This document discusses the interaction of photons with matter. It describes ionizing radiation as radiation with enough energy to eject electrons from atoms or molecules, and non-ionizing radiation as lacking this energy. The document outlines the types of ionizing radiation, including electromagnetic (x-rays) and particulate (electrons, protons, etc.). It then explains various processes of photon absorption like the photoelectric effect and Compton scattering. The document also discusses the direct and indirect effects of radiation and uses of radiation in areas like PET imaging.

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