Radiographic Testing

• Radiographic Testing


  One of the earliest NDT methods, but alternative methods are replacing it in some applications due to health and safety implications. Remains one of the two main volumetric NDT methods.
• Radiography


  • One of the two main volumetric testing methods, along with ultrasonics
  • Used when materials are difficult to penetrate with ultrasonics or a permanent visual record is required

  
• Radiographic testing


  1. Producing short-wave X- or gamma radiation

  2. Directing radiation at an object

  3. Radiation penetrates matter to a certain extent

  4. Radiation captured on photographic film

  
• High density object


  Absorbs more radiation, causing less radiation to hit the film, producing a lighter image
• Low density object


  Absorbs less radiation, causing more radiation to hit the film, producing a darker image
• Radiographic image quality


  • Density
  • Contrast
  • Definition & Sensitivity
• Density


  Degree of blackness on the radiograph. There are minimum and maximum amounts to make the radiograph readable and give the required sensitivity.

  
• Radiographic contrast


  Degree of difference between density fields on a radiograph. High contrast has only blacks and whites, low contrast has similar density tones.
• Radiographic contrast


  • Film contrast (dependent on film type and developing process)
  • Subject contrast (dependent on subject, screens, filters, and radiation wavelength)
• Radiographic screens


  • Metal foil screens (e.g. lead foil)
  • Fluorescent salt screens
• Metal foil screens


  Improve photographic action, absorb soft and scattered radiation, increase image quality, and reduce exposure time
• Fluorescent intensifying or salt screens


  Consist of a thin layer of fluorescent material that emits visible or UV rays when exposed to radiation, greatly reducing exposure time
• Radiographic definition and sensitivity

  Measures of the quality of an image in terms of the smallest detail or discontinuity that may be detected
• 1. rays


  • Produced by an X-ray set with controls for kilovoltage, milliamperage, and time
  • Produce a mixture of wavelengths

  
• Gamma rays


  • Produced when unstable radioactive isotopes disintegrate to become stable
  • Produce discrete wavelengths of radiation
  • Amount of radiation depends on the size (activity) of the source and the half-life of the isotope
• 1. ray machines are usually static in an inspection area, while gamma sources are more portable

  
• Safety considerations for radiography


  • Radiation destroys human cells, so it must be used in a controlled and safe manner
  • Radiation cannot be detected by human senses, so instruments are used to detect and measure it
• Radiation detection instruments


  • Personal dosemeters (film badges/TLDs)
  • Survey meters (dose rate meters)
  • Audible alarms
  • Pocket dosemeters (exposure meters)
  • Geiger counters
  • Audio/visual alarms

  
• Pocket dosemeters (exposure meters)


  Meters that record an accumulative amount of radiation, and can be used for measuring the dose received, for instance over one day, instead of waiting for the monthly badge results

  
• Monitoring radiation dosage


  • Pocket dosemeters
  • Thermoluminescent dosimeters
  • Direct reading / Pen dosimeters

  
• Geiger counters


  Radiation detectors that are often seen on TV clicking away in a radioactive situation; however, they are rarely used in radiography as they can only detect very low intensities–larger intensities would jam the instrument. They are often used for checking for radiation leakage at radiography cell doors or radiography cabinets. They are very sensitive.
• Audio/visual alarms


  Alarms, such as the 'Gamma Alert', that are placed inside the radiation area, and normally have an amber flashing light, which changes to red when the source is exposed, and radiation is present. This is also accompanied by an audible warning.
• None of these instruments are perfect on their own but, if a combination is used, the radiographer can measure, and record the radiation doses, and work safely.

  
• Radiography should be carried out wherever possible in a proper enclosure, called a radiography cell. These are rooms constructed with very thick walls or lined with high-density materials such as lead, which will reduce the level of radiation at the outside of the wall to an acceptable level.
• Where it is not possible to carry out radiography in a cell, a controlled area must be set up, and barriers established at positions where the radiation levels are considered to be safe for non-classified personnel. The present legal requirement is 7.5 μSv/hr–1.

  
• The size of the controlled area can be reduced, by using shielding or collimators at the source. The intensity of the radiation is inversely proportional to the square of the distance from the source, i.e., if you double the distance away from the source, the radiation level is reduced by a factor of four.
• Producing a radiograph


  1. Expose film

  2. Develop latent image

  3. Fix film

  4. Wash film

  5. Dry film
• Developer


  An alkali solution, e.g.: Hydroquinone or Phenidone, that reduces the silver halide grains that have been exposed to radiation to metallic silver
• Stop bath


  A mild acidic solution (normally 2%) that arrests any further development and removes the developing chemicals from the film
• Fixer tank


  Removes the undeveloped silver halide crystals and fixes the remaining developed crystals. The fixer also hardens the film to make it easier to handle.
• Final wash


  Removes the fixing chemicals from the film to prevent deterioration
• Wetting agent


  Promotes even drying of the film
• Radiographic film


  Has an emulsion coating on both sides to allow more silver halide crystals, which produce the dark images, without building up too thick a layer
• After exposure, the image on the film cannot initially be seen; this is called the latent image, and can only be seen after the film has been processed.
• Discontinuity (void or inclusion)


  Appears in contrast compared to its surroundings on the radiograph
• Interpretation of radiographic results

  • Gas porosity - Appears as round or elongated dark spots
  • Slag inclusion - Appears as dark irregular shapes
  • Incomplete penetration of weld joint - Appears as continuous or intermittent dark lines of uniform width, occurring in the middle of the weld
  • Cracks - Appear as sharp lines with tapered ends
  • Foreign metal inclusion - Appears as white areas of round or irregular shapes
• Digital radiography


  Uses large thin-film transistor arrays and charged-coupled device (CCD) cameras to capture radiographic images digitally, allowing enhancement, magnification, and storage of the images
• Real-time radiography


  Transfers the radiographic image to a screen display, allowing the radiographer to view it as it happens
• Computerized tomography


  Shows a slice through an image instead of a flat two-dimensional image, allowing viewing of interior regions of interest without interference
• Neutron radiography


  Uses neutrons instead of X- or gamma rays to pass through the object and expose the film, allowing detection of light materials inside dense materials