RADIOGRAPHIC TECHNIQUES
Introduction •
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This module presents information on the NDT method of radiographic inspection or radiography. Radiography uses penetrating radiation that is directed towards a component. The component stops some of the radiation. The amount that is stopped or absorbed is affected by material density and thickness differences. These differences in “absorption” can be recorded on film, or electronically.
Introduction •
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This module presents information on the NDT method of radiographic inspection or radiography. Radiography uses penetrating radiation that is directed towards a component. The component stops some of the radiation. The amount that is stopped or absorbed is affected by material density and thickness differences. These differences in “absorption” can be recorded on film, or electronically.
Electromagnetic Electromagnetic Radiation The radiation used in Radiography Testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see every day. day. Visible light is in the same family as x-rays and gamma rays.
General Principles of Radiography The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation. r adiation.
X-ray film
The film darkness (density) will vary with the amount of radiation reaching the film through the test object.
= less exposure = more exposure Top view of developed film
General Principles of Radiography •
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The energy of the radiation affects its penetrating power. Higher energy radiation can penetrate thicker and more dense materials. The radiation energy and/or exposure time must be controlled to properly image the region of interest. Thin Walled Area
Low Energy Radiation
High Energy Radiation
GEOMETRIC UNSHARPNESS refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area.
For the case, such as that shown, where a sample of significant thickness is placed adjacent to the detector, the following formula is used to calculate the maximum amount of unsharpness due to specimen thickness: Ug = f x b / a f = source focal-spot size ( 2 – 3mm ) a = distance from the source to front surface of the object
For the case when the detector is not placed next to the sample, such as when geometric magnification is being used, the calculation becomes: Ug = f x b / a f = source focal-spot size. a = distance from x-ray source to front surface of material/object b = distance from the front surface of the object to the
FLAW ORIENTATION
IDL 2001
Flaw Orientation (cont.) Since the angle between the radiation beam and a crack or other linear defect is so critical, the orientation of defect must be well known if radiography is going to be used to perform the inspection.
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If the radiation is not parallel with the discontinuity, the feature will appear distorted, out of position and less defined in the image.
Each radionuclide decays at its own unique rate which cannot be altered by any chemical or physical process. A useful measure of this rate is the half-life of the radionuclide. Half-life is defined as the time required for the activity of any particular radionuclide to decrease to one-half of its initial value. In other words one-half of the atoms have reverted to a more stable state material. Half-lives of radionuclides range from microseconds to billions of years. Half-life of two widely used industrial isotopes are 74 days
Radiation Sources Two of the most commonly used sources of radiation in industrial radiography are x-ray generators and gamma ray sources. Industrial radiography is often subdivided into “X-ray Radiography” or “Gamma Radiography”, depending on the source of radiation used.
Gamma Radiography •
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Gamma rays are produced by a radioisotope. A radioisotope has an unstable nuclei that does not have enough binding energy to hold the nucleus together. The spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter is known as radioactive decay.
Gamma Radiography (cont.) •
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Most of the radioactive material used in industrial radiography is artificially produced. This is done by subjecting stable material to a source of neutrons in a special nuclear reactor. This process is called activation.
The Isotope Material is loaded into a Stainless Steel Capsule and sealed by welding. The Capsule is attached to short flexible cable called a Pigtail
The Source Capsule and the Pigtail is housed in a Shielding Device referred to as an Exposure Device or Camera. Depleted uranium is often used as a shielding material for sources. The exposure device for iridium-192 and cobalt-60 sources will contain 45 pounds and 500 pounds of shielding materials, respectively. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. When the source is not being used to make an exposure, it is locked inside the exposure device.
Gamma Radiography (cont.) A hose-like device called a guide tube is connected to a threaded hole called an “exit port” in the camera. The radioactive material will leave and return to the camera through this opening when performing an exposure!
The end of the guide tube is secured in the location where the radiation source needs to be to produce the radiograph.
The crank-out cable is stretched as far as possible to put as much distance as possible between the exposure device
Gamma Radiography Source Connector - The Source cannot be exposed unless the Pigtail is connected to the Cable.
To make a radiographic exposure, a crank-out mechanism and a guide tube are attached to opposite ends of the exposure device. The guide tube often has a collimator at the end to shield the radiation except in the direction necessary to make the exposure. The end of the guide tube is secured in the location where the radiation source needs to be to produce the radiograph. The crank-out cable is stretched as far as possible to put as much distance as possible between the exposure device and the radiographer.
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To make the exposure, the radiographer quickly cranks the source out of the exposure device and into position in the collimator at the end of the guide tube. At the end of the exposure time, the source is cranked back into the exposure
One of the methods of controlling the quality of a radiograph is through the use of Image Quality Indicators (IQIs), which are also referred to as penetrameters. The IQI indicates that a specified amount of change in material thickness will be detectable in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference point, consistency and quality could not
FILM EXPOSURE CALCULATIONS Properly exposing a radiograph is often a trial and error process, as there are many variables that affect the final radiograph. Some of the variables that affect the density of the radiograph include: 1. The exposure time. 2. The distance between the radiation source and the film. 3. The material of the component being radiographed. 4. The thickness of the material that the radiation must travel through. 5. The amount of scattered radiation reaching the film. 6. The film being used. 7. The concentration of the film processing chemicals and the contact time. EXPOSURE TIME = SOURCE TO FILM DISTANCE X EXPOSURE FACTOR X FILM FACTOR Exposure Factor for 1” thick material is 0.29. Film Factor for AGFA D7 is 1.
This process may begin using published exposure charts to determine a starting exposure, which usually requires some refinement.
RADIOGRAPHIC DENSITY Radiographic Density, or Film Density, is a measure of the degree of film darkening. Industrial codes and standards typically require a Radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common Film Viewers. Above 4.0, extremely bright viewing lights is necessary for evaluation. Film density is measured with a
STRUCTURIX CERTIFIED DENSTEP is an X-Ray Film Step Tablet for the Calibration of Optical Transmission Densitometers or used for judging Film Density.
X-ray Radiography Unlike gamma rays, x-rays are produced by an X-ray generator system. These systems typically include an X-ray tube head, a high voltage generator, and a control console.
X-ray Radiography (cont.) •
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X-rays are produced by establishing a very high voltage between two electrodes, called the anode and the cathode. To prevent arcing, the anode and cathode are located inside a vacuum tube, which is protected by a metal housing.
X-ray Radiography (cont.) High Electrical Potential •
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The cathode contains a small filament much the same as in a light bulb. Current is passed through the filament which heats it. The heat causes electrons to be stripped off. The high voltage causes these “free” electrons to be pulled toward a target material (usually made of tungsten) located in the anode. The electrons impact against the target. This impact causes an energy exchange which causes x-rays to be created.
Electrons +
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X-ray Generator or Radioactive Source Creates Radiation
Radiation Penetrate the Sample
Exposure Recording Device
X-rays are generated by directing a stream of high speed electrons at a target material such as tungsten, which has a high atomic number. When the electrons are slowed or stopped by the interaction with the atomic particles of the target, X-radiation is produced. This is accomplished in an X-ray tube. A focusing cup is used to concentrate the stream of electrons to a small area of the target called the focal spot.
The major components of an X-ray generator are the tube, the high voltage generator, the control console, and the cooling system.
Film Radiography •
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One of the most widely used and oldest imaging mediums in industrial radiography is radiographic film. Film contains microscopic material called silver bromide. Once exposed to radiation and developed in a darkroom, silver bromide turns to black metallic silver which forms the image.
Film Radiography (cont.) •
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Film must be protected from visible light. Light, just like x-rays and gamma rays, can expose film. Film is loaded in a “light proof” cassette in a darkroom. This cassette is then placed on the specimen opposite the source of radiation. Film is often placed between screens to intensify radiation.
Film Radiography (cont.) •
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In order for the image to be viewed, the film must be “developed” in a darkroom. The process is very similar to photographic film development. Film processing can either be performed manually in open tanks or in an automatic processor.
Film Radiography (cont.) Once developed, the film is typically referred to as a “radiograph.”
Radiation Safety Use of radiation sources in industrial radiography is heavily regulated by state and federal organizations due to potential public and personal risks.
Sources of High Energy Radiation
Radiation Safety (cont.) X-rays and gamma rays are forms of ionizing radiation, which means that they have the ability to form ions in the material that is penetrated. All living organisms are sensitive to the effects of ionizing radiation (radiation burns, x-ray food pasteurization, etc.)
X-rays and gamma rays have enough energy to liberate electrons from atoms and damage the molecular structure of cells. This can cause radiation burns or cancer.
Radiation Safety (cont.) Technicians who work with radiation must wear monitoring devices that keep track of their total absorption, and alert them when they are in a high radiation area.
Survey Meter
Pocket Dosimeter
Radiation Alarm
Radiation Badge
CONTROLLING RADIATION EXPOSURE
A survey must be performed occasionally to verify that vaults are not "leaking" radiation and that the safety devices are performing properly and when conducting radiography in the field, with a survey meter measurement.
SURVEY METERS
Half-Value Layer, mm (inch) Source Concrete Steel Lead Tungsten Uranium Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11) Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)
The thickness of any given material where 50% of the incident energy has been decreased is the half-value layer (HVL). In a shielding calculation, if the thickness of one HVL is known, it is possible to quickly determine how much
CELL RADIOSENSITIVITY High Radiosensitivity Lymphoid organs, bone marrow, blood, testes, ovaries, intestines
Fairly High Radiosensitivity Skin and other organs with epithelial cell lining (cornea, oral cavity, esophagus, rectum, bladder, vagina, uterine cervix, ureters)
Moderate Radiosensitivity Optic lens, stomach, growing cartilage, fine vasculature, growing bone
Fairly Low Radiosensitivity Mature cartilage or bones, salivary glands, respiratory organs, kidneys, liver, pancreas, thyroid, adrenal and pituitary glands
Low Radiosensitivity Muscle, brain, spinal cord
EXPOSURE SYMPTOMS • 0-25 No injury evident. First detectable blood change at 5 rem. • 25-50 Definite blood change at 25 rem. No serious injury. • 50-100 Some injury possible. • 100-200 Injury and possible disability. • 200-400 Injury and disability likely, death possible. • 400-500 Median Lethal Dose (MLD) 50% of exposures are fatal. • 500-1,000 Up to 100% of exposures are fatal. • 1,000-over 100% likely fatal.
Dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period. 100 200 Rem First Day No definite symptoms First Week No definite symptoms Second No definite symptoms Week Loss of appetite, malaise, sore throat Third Week and diarrhea Recovery is likely in a few months Fourth unless complications develop because Week of poor health
400 500 Rem Nausea, vomiting and diarrhea, usually in the first few hours First Week Symptoms may continue Second Epilation, loss off appetite Week Hemorrhage, nosebleeds, inflammation Third Week of mouth and throat, diarrhea, emaciation Fourth Rapid emaciation and mortality rate Week around 50% First Day
AUDIBLE ALARM RATE METERS
FILM BADGES
