Radiographic Testing (RT) - Level II Study Guide
Core concepts
Radiography uses X-rays or gamma radiation from isotopes such as iridium-192, cobalt-60, selenium-75, or cesium-137 to penetrate material; differential attenuation produces a latent image on film or a digital detector. Know source selection and half-life, the inverse square law, geometric unsharpness given by Ug = F times t divided by d, where F is source size, t is object-to-film distance, and d is source-to-object distance. Understand film density (about 2.0 to 4.0 is typical), the characteristic curve, contrast versus definition, lead screens, and image quality indicators of the hole and wire types.
Traps to expect
- Density is not sensitivity. The essential wire or hole must be visible; higher density alone does not make a better image.
- Unsharpness grows with a larger source and with greater object-to-film distance, and shrinks with greater source-to-object distance.
- Inverse square law: doubling distance cuts intensity to one quarter.
- Film artifacts such as static marks, crimp marks, and water spots are often mistaken for defects.
- Radiation safety: ALARA, dosimeter badges, and time-distance-shielding principles.
Rule of thumb
If source-to-film distance doubles, exposure time must increase fourfold to keep the same density.
Worked example: an exposure of 2 minutes at 20 in becomes 8 minutes at 40 in, assuming all other factors are unchanged.