Gamma Radiography

Radiography (RT) is an NDT method that utilizes electromagnetic energy in the form of x-rays or gamma rays. The radiograph is a two-dimensional negative image of the object on a film that is sensitive to radiation exposure. Radiography has the ability to detect surface and subsurface defects and/or discontinuities.

Cobalt 60 (Co60) and Iridium 192 (Ir192) are the most commonly used industrial gamma ray isotopes which continuously emit gamma rays and are stored and transported in secured shielded exposure devices. An x-ray generator (also known as an x-ray tube) transforms electrical energy to produce x-rays, and is controlled to stop x-ray emission when the equipment electrical power is turned off.

Radiography is performed on most metallic and non-metallic materials including welds, weld overlays, castings, forgings, fittings, valves and components, pipe, machined parts, pressure vessels, oil storage tanks and structural steel. Common industries radiography is performed in include; power generation facilities, gas production facilities, pulp and paper mills, oil refineries and upgraders, pipelines and foundries. Radiography is also used extensively for determining the extent of internal and/or external corrosion/erosion in process piping, pressure vessels and valves. The radiographic film provides a permanent record of the object examined.

Due to the radiographic image being two-dimensional, the depth of a defect and/or discontinuity requires additional NDT which is typically performed using ultrasonic inspection. Access to both sides of the object is required, and the processed film image is viewed and interpreted under a variable high intensity white light.

The practical thickness limit for carbon steel gamma radiography is approximately 2.5" for Iridium 192 (Ir-192), and 9" for Cobalt 60 (Co-60), whereas the x-ray generator energy determines the depth of penetration which is typically up to an average of 1.5” with portable equipment. Materials thicknesses in excess of 1.5” require high energy x-ray generator fixed equipment.

Safety is a critical consideration when using either gamma ray devices or x-ray generators. Clearance of large areas adjacent to the subject material is required to ensure the safety of NDT personnel and the general public.