Published 19.02.2008 , updated: 13.11.2009, 14:17
Non-destructive test methodsIndustrial radiography is a method for Non Destructive Testing, or NDT; that is, the material qualities of an object can be examined and studied without destroying the object.Photographing the object with the help of x-rays or gamma radiation makes it possible to discover different material defects such as poor welding seams or cracks. The method resembles medical use of x-rays to discover bone fractures.The picture below shows a typical situation in radiography where a gamma source is being used to inspect a welded seam on a pipe. There are about 100 companies in Norway authorised to carry out industrial radiography.Radioactive sources and physical protectionIn gamma radiography, the radiation comes from a radioactive source, such as Iridium-192, for example. The radioactive source is placed in a portable protective container during storage and transport, as shown in the picture. When in use, the source can be pushed forward to the radiating position with the help of a mechanical crank. After use, the source is then returned to its shielded position.Industrial radiography usually has to be carried out outdoors or within shielded enclosures, as shown in the figure. The surrounding area must then be cordoned off and the dose rate outside the barred area should not exceed 7.5 µSv/hour. The dose rate is controlled by the operators through the use of a hand monitor. In order to meet the barrier requirements, it is essential that the primary beam from the radioactive source or x-ray machine is shielded, because of the high radiation intensity in the beam. There will be radiation also in the area immediately outside the primary beam, and this is known as scattered radiation or secondary radiation. However, the intensity of this radiation is much lower, on the order of 1/1000 the intensity of the primary beam.A radioactive source can never be turned off; hence, upon the completion of exposure, the source must be redrawn into its shielded container. The operator must them check with the hand monitor that the source is safely back in its shielded position. An x-ray machine on the other hand ceases to emit radiation once the power has been shut off, and hence does not need any kind of protective shielding.