X-Ray Physics and Pair Production Concepts

1-1 History of Radiography

• X-rays discovered in 1895 by Wilhelm Conrad Roentgen
• Roentgen observed a fluorescent glow from crystals near a cathode ray tube
• Roentgen shielded the tube to discover a green fluorescent glow
• A new type of ray was being emitted from the tube
• The ray could pass through most substances, but not bone or metal
• Roentgen's discovery was a scientific bombshell
• Scientists duplicated his experiments quickly
• Newspapers and magazines published stories about the properties of the rays
• X-rays used in medicine and industry within months of the discovery
• X-rays used to locate bullets in wounded soldiers in 1896

A Second Source of Radiation

• Henri Becquerel discovered natural radioactivity in 1896
• Becquerel's research involved uranium compounds and fluorescence
• He found that the uranium compounds emitted radiation
• Becquerel's discovery attracted little attention initially
• Marie Curie and Pierre Curie became interested in Becquerel's work, discovered polonium (1898), and radium (1898)
• Man-made sources such as cobalt and iridium became available for radiography in 1946, replacing radium sources.

Health Concerns

• The widespread use of X-rays led to serious injuries in early experimenters
• Early experimenters did not initially suspect X-rays as a cause of injury
• Warning signs about X-ray damage reported by Thomas Edison, William J. Morton, and Nikola Tesla in the early 1900s
• Radiation considered a leading cause for illness
• Extensive investigation into the mechanisms of radiation damage

1-2 Present State of Radiography

• Radiography has changed little from its early days
• Still uses film, but now with higher quality and greater sensitivity
• Automation of film processing
• Digital capturing of images
• Smaller, lighter, portable equipment for the production of high-quality X-rays.

1-3 Future Direction of Radiographic Education

• Use of computers in radiographic inspection
• Digitally captured images sent to customers
• Computer systems for film analysis and report generation
• 3-D radiographic images, like computer-generated ultrasound scans.

2. Physics of Radiography

• 2-1 Nature of Penetrating Radiation
  • X-rays and gamma rays are electromagnetic radiation
  • Have very short wavelengths and high energy
  • Not easily detectable by human senses

2-2 X-Radiation

• X-rays are a type of electromagnetic radiation
• Produced by accelerating electrons into a heavy target
• Bremsstrahlung (breaking radiation), occurs during the acceleration of electrons
• K-shell emission, occurs when inner-shell electrons are knocked out of their orbit and replaced by outer-shell electrons

2-3 Gamma Radiation

• Gamma rays are a type of electromagnetic radiation
• Emitted by radioactive atoms undergoing decay.
• Gamma rays have high energy, short wavelengths, and are more penetrating

3. Equipment's and Materials

• 3-1 X-ray Generators are composed of
  • X-ray tubes (produce X-rays)
  • High-voltage generators
  • Control consoles
  • Cooling system
• Control Console
• Allows for control over voltage, current, and exposure times.
• Tube Cathode (Filament) - Low voltage current heats up the filament of wires that allows electrons to be emitted.
• High Voltage Generator
• Creates a high voltage between the anode and cathode to accelerate the flow of electrons
• Focal Spot - Area on the target where electrons strike and produce X-rays
• Focusing Cup - Concentrates the stream of electrons toward a small area on the target.

3-2 Radio Isotope (Gamma) Sources

• Man-made radioactive materials, such as iridium-192 and cobalt-60
• Emit gamma rays, which are electromagnetic radiation
• Used for industrial radiography due to their portability and ability to penetrate various materials

3-3 Radiographic Film

• Light-sensitive silver halide crystals encased in gelatin and a clear base.
• X-rays or gamma rays liberate the bromide ions, allowing for image formation.

3-4 Exposure Vaults & Cabinets

• Provide a safe area to perform radiographic examinations
• Usually, large walk-in rooms with high-density concrete and lead shielding

4. Techniques and Calibrations

• 4-1 Image Considerations
  • Contrast and definition are crucial
  • Contrast shows contrast between different parts of the radiographic image
  • Geometric factors affect definition (source size, source-to-film distance, object-to-film distance).

4-2 Radiographic Contrast

• Differences in photographic density in a radiograph
• Subject contrast and detector (film) contrast.

4-3 Definition

• Sharpness in a radiograph caused by the abrupt change between high and low densities.
• Geometric factors or material characteristics can affect the definition

4-4 Radiographic Density

• Measures the degree of film darkening, related to the intensity of light reaching the film.
• Uses a logarithmic scale, with higher numerical values indicating darker areas on the film.

4-5 Film Characteristic Curves

• Used to determine the relative exposure required to create film of a specific density.
• Shows the relationship between film density and relative exposure.

4-6 Exposure Calculations

• Shows the relationship between various factors such as exposure time, voltage, current and thickness of the material in creating an image of a required density .

4-7 Controlling Radiographic Quality

• Image quality indicator (IQI)
• Helps with contrast and definition assessment in a radiograph.
• Types: Hole and Wire IQI.

4-8 Film Processing

• Five steps involved
  • Development
  • Stopping
  • Fixing
  • Washing
  • Drying

4-9 Viewing Radiographs

• Important viewing conditions
• clean area
• low ambient light
• film viewers
• subdued light, not total darkness.

4-10 Radiograph Interpretation – Welds and Castings

• Identifying quality defects in weldings and castings by inspecting the radiographic images
• Defects in welds and castings are recognized based on their appearance on the radiograph:
  • Porosity
  • Undercut
  • Incomplete penetration
  • Incomplete fusion 
  • Melt-through
  • Burn-through 
  • Slag inclusions
  • cracks and cavities

5. Advanced Techniques

• 5-1 Real-time Radiography (RTR)
  • An NDT method for dynamic monitoring of objects
  • Images displayed on a screen in real time without delay
• 5-2 Computed Tomography (CT)
  • A non-destructive evaluation technique
  • Generates 2-D and 3-D cross-sectional images of objects
  • Using X-rays.