Thursday, 8 October 2015


                The principle of detecting defects is same as X-ray radiography. Gamma rays are emitted during the disintegration of radio active material and X-rays are electromagnetic radiation.Gamma rays have shorter wavelengths and are more penetrating than X-rays. The source for gamma radiations is usually the radioactive isotopes of cobalt-60 enclosed in a special container or capsule. Gamma rays radiography give better results for thicker materials. Now a days cheap radioisotopes are available and this test can be performed in a very short time and therefore this method is becoming more popular. However, there are some limitations of this method, e.g., Handling of radioisotopes and precautions required.

Gamma-ray radiography differs from X-ray radiography in the following aspects:

  1. The apparatus for Gamma-ray radiography is very simple and less costly than X-ray unit.
  2. Unlike X-rays, Gamma rays from its source are emitted in all directions, therefore, a number of separate castings having cassette containing film, fastened to the back of each casting, are disposed in a circle around the equipment placed in a central position. This way, many castings can be radiographed simultaneously and overnight exposures may be taken without continues supervision.
  3. X-rays are better than gamma rays for detecting small defects in casting sections less than about 50 mm.
  4. Gamma rays are used for detecting defects in castings thicker than those inspected by X-rays.
  5. X-ray method is much more rapid than Gamma-ray method, it requires seconds or minutes instead of hours.    

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