Principles of Radiography and Radiation Physics
Where photons come from
X-rays are produced when electrons are accelerated across a tube and stopped in a target: a continuous bremsstrahlung spectrum plus sharp characteristic lines that appear only above the target element's excitation energy. Gamma rays come from nuclear decay and have discrete energies fixed by the isotope (Ir-192, Co-60, Se-75, Yb-169). As a working relation, wavelength in ångström is about 12.4 divided by photon energy in keV, so 100 keV corresponds to roughly 0.124 Å: shorter wavelength means harder, more penetrating radiation.
Attenuation and interaction
For a narrow beam, I = I0·exp(−μx); μ depends on density, atomic number and photon energy. Photoelectric absorption dominates at low energy and rises steeply with atomic number, Compton scattering dominates in the middle range typical of industrial radiography and depends mainly on electron density, and pair production needs at least 1.022 MeV. Image contrast comes from differential absorption between a flaw and its surroundings, not from absolute absorption. Half-value layer (HVL) is the thickness that halves intensity and is a practical index of beam quality; added filtration hardens the beam and raises HVL.
Traps
Questions like to imply a denser material always attenuates more: at high energy, atomic number matters less than density. Another trap is treating kV and mA as interchangeable - kV sets quality (penetration, HVL) and mA sets quantity (intensity, exposure time).
Worked example
A steel section with HVL = 10 mm at the working energy. Going from 10 mm to 40 mm is three half-value layers, so transmitted intensity falls by 2³ = 8 and exposure time must increase eightfold. Applying the inverse-square law to this thickness step is a common error.