Radiography using gamma rays, instead of the normal X-rays. Gamma rays are emitted by radioactive bodies and have a shorter wavelength than X-rays—i.e., around 10-10 cm.—they are therefore more penetrating. This form of radiography is now extensively used in industrial non-destructive examination of castings, welds, and moulded parts.
The most widely employed sources of gamma radiation are radium and the gamma-active isotopes produced in atomic reactors. These are supplied in the form of cylinders of 1 to 6 mm. of isotopes of metals such as antimony, cobalt, iridium and tantalum. The gamma energy of the source and its other properties—e.g., half-life and activity per grain—are specified according to the type of specimen to be inspected.
One of the principal advantages of isotopes as a source is that they can be used in positions ' where it would be impossible to accommodate an X-ray tube—e.g., inside pipes and on welded girder-work.
The technique is simple: an X-ray type of film in a holder, usually sandwiched between lead foil intensifying screens, is placed against the subject, and the isotope is uncovered on the opposite side so that the radiations pass through the subject before falling on the film. By suitably choosing the characteristics of the source and the time of exposure, the resulting image reveals variations in the internal structure of the specimen—e.g., flaws, scale inclusions, and cavities hi welds and castings.
Where a number of small items have to be examined in this way, the isotope is simply surrounded by any number of specimens, each one associated with its own film and screens. All are then exposed at the same time.
Isotopes for gamma radiography are dangerous to handle; they are normally supplied in lead bombs which are opened from a distance with tongs or other remote handling equipment only when everything has been placed in position and no one is left within range. The exposure is made by uncovering the isotope and leaving it—often for a day or more—in a locked room surrounded by the specimens to be radiographed. At the end of the calculated exposure time the bomb is again closed up before anyone is allowed near the specimens.
Gamma radiations are used in the treatment of cancer but not normally for the radio-graphic examination of the body.