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Mobile radionuclide gauges


There are several different kinds of mobile measuring equipment that utilise ionising radiation in the form of x-rays or gamma, neutron, alpha or beta radiation. Generally speaking, the radioactive sources used in this equipment are medium and low activity sealed sources, and the procurement, use and handling is required to be formally notified to NRPA in accordance with radiation protection regulations.

There are several different kinds of mobile measuring equipment that utilise ionising radiation in the form of x-rays or gamma, neutron, alpha or beta radiation. Common to this type of measuring equipment is the utilisation of the information that lies in the ability of different substances to absorb or reflect radiation. The radioactive sealed source as a rule constitutes part of the measuring instruments themselves, and the beam is directed towards the object or sample that is to be analysed. Generally speaking, the radioactive sources used in this equipment are medium and low activity sealed sources, and the procurement, use and handling is required to be formally notified to NRPA in accordance with radiation protection regulations.

Undertakings wishing to procure use or handle ionising radiation sources for purposes and in contexts other those mentioned under section 5 shall notify the Norwegian Radiation Protection Authority.

Field measurements
The following section gives a closer description of three different kinds of portable measuring devices with radioactive sealed sources intended for so-called field measurements. There are however a number of other kinds of instruments that utilise ionising radiation, both for use in the field and as semi-stationary laboratory instrumentation. The IAEA category of the sources is indicated in the overview. This is an international 5-level categorization scale which gives the level of risk associated with use of the source. The most dangerous sources are classified in Category 1, while the least dangerous are in Category 5.

Instrument for measuring thickness and moisture in road surfaces
The sources used in this type of work are usually the radioactive isotopes Cesium-137 and/or Americum-241 built into a portable instrument. When in use, this instrument is placed on top of the road surface and the radiation can then be sent down into road surface. Alternatively, the radioactive source itself can be located at the end of a long rod which can be guided down into the road base. The thickness and/or moisture content is gauged by analysing the radiation that is picked up by the detector inside the instrument.

Instrument for measuring thickness and moisture in road surfaces. Photo: NRPA.

There will always be a certain level of radiation around such an instrument when it is in use, but this decreases rapidly with distance. Spending time unnecessarily in close proximity to the instrument while it is in use should be avoided.

The radioactive sealed sources for this area of use are typically classified in Category 4.

Liquid level gauges
Measuring the level of liquids in different small containers, or the filling level in fire extinguishers and so forth can be accomplished with the help of small mobile gamma sources. The gamma source, such as for example 18 MBq Co-60, can be lined up on the one side of the container while the detector is lined up in parallel with it on the other side. The illustration shows a typical example of a liquid gauge.

During the use of such equipment, the operator will be exposed to a relatively minor amount of radiation. It is important nevertheless that the operator still maintains the greatest possible distance between himself and the radioactive source when using the equipment. When being stored, the radioactive source is placed in its shielded container.

These types of sources are typically classified as Category 4.

X-ray fluorescence analyses
These are small handheld instruments for the purpose of identifying elements, i.e. determining what chemical elements are present in a sample. The principle for carrying out the measurement is based on the idea that the use of ionising radiation ”excites” the atoms in the unknown substance. The substance will then emit so-called characteristic x-rays that can be analysed in the instrument (x-ray fluorescence). This radiation is unique to and characteristic of each individual element, and thus serves as a fingerprint for the individual element.

Both x-rays and different kinds of radioactive sealed sources can be used as an exciting source inside the instrument. However, it is sources with extremely low energy x-rays or gamma rays that are used in practice, as these are the best suited for inducing x ray fluorescence. When the device is not in use, the radiation level on the outside of the instrument will hardly be measurable. During use, the hands of the operator may be exposed to radiation doses of typically less than 1 µSv/hour.

This area of use falls in the lowest IAEA class, that is, Category 5.