interaction of radiation with matter

1. INTERACTIONS OF
RADIATION WITH
MATTER
DR VIJAY KUMAR
DNB PGT
DEPT OF RADIATION ONCOLOGY

2. Basic Concepts Of Interaction

3. Attenuation
 The intensity reduction of x-ray photons as
they pass through matter
 Primary radiation – attenuation = remnant or
exit radiation
 Photon attenuation is characterized by
attenuation coefficient μ.
 For a narrow mono energetic beam,
attenuation coefficient -μx
is : I(x)=Io e
 And hence HVL= 0.693/μ

4. Attenuation Of An X-Ray
Photon

5. The Five Interactions Of X and
Gamma Rays With Matter
 Photoelectric effect
 Very important in diagnostic radiology
 Compton scatter
 Very important in radiotherapy
 Coherent scatter
 Not important in diagnostic or therapeutic radiology
 Pair production
 important in diagnostic radiology
 Photodisintegration
 Neutron contamination of therapy beams

6. Photoelectric Effect
 All of the energy of the
incoming photon is totally
transferred to the atom
 Following interaction, the
photon ceases to exist
 The incoming photon
interacts with an orbital
electron in an inner shell –
usually K
 The orbital electron is
dislodged
 To dislodge the electron, the
energy of the incoming
photon must be equal to, or
greater than the electron’s
energy

7. Photoelectric Effect
 The incoming photon gives up all its energy, and
ceases to exist
 The ejected electron is now a photoelectron
 This photoelectron energy =
energy of the incoming photon- the binding
energy of the electron shell
 This photoelectron can interact with other atoms
until all its energy is spent
 These interactions result in increased patient
dose, contributing to biological damage

8. Photoelectric Effect
 A vacancy now exists in the inner shell
 To fill this gap, an electron from an outer shell
drops down to fill the gap
 Once the gap is filled, the electron releases its
energy in the form of a characteristic photon
 This process continues, with each electron
emitting characteristic photons, until the atom is
stable
 The characteristic photon produces relatively
low energies and is generally absorbed in tissue

9. The Byproducts of the
Photoelectric Effect
 Photoelectrons
 Characteristic photons

10. The Probability of Occurrence
 Depends on the following:
 Mass photoelectric coefficient is 2 ª 3
Z/E
 It increases as the photon energy decreases, and the
atomic number of the irradiated object increases
 When the incident photon’s energy is more or close to
the binding energy of the orbital electron
 In water or soft tissue This type of interaction is
prevalent in the diagnostic kVp range – 10-25keV(30-
75kVp)

11. What Does This All Mean?
 Bones are more likely to absorb radiation
 This is why they appear white on the film
 Soft tissue allows more radiation to pass
through than bone
 These structures will appear gray on the film
 Air-containing structures allow more radiation to
pass through
 These structures will appear black on the film

12. Compton Scattering
 An incoming photon is
partially absorbed in an
outer shell electron
 The electron absorbs
enough energy to break
the binding energy, and is
ejected
 The ejected electron is
now a Compton electron
 Not much energy is
needed to eject an
electron from an outer
shell
 The incoming photon,
continues on a different
path with less energy as
scattered radiation

13. Byproducts Of Compton Scatter
 Compton scattered electron
 causes projectile damage in the tissue.
 Possesses kinetic energy and is capable of ionizing
atoms.
 The atom becomes a free radical, causing biological
damage in the tissue
 Scattered x-ray photon with lower energy
 Continues on its way, but in a different direction
 It can interact with other atoms, either by photoelectric
or Compton scattering
 It may emerge from the patient as scatter

14. Probability Of Compton Scatter
Occurring
 Probability of a Compton interaction is inversely
proportional to energy of the incoming photon.
 In water More probable at kVp ranges of 10-150. and
decreases further with increase in energy.
 Most dominant interaction in tissues at treatment
energies(30keV-24MeV).
 It is independent of atomic number, so at treatment
energies, bone and soft-tissue interfaces are barely
distinguishable (= poor contrast)
 At diagnostic x-ray energies, Compton Scattering
direction is fairly random; at treatment x-ray energies, it
is forward-peaked

15. Coherent Scatter
 Only significant at lowest diagnostic x-ray energies
(<5% interactions)
 Incoming photon is deflected (absorbed and
immediately re-emitted), with minimal direction and
energy change
 May result in radiographic film fog

16. Pair Production
 Occurs only at high photon energies
(>1.02 MeV) and preferentially in high-
Z tissues
 Incoming photon (energy) is converted
to mass (electron and positron) in the
vicinity of atomic nucleus via E=mc2

17. Pair Production
 An incoming photon of
1.02 MeV or greater
interacts with the
nucleus of an atom
 The incoming photon
disappears
 The transformation of
energy results in the
formation of two
particles
 Negatron
 Possesses negative
charge
 Positron
 Possesses a positive
charge

18. Positrons
 Will interact with the first electron they encounter
 An electron and the positron destroy each other
during interaction
 Known as the annihilation reaction
 This converts matter back into energy
 Both the positron and electron disappear
 Two gamma photons are released with an energy
of .51 MeV and travel at an angle of 180º. A
simultaneous detection of gamma ray photons in
two detectors places the source on a line
between those detectors (PET SCAN: where
radioisotopes used for positron emission).

19. Pair Production
 Electron causes projectile damage in the
tissue
 Significant pair production can be seen in
blocking of the oncoming beam, since
blocks are high-Z materials (for lead, this is
the main effect at energies >5 MeV)

20. Table 5.2 Relative Importance of Photoelectric (τ),
Compton (σ), and Pair Production (Π) Processes in Water
Photon Energy (MeV)
Relative Number of Interactions (%)
τ σ Π
0.01 95 5 0
0.026 50 50 0
0.060 7 93 0
0.150 0 100 0
4.00 0 94 6
10.00 0 77 23
24.00 0 50 50
100.00 0 16 84
Data from Johns HE, Cunningham JR. The Phy s ic s o f Ra d io lo g y . 3rd ed. Springfield, IL: Charles
C Thomas; 1969.

21. Photodisintegration
 Occurs at above 10 MeV
 A high energy photon is
absorbed by the nucleus
 The nucleus becomes
excited and becomes
radioactive
 To become stable, the
nucleus emits negatrons,
protons, alpha particles,
clusters of fragments, or
gamma rays
 Source of low-level
neutron production

22. Interactions Of Particulate
Radiation With Matter
 Electrons, protons, neutrons, alpha particles,
beta paticles are examples of particle
radiation.
 Charged particle interaction or collisions
mediated by coulomb force between the
electric field of travelling particle and electric
fields of orbital electrons and nuclei of atoms
of the material.
 They interact primarily by ionization or
excitation.
 All particles exhibit Bragg peak near end
except electrons due to excessive scattering.

23. Electrons
 Two fundamental interactions:
 Radiation (Bremsstrahlung) - bending of electrons
around nucleus => shedding of energy as EM x-rays
 Ionization (Characteristic X-rays) - impact with
orbital electron => electron release => vacancy fill
=> shedding of energy as Characteristic x-rays

24. Protons
 Incoming protons also lose energy mainly by interacting with
orbital electrons; however, since they are much heavier
(~1800x), they only lose very small fraction of their kinetic
energy with each interaction, and thus scatter only minimally
 The interactions (and thus energy loss) become more
frequent at slower energies. Thus the slower the proton
moves, the more energy it loses to the tissue electrons, in a
feed-forward loop, until it abruptly loses all energy. This
region of rapid energy loss (and its deposition into the tissue)
is called the Bragg peak.
 The distance at which Bragg peak occurs, and the energy is
deposited, can be calculated very precisely (unlike electrons).
The rapid drop-off in dose make it ideal for delivering dose
precisely to the tumor, and not to the healty tissue beyond the
tumor.
 Incoming protons also rarely interact with the nucleus, and
may enhance cell kill by ~10%

25. Neutrons
 Interact by ejecting recoil protons from
hydrogen and recoiling heavy nuclei from
other elements or by producing nuclear
disintegrations.
 Lead is an efficient absorber of x-rays but not
of neutrons.
 The most efficient absorber of neutrons is a
hydrogenous material such as water, paraffin
wax, and polyethylene.

26. Heavy ions
 Stopping power of ionization interactions is
proportional to square of particle charge and
inversly to square of its velocity
 They interact with tissue similarly to protons,
but since they are heavier still, they scatter
less initially, and have a faster dose fall-off
(Bragg peak) at the end.