Gamma radiography was introduced twenty years or so ago as a supplement to X-ray radiography to provide a means of inspecting castings, welded assemblies and other engineering structures for internal defects such as blowholes and cracks. This method of nondestructive testing has been developed alongside X-ray inspection and in general has been of special service where

Originally the source of radiation was of course a naturally radioactive material. In the early days of gamma radiography it was usual to employ either permanently active sealed radium sources or short-lived sources filled with the radioactive gas radon drawn from a radium source. The radiation emitted from radium or radon is of a strongly penetrating character and experience showed that it was particularly suitable for the examination of thick castings of steel or copper alloys.

Both these sources are still available but, since radioactive isotopes made artificially by irradiation in an atomic pile have become available, it has been more customary to employ cobalt 60, which is an isotope of relatively long half life (5.3 years) and has proved a most attractive alternative to radium. The radiation emitted from cobalt 60 approximates to X-radiation generated at million volts and, like that from radium, is suitable for examining heavy steel and iron castings up to six inches in section.

More recently other radioactive isotopes have become available, including iridium 192, tantalum 182, caesium 137 and thulium 170. Of all these, the isotope iridium 192 has been found to have many particularly attractive characteristics. It is prepared by a neutrongamma reaction which takes place when a sample of metallic iridium of the required shape is inserted in the atomic pile. At the same time iridium 194 is also produced but, as this has a half life of only 19 hours, the radiation from this isotope becomes negligible at the end of about one week. Iridium 192 which then remains, has a half life of 74.4 days.

The nature of the radiation emitted by iridium 192 has been discussed by R. Halmshaw (1) who points out that the energy distribution in the gamma ray spectrum is complex and is probably not far removed from that quoted by Muller (2) and his colleagues and shown in the table.

The work of Halmshaw suggests that a slightly larger proportion of the higher energy spectrum lines than are indicated by this table may be present, and from experiments with exposure curves he concludes that the radiation from iridium 192 is roughly equivalent to that from an X-ray tube running at a peak voltage of 800 to 900 kV.

From theoretical reasoning it is very difficult to compare the value of different sources of gamma radiation, but Halmshaw has shown that the radiation from iridium 192 is, as would be expected, inferior to 200 kV or 400 kV X-rays for detection of fine cracks, but that the flaw sensitivity, when considered in terms of contrast alone, is actually comparable with 400 kV X-rays for thicknesses from I to 3 inches of steel and, although considerably less than that with 200 kV X-rays, remains reasonable for thicknesses down to about inch steel provided that good film processing technique is followed.

More recently, W. H. Sansom (3) has confirmed from considerable experience that there is “a relatively high, and rather unexpected, degree of image contrast obtained when radiographing comparatively thin sections of steel [with iridium 192]—even down to 7 lb plate, which, with reinforcement, is approximately inch”. Sansom reports that in the radiographic examination of the shell or hull of a ship the use of an iridium 192 source “in conjunction with a medium-speed non-screen film between four and six thousandths lead screens, has produced results which, for all practical purposes, bear very favourable comparison with exographs taken under similar conditions. In the case of steel sections in excess of one inch, there becomes progressively less and less to choose between the exograph and gammagraph in terms of contrast and sensitivity and, if time allows, with the use of a fine-grain non-screen film for the gammagraph, the difference becomes negligible”.

He further points out that “it may be said from experience, with some degree of certainty, that, in spite of the considerably longer exposures necessary with an iridium 192 source of medium or low strength (the exposure times with a high-strength source may be very close to the corresponding X-ray exposure), the actual volume of work over a given period is little different from that achieved by the use of X-rays—in some cases, it may actually be higher. This comparison is set in terms of a relatively small department with a staff of three or, at the most, four operators working each unit. The reason for this is to be found in the great bulk and weight of an X-ray unit, with its necessary ancillary services of coolant and power, as compared to an isotope container or bomb”.