Safety · Interactions with Matter

Photon interactions with matter: photoelectric, Compton & coherent

After x-rays leave the tube, photons interact in the patient. Three interactions matter on the registry: photoelectric (total absorption—contrast and patient dose), Compton (scatter—fog and your occupational exposure), and coherent (direction change, little diagnostic role). This diagram shows what each does and which wins as energy rises.

Diagram comparing photoelectric absorption, Compton scatter, and coherent scatter of an x-ray photon in tissue

Photoelectric effect

The photon is totally absorbed; nothing continues as that photon. Needs at least the electron’s binding energy. Probability rises with Z³ (why bone/contrast media show up) and falls steeply with energy (1/E³).

Compton scatter

The photon deflects, loses energy, and ionizes the atom. Largely independent of Z—so it doesn’t create subject contrast; it fogs the image and irradiates the room. Probability falls more slowly with energy (≈1/E) than photoelectric.

Coherent (classical) scatter

Low-energy, change of direction without ionization/energy transfer. Minor in the diagnostic range.

Which interaction predominates

As kVp rises, photoelectric falls off faster than Compton, so Compton’s share grows even though both decline—classic “higher kVp, longer gray scale / more scatter fog” reasoning. Soft-tissue crossover talk (~20 keV in many teaching models) is a study aid; exam stems care about the trend and the consequences (contrast vs scatter).

Frequently asked questions

Which photon interaction is responsible for radiographic contrast?

The photoelectric effect—total absorption that varies strongly with atomic number (Z³). Compton does not provide subject contrast; it adds fog.

Where does most occupational exposure in fluoroscopy come from?

Compton scatter from the patient. That is why distance, shielding, and scatter control matter.

Why does raising kVp reduce contrast and increase the relative importance of Compton?

Photoelectric probability falls much faster with energy than Compton, so a larger share of remaining interactions are Compton scatter, which lengthens gray scale and adds fog.

Interactive diagram

Free layer covers the three interactions and dominance trends. Unlock full access to scrub energy/tissue and linked practice.

30 keV in soft tissue — Compton predominates

Above 20 keV, Compton scattering is the more likely interaction in soft tissue. Compton does not care about atomic number, so it carries no contrast information — it just fogs the image and puts scatter in the room. Photoelectric probability has fallen away as 1/E³ while Compton has only fallen as 1/E, which is why raising kVp trades contrast for penetration.

The crossover — where the two are equally probable — is at 20 keV in soft tissue.

Coherent scatteringalso called classical or Thompson scattering

Happens below about 10 keV — under the diagnostic range
The x-ray changes direction but not wavelength, so not energy
No energy is transferred, so no ionization occurs
Most coherently scattered x-rays travel on forwards

Photoelectric effecttotal absorption of the incident photon

The incident x-ray is completely absorbed — nothing carries on
It cannot happen unless the x-ray has at least the electron’s binding energy
Probability rises with the cube of atomic number (Z³) — this is what makes contrast
Probability falls with the cube of energy (1/E³), so it drops away fast as kVp rises

Compton scatteringthe interaction that produces scatterPredominates here

The x-ray is deflected and loses energy, and the atom is ionized
Independent of atomic number — as likely in bone as in soft tissue
Probability falls with energy (1/E), far more slowly than photoelectric
Even at 180 degrees the scattered x-ray keeps about two-thirds of its energy

Safety lessonInteractions with matter