1. INTERACTION OF PHOTONS
By- Dr. Aakriti Bhardwaj
Moderator- Dr. Umesh V

2. INTRODUCTION
• Absorption of energy from radiation in biological material may lead to
excitation or ionization
• IONIZING RADIATION : Radiation with sufficient energy to eject ≥1
orbital electrons from atom or molecules
• NON-IONIZING RADIATION: Radiation that doesn’t carry enough
energy to ionize atoms or molecules

3. Types of Ionizing Radiations
• Electromagnetic
• Particulate

4. Electromagnetic radiation
• Electromagnetic waves have both electric and magnetic components.
They are at right angles to each other, vary with time.
• The wave moves forward with velocity ‘c’, which in vacuum = 3 x
10^10 cm/s. The distance between successive peaks of wave ‘λ‘, is
wavelength. The number of waves passing a fixed point per second is
frequency ‘v’.
• c= λv

6. X-RAYS
• Produced extranuclearly
• Waves of electrical and magnetic energy in fields perpendicular to
each other.
• Streams of photons of packets of energy.
• Potency of X-rays depends on the size of each individual packets ; not
on the total energy absorbed.

7. Properties of photons
• A photon is a “packet” of electromagnetic energy.
• It has no mass, no charge, and travels in a straight line at the speed of
light.
• Photons interact differently in matter than charged particles because
photons have no electrical charge.
• In contrast to charged particles, photons do not continuously lose
energy when they travel through matter.

8. • When photons interact, they transfer energy to charged particles
(usually electrons) and the charged particles give up their energy via
secondary interactions (mostly ionization).
• The interaction of photons with matter is probabilistic, while the
interaction of charged particles is certain.

9. Particulate radiation
• This constitutes of electrons, protons, alpha particles, neutrons,
negative π-mesons, and heavy charged ions.
• Electrons are small, negatively charged particles that can be
accelerated to high energy to a speed close to that of light by means
of an electrical device, such as a betatron or linear accelerator. They
are widely used for cancer therapy.

10. • Protons are positively charged particles and are relatively massive,
having a mass almost 2,000 times greater than that of an electron.
Because of their mass, they require more complex and more
expensive equipment, such as a cyclotron, to accelerate them to
useful energies, but they are increasingly used for cancer treatment in
specialized centers.

11. Absorption of X-RAY
• Coherent Scatter
• Photoelectric Effect
• Compton Effect
• Pair Production
• Photo Disintegration

13. Photoelectric process

14. Compton Process

15. Pair Production

16. • Radioactive tracer injected into body gets trapped within tissue of
interest.
• Tracer emits positrons which interact with neighbouring electrons.
• Two annihilation photons are produced which is measured by PET.

17. • Positron emitting used are : 18-F ,11-C,13-N,15-O ; can be
incorporated into a biological substrate without altering their
biological activity
• Provides molecular imaging of a biological function in stead of
anatomy
• The detection of both annihilation photons in coincidence increase
the sensitivity of PET.However images are much blurier compared to
CT/MRI

18. PHOTO DISINTEGRATION

19. • At energies above 8-16 MeV
• Emission of neutron – more late toxicity
• Not a major contributor to therapeutics
• Main source of neutron contamination

21. Direct and Indirect action of Radiation

22. Indirect action of Radiation
• H2O → H2O+ + e−
• H2O+ + H2O → H3O+ + OH∙

23. • Incident x-ray photon
• ↓
• Fast electron (e−)
• ↓
• Ion radical
• ↓
• Free radical
• ↓
• Chemical changes from the breakage of bonds ↓ Biologic effects

24. Summary
• Types of radiation-ionizing and non-ionizing
• Types of ionizing radiation-electromagnetic and particulate
• Absorption of X-rays: coherent scatter, photoelectric effect, compton
effect, pair production, photo disintegration
• Direct and indirect effect of radiation

25. Reference
• Radiobiology for the Radiologist by Eric J.Hall, Amato J.Giaccia
• Goodwin PN, Quimby EH, Morgan RH. Physical Foundations of
Radiology. 4th ed. New York, NY: Harper & Row; 1970.
• Johns HE, Cunningham JR. The Physics of Radiology. Springfield, IL:
Charles C Thomas; 1969.
• Rossi HH. Neutron and heavy particle dosimetry. In: Reed GW, ed.
Radiation Dosimetry: Proceedings of the International School of
Physics. New York, NY: Academic Press; 1964:98–107.
• Smith VP, ed. Radiation Particle Therapy. Philadelphia, PA: American
College of Radiology; 1976.

26. THANK YOU