Nuclear gauges in industry

Nucleonic gauges have several applications in modern industry. The following are some examples.

LEVEL GAUGES
In the case of a dipstick, the source is placed on one side of a container to be filled and the sensor against the opposite wall, as shown in Figure 1. When the liquid rises to intercept the line between the two instruments, the amount of radiation received by the detector drops sharply. This technique is used for example to fill cans of soda or lighters. The attenuation of the signal on the detector can determine the level of filling of the container and can cause automatic shutdown, further filling, or an alarm. The radionuclides used depend on the characteristics of the container and the content. As applicable, Am-241 (activity around 1-2 GBq), Cs-137 (activity around tens MBq) are generally used.

Figure 1 - Operation mode of liquid fill height gauges.

Nuclear density and moisture gauges
Nuclear density gauges are used in civil construction and the petroleum industry, as well as for mining, archaeology purposes and agronomy soil studies. Effective control of compaction of soil and stone layers is an important factor in the construction of roads and other types of foundations for civil engineering structures. In building a road there are many layers that help make it strong enough to support big trucks and cars. Each layer must have the right density and layers of gravel must have the right amount of moisture. Construction crews measure density and moisture with portable nuclear gauges. Gauging systems based on neutrons have been developed to determine moisture in soil. In addition, devices containing a neutron source and a gamma source are used to perform simultaneous measurements of soil bulk density and water content, as shown in Figure 2. Such devices have a

fast neutron source (often Am-241/Be) with a slow neutron detector (e.g. ³He chamber), and a gamma ray source (often Cs-137) with a Geiger-Mueller-type detector. Depth probes and surface probes are available. In this case, shields are generally made of lead and a hydrogen-containing material. With respect to conventional methods, they are easy to use and enable to perform fast, non-destructive and repeatable analyses.

Figure 2 - Portable nucleonic gauge used to measure moisture and density typically in soil and asphalt.

THICKNESS GAUGES
Gauges based on radioisotopes are widespread also in industries producing plates of different materials. For example, the modern steel factories use such nucleonic gauges to measure the thickness of the metal plates coming out of rolling mills at each stage of production. Without radioisotopes, it is unlikely that automation could have developed in these industries. For metals, sources are made of Cs-137 with an activity of about 1 TBq. In the case of plastic films, paper, gum sheet, clothes and aluminium plates, thickness can be measured by beta radiation. This is widely used in paper making industry for the control of the weights of paper: a beta electron beam passes through the paper and is received on a detector located vis-à-vis. The attenuation of the signal on that detector measures the paper density and thus its weight. The sources used are generally Kr-85, Pm-147 and Am-241 with activity levels lower than 3 GBq.

INDUSTRIAL DUST GAUGES
To measure the concentration of dust in the air, nucleonic gauges uses the attenuation of beta particles by deposited dust on a filter paper. In these dust monitors, Pm-147 or C-14 beta sources of less than 3.7 MBq are employed. Nucleonic gauges with such low activity sources have been increasingly employed in recent years in mining, environmental monitoring and construction works in the civil engineering.

WELL LOGGING
Nucleonic gauges are widely exploited also in the oil and gas industry, metallic ores prospecting and exploitation, hydrogeology, in specific borehole logging tools. Well logging is an evaluation technique which provides fast and detailed data on geological formations for the exploration of mineral resources, such as oil, gas and coal. The well logging operation involves the use of sealed radioactive sources and portable mini-neutron generators in a suitable logging tool lowered into the boreholes drilled for exploration. The commonly used gamma sources for this purpose are Cs-137, Co-60, Am-241, while the neutron sources are Am-241/Be and Pu-239/Be. During measurements, the probe is lowered to the desired depth in the soil inside an aluminium access tube that is “transparent” to fast neutrons, which are scattered by the soil within 30 to 50 cm of the source. As a result of this scattering, the neutrons lose energy and are slowed down. This interaction is used to estimate moisture content or other parameters. In the oil and gas industry portable nucleonic gauges find a lot of applications: they can be used on-site to analyze the walls of dug holes to identify mineral deposits, or to search for underground caves or other formations that could make a building site unstable.
When a portable radioactive gauge is used at a different site, a person responsible for the use, the secure storage and the emergency procedure has to be defined. Since they contain a radioactive source, portable gauges must be carried in transport packages that comply with national and international regulations. In particular, packages as well as the vehicles carrying nucleonic gauges must be labelled to indicate the dose rate on the surface, and at 1 m from the package (Figure 3).

Figure 3 - Transport package and labels for the transport of nucleonic gauges.