarm.

Gamma-ray absorption and the same principle are also used to measure the density and mass of materials contained inside a vessel.

The use of thickness gauges based on radionuclides is widespread in industries producing sheets of material. Without these radioelements, it is unlikely that automation could have developed in these industries. Modern steelworks use such gauges to measure the thickness of metal plates leaving rolling mills at every stage of production.

This is also the case in paper manufacturing, including the precise measurement of wet pulp during the first stages of production. For the basis weight of paper sheets, beta rays are used because gamma rays would be too weakly absorbed by the thinness of the sheets. A beam of beta electrons passes through the paper and is received by a detector positioned opposite. The attenuation of the signal on this detector makes it possible to determine the density of the paper and therefore its basis weight.

According to the Nuclear Safety Authority, the radioelements most commonly used for these applications are krypton 85, cesium 137, americium 241, cobalt 60 and promethium 147. The activities of the sources range from a few thousand (kBq) to several billion becquerels (GBq).