Interactions of EM Radiation with Matter Manos Papadopoulos Nuclear Medicine Department Castle Hill Hospital Hull & East Yorkshire Hospitals NHS Trust

ELECTROMAGNETIC RADIATION Light is electromagnetic radiation a form of energy Has both electric and magnetic components Characterised by wavelength (λ) frequency (ν)

WAVE CHARACTERISTICS Wavelength (λ): The distance between two consecutive peaks in the wave

WAVE CHARACTERISTICS Frequency (ν): The number of waves (or cycles) per unit time

WAVE CHARACTERISTICS The product of wavelength (λ) and frequency (ν) is constant

PARTICLE CHARACTERISTICS Particle-like properties Photons or quanta Ε = hν = hc/λ where h is Planck’s constant For a typical diagnostic X-ray λ = 2·10-11 m  photon energy is 62 keV

ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC SPECTRUM A triangular prism dispersing light

ELECTROMAGNETIC SPECTRUM Name  (m)  (Hz) Interesting Facts Radio/TV 10-1 – 10-4 109 – 104 Low “”are reflected from earth’s atmosphere Microwaves 10-3 – 10-1 1011 – 109 Cellular phones, Radar Infrared 10-7 – 10-3 1014 – 109 “Heat” radiation Visible 4·10-7 – 7·10-7 7.5·1014 – 4.3·1014 ~ 1/40 of total spectrum Ultraviolet 10-8 – 7x10-7 1016 – 1014 “Burning rays” of sun X-rays 10-11 – 10-8 1019 – 1016 tissue damage, ionisation Gamma rays <10-11 >1019

ATMOSPHERIC OPACITY

GENERAL PROPERTIES Intensity (I) of a beam of radiation rate of flow of energy per unit area (A) perpendicular to the beam Reduction in intensity by the inverse square law attenuation by interaction with matter

INVERSE SQUARE LAW The intensity of a beam of radiation decreases as the inverse of the square of the distance (r) from that source where E is the rate of energy emission of the source Applies to all radiations under defined conditions for a point source in the absence of attenuation

INVERSE SQUARE LAW

PHOTON ATTENUATION The removal of photons from a beam of photons as it passes through matter Attenuation is caused by absorption scattering of primary beam

ATTENUATION COEFFICIENT Linear Attenuation Coefficient (μ) is defined as the fraction of photons removed from a beam of X- or γ- rays per unit thickness n: number of photons removed from the beam N: number of photons incident on the material Δx: thickness of the material (cm)

ATTENUATION COEFFICIENT Linear attenuation coefficients (in cm-1) for a range of materials at γ-ray energies of 100-, 200- and 300 keV Absorber 100 keV 200 keV 500 keV Air 0.000195 0.000159 0.000112 Water 0.167 0.136 0.097 Carbon 0.335 0.274 0.196 Aluminium 0.435 0.324 0.227 Iron 2.72 1.09 0.655 Copper 3.8 1.309 0.73 Lead 59.7 10.15 1.64

PHOTON ATTENUATION

HALF-VALUE LAYER The half-value layer (HVL) is defined as: the thickness of material required to reduce the intensity of a beam to one half of its initial value μ and HVL are related as follows: HVL is a function of photon energy attenuating material geometry

HALF-VALUE THICKNESS

INTERACTIONS WITH MATTER Rayleigh scattering Compton scattering Photoelectric effect Pair production

RAYLEIGH SCATTERING Incident photon interacts with and excites an atom Atom is excited  emission of a photon Emitted photon same energy different direction  scattered photon

RAYLEIGH SCATTERING

RAYLEIGH SCATTERING

RAYLEIGH SCATTERING

RAYLEIGH SCATTERING Electrons are not ejected In medical imaging no ionisation In medical imaging detection of scattered photons impairs image quality Scattering angle increases as the photon energy decreases Occurs with very low-energetic diagnostic X-rays Low probability of occurrence in diagnostic energies ~ 12% of interactions at 30 keV ~ 5% of interactions above 70 keV

COMPTON SCATTERING Inelastic scattering Photon interacts with an outer-shell (valence) electron scattered photon – reduced energy Compton electron Through conservation of energy:

COMPTON SCATTERING

COMPTON SCATTERING Compton electron loses its kinetic energy through excitation and ionisation of surrounding atoms Scattered photon may traverse the medium without interaction or may undergo subsequent interactions Scattered photons detected by image receptor image quality is impaired

COMPTON SCATTERING Incident photon energy increases scattered photons Compton electrons For higher energy incident photons majority of energy transferred to scattered electron Probability of a Compton interaction increases with the incident photon energy (E) is independent of atomic number (Z) scattered more towards the forward direction

PHOTOELECTRIC EFFECT Photon interacts with orbital electron Electron absorbs all of photon energy Electron is ejected now called a photoelectron Through conservation of energy

PHOTOELECTRIC EFFECT

PHOTOELECTRIC EFFECT The incident photon energy must be ≥ to the binding energy of the ejected electron Following a photoelectric interaction the atom is ionised a vacancy is created  electron cascade Characteristic X-rays or Auger electrons Probability of a photoelectric interaction decreases with increasing photon energy (E) increases with atomic number (Z)

PAIR PRODUCTION X- or γ-ray photon interacts with electric field of nucleus energy of photon transformed into an electron-positron pair Pair production has a threshold energy equal to 1.022 MeV - the rest mass energies of the β-particles The beta particles lose their kinetic energy via excitation and ionisation When the positron comes to rest interacts with an electron  annihilation radiation

PAIR PRODUCTION

DOMINANT REGIONS

SUMMARY I Light is electromagnetic radiation energy propagated as a pair of electric and magnetic fields Duality of light wave-properties particle-properties Reduction in intensity by the inverse square law attenuation by interaction with matter

SUMMARY II Interactions of photons with matter Rayleigh scattering incident photon excites the entirety of the atom Compton scattering part of the incident photon’s electron absorbed by free electron Photoelectric effect all of incident electron absorbed by inner-shell electron Pair production X- or γ-ray photon interacts with electric field of nucleus electron – positron pair created

THE END Any questions ?