1.3 Photon Interactions with Matter: Photoelectric, Compton, Coherent & Pair Production

1.3 Photon Interactions with Matter: Photoelectric, Compton, Coherent & Pair Production

The Enhanced Table of Specifications devotes a full 10 items of the Radiation Physics subject to Application of X-ray Interaction with Matter — more than any other single sub-topic in that subject. The Board is not asking you to recite definitions; the competencies say apply the clinical significance of photoelectric and modified scattering interactions, explain ionization and excitation, explain the relationship of energy, wavelength and frequency, and assess the production of bremsstrahlung and characteristic radiations. This section is written to that standard.

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1. Why Interaction Physics Decides Everything on the Image

A radiograph is a map of differential attenuation. Every photon leaving the tube does exactly one of three things:

1. Passes through the patient unaffected (transmission). These photons carry the "black" of the image — they reach the image receptor and expose it.
2. Is absorbed in the patient. These photons never reach the receptor and produce the "white" of the image. They also deposit dose.
3. Is scattered. These photons change direction; if they still reach the receptor they add uniform fog with no anatomical information, and if they leave the patient sideways they irradiate the room and the staff.

Subject contrast is created by categories 1 and 2. Category 3 destroys it. That single sentence explains grids, collimation, air gaps, lead aprons, and most of radiation protection.

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2. The Five Interactions

Coherent scattering involves no ionization at all. A low-energy photon sets the whole atom vibrating; the atom re-radiates a photon of identical wavelength in a new direction. Because energy is unchanged, it is also called unmodified scattering. Compton scattering, by contrast, is modified scattering — the scattered photon has less energy than the incident photon.

Ionization versus excitation

These two terms are separately examinable.

• Ionization removes an electron completely from the atom, leaving an ion pair: the negative ejected electron and the positive residual atom. Photoelectric and Compton interactions are both ionizing.
• Excitation raises an electron to a higher-energy shell without ejecting it. The atom returns to ground state by emitting the excess as a lower-energy photon. Excitation is the mechanism behind fluorescence in intensifying screens and behind luminescence in scintillation detectors.

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3. The Photoelectric Effect — The Contrast Maker

An incident photon strikes an inner-shell (usually K-shell) electron, is totally absorbed, and ejects that electron as a photoelectron with kinetic energy equal to the incident photon energy minus the electron's binding energy. The vacancy is filled from an outer shell, and the energy difference is released as characteristic (secondary) radiation — in soft tissue this is only a few hundred eV and is absorbed within a millimetre.

Two conditions must both hold:

1. The photon energy must be equal to or slightly greater than the binding energy of the shell electron.
2. The interaction becomes overwhelmingly more likely as the photon energy approaches that binding energy from above.

The two proportionalities you must be able to use:

• Probability rises roughly with the cube of the effective atomic number (Z-effective cubed).
• Probability falls roughly with the cube of the photon energy (1 / E cubed).

Work an example. Cortical bone has an effective atomic number of about 13.8; soft tissue about 7.4. The ratio 13.8/7.4 is about 1.86, and 1.86 cubed is about 6.5. Bone therefore absorbs photoelectrically about six to seven times more strongly than an equal thickness of soft tissue at the same photon energy — which is precisely why the skeleton is visible. Now drop the kVp: because the probability also scales as the inverse cube of energy, halving the effective photon energy multiplies photoelectric absorption roughly eightfold, sharpening the bone-to-tissue difference. Low kVp equals high subject contrast, and also high patient dose. That trade-off is the entire logic of the ALARA-versus-image-quality balance.

Effective atomic numbers worth memorising

K-edge physics and contrast media

The K-edge is the binding energy of the K-shell electron. Just below it, photons cannot eject a K-shell electron and photoelectric absorption is low; just above it, absorption jumps discontinuously — often by a factor of five or more.

• Iodine: K-edge 33.2 keV.
• Barium: K-edge 37.4 keV.

A typical 70-80 kVp beam has an average photon energy of roughly 30-45 keV, so a large fraction of the beam sits just above both K-edges. That is the physical reason iodine and barium became the universal radiographic contrast agents rather than some other dense element. It is also why excessively high kVp wastes contrast media: push the beam to 120 kVp and most photons are far above the K-edge, where absorption has fallen away again.

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4. Compton Scattering — The Contrast Destroyer

An incident photon strikes a loosely bound outer-shell electron, ejects it (the Compton or recoil electron), and continues in a new direction with reduced energy. The relationship is a partition:

incident photon energy = binding energy + kinetic energy of the recoil electron + energy of the scattered photon

Key behaviours:

• Scatter angle and energy. Deflection can be anywhere from 0 to 180 degrees. At small angles the scattered photon retains nearly all its energy. At 180 degrees (backscatter) the photon retains the least energy — but even a 180-degree scatter of a 70 keV photon still carries roughly 50 keV, more than enough to penetrate the room and reach staff. This is why "the scattered photon is harmless" is always a wrong option.
• Atomic number independence. The probability depends on the number of electrons per gram, which is remarkably constant across biological materials (hydrogen is the outlier, with roughly twice the electrons per gram of everything else). Compton scatter therefore differentiates mass density, not composition, and produces essentially no useful subject contrast.
• Relative share rises with kVp. As kVp increases, photoelectric absorption collapses (inverse cube of energy) while Compton falls only gently. The fraction of interactions that are Compton therefore climbs steeply — which is why high-kVp technique yields low contrast and heavy fog.
• Occupational dose. The patient is the principal scattering source in the room. Scatter intensity at 1 metre from the patient is roughly 0.1% (1/1000) of the intensity of the primary beam at the patient's entrance surface — the classic figure quoted for fluoroscopy and mobile radiography.

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5. Pair Production and Photodisintegration

• Pair production. A photon of at least 1.02 MeV passes close to a nucleus, disappears, and its energy converts into an electron-positron pair (each with a rest-mass equivalent of 0.51 MeV). The positron subsequently annihilates with an electron, producing two 0.511 MeV photons emitted at 180 degrees — the physical basis of PET imaging. Because no diagnostic x-ray tube reaches 1.02 MeV, pair production never occurs in radiography.
• Photodisintegration. Above roughly 10 MeV, the photon is absorbed directly by the nucleus, which becomes excited and emits a nucleon (usually a neutron). This matters only for high-energy therapy linear accelerators, where it drives neutron-shielding requirements in the treatment vault maze.

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6. Production Revisited: Bremsstrahlung and Characteristic Radiation

The TOS asks you to assess the production of both, so keep the distinctions crisp.

At 65 kVp the beam is 100% bremsstrahlung, because the projectile electrons cannot supply the 69.5 keV needed to eject a tungsten K-shell electron. That single fact is a frequent item.

Energy, wavelength and frequency

For electromagnetic radiation, energy is directly proportional to frequency and inversely proportional to wavelength. Increase kVp and you shorten the wavelength, raise the frequency, and raise the energy — a "harder", more penetrating beam. Filtration does the same thing selectively, by preferentially removing the long-wavelength (low-energy) photons.

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7. Putting It Together at the Console