PHOTON INTERACTIONWITH MATTERPhotoelectricCompton Scattering

SYAZWANI AFIQAH BINTI MOHD NOHZULAIKHA BINTI MOHD DZAHERSITI ADIBAH BINTI MISRANFARAH NOR SHAHIRAH BINTI AMINMUHAMMAD TARMIZI BIN OMARMOHD HAZIM BIN ISHAK

Photon Interaction With Matter

• Photons are electromagnetic radiation with zero charge & mass

• velocity = the speed of light (3x10⁸ m/s)

• Neutral – do not lose energy via coulombic interactions with atomic electrons, as do charged particles.

Photon Interaction With Matter

• Photons travel some distance before undergoing a more “catastrophic” interaction leading to partial or total transfer of the photon energy to electron energy

• These electrons will ultimately deposit their energy in the medium.

• Photons are far more penetrating than charged particles of similar energy.

PHOTOELECTRIC

• Discovered by Albert Einstein in 1905• Occurs when energy of incident photon is equal

or slightly greater than electron binding energy• It transfers all the energy to the electron in one

of the shell• As complete absorption occur, e ejected as ˉ

photoelectron – kinetic energy• Vacancy then filled by outer e - characteristic ˉ

radiation produced

PHOTOELECTRIC

Photoelectric increases contrast in radiographic image

• Probability decreases as Eᵢ increases

• Probability increases as Z number of absorber increases

• As Z number of material increases– more electrons per atom– Inner-shells electron are held tightly

so interaction occur more often• Lead (Z=82) , tungsten (Z=74) –

shielding• Barium (Z=56) , iodine (Z=54) –

contrast agents

Characteristics of material that affect the probability of photoelectric interactions

PRO

• No scatter radiation• Enhances images contrast

CONS

• Contributes film fog• Radiation dose

Characteristics of photoelectric interactions

thus kVp must be adjusted carefully to produce desirable images but minimized dose to pt

COMPTON

• Energy range 30 keV to 10 MeV• A higher-energy photon within the range

ejects an outer shell electron• Only a portion of total energy is consumed

in the process• The photon ejected away from original

direction - results in lower-energy scattered radiation

COMPTON

• The greater scattering angle, the lower the energy of scatter

• Lowest energy at 180° - back scatter• As Eᵢ increases, energy of scatter radiation

increases• At 10 keV, probability of photon being scattered

forward and back scattered almost equal• At ˃1MeV, probability of scattering at large

angles increases – extreme penetrating

SCATTER RADIATION

• Scattered radiation – photons undergo a change in direction after interacting with atom

• Secondary radiation – radiation emitted from an atom after it has absorbed a photon and included:– Characteristics x-rays aka fluorescence– Ejected photoelectrons (result of photoelectric interaction)– Compton-scattered/recoil electrons

Difference between Scattered Radiation and Secondary Radiation

Although scattered radiation may be considered under secondary radiation, not all secondary radiation is scattered radiation

Features of Compton Scattering

• As x-ray energy increases:– Increased penetration through tissue without

interaction– Increased Compton scattering relative to photoelectric

effect

• As Z number of absorber increases – No effect on Compton scattering

• As mass density of absorber increases – Proportional increase in Compton scattering

Compton effect reduces contrast in an x-ray image.

The probablility of the Compton effect is inversely proportional to energy (1/E) and independent of atomic number

Some of the radiation after entering the tissue will scatter back toward the surface – backscatter radiation

CONCLUSION

• In diagnostic energy range, Photoelectric absorption & Compton scattering are the most important methods

• Compton scattering is the primary mode of interaction of diagnostic x-rays in tissue

• The different shades in radiograph are the result of varying degrees and types of interactions occur

• Because as beam passes through body tissues, different absorption happens