00 Foundations
Three ways a photon can vanish — and why the one that wins depends on energy and atomic number.
For Residents · RTT students · anyone opening a spectrum·8 min
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Chapter 01
A diagnostic x-ray tube and a linac both throw photons at a patient. What happens next is not a mystery: the photon either disappears, scatters, or walks through. The mix of those three fates is the entire contrast of a chest film and the entire build-up of a 6 MV beam.
Keep thisPhotons do not ‘dose’. Electrons they set in motion do.
Chapter 02
Photoelectric: the photon vanishes, a bound electron leaves. Probability climbs steeply with Z and falls hard with energy — bone lights up on a 70 kV film. Compton: a glancing blow, wavelength stretches with angle; this is radiotherapy’s workhorse. Pair production: above 1.022 MeV the photon becomes electron plus positron in the nuclear field.
Energy chooses the death. Z weights photoelectric.
Simulation
Compton scattered photon energy — Change the numbers; the scene follows.
Variables
E′ = 83.6334 keV
Keep thisEnergy chooses the death. Z weights photoelectric.
Open the calculator — Compton scattered photon energyChapter 03
A slab does not remove a fixed number of photons. It removes a fraction. That is why we talk in half-value layers: one HVL halves the beam, two leave a quarter. Open the live curve. Raise mu and watch the line dive. That dive is every barrier, every patient thickness, every TVL you will ever specify.
Never add millimetres linearly. Always think in HVLs.
Simulation
Linear and mass attenuation — Change the numbers; the scene follows.
Variables
μ = 1.695 cm⁻¹
Keep thisNever add millimetres linearly. Always think in HVLs.
Open the calculator — Linear and mass attenuationIn this specialty