photon interaction with matter

1. PHOTON INTERACTION WITH
MATTER
DARSHAN BS
MEDICAL IMAGING TECHNOLOGIST
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY BANGALORE

2. content
• Nature of radiation,Types.
• Attenuation
• Absorption
• scattering
• Photon Interaction with matter

3. NATURE OF RADIATION
• The term radiation applies to the emission &propagation of energy
through space or material
• TYPES OF RADIATION
1)Electromagnetic Radiation(ex .light wave,heat waves,xray,Y rays )
-defined by Maxwell in terms of oscillating electrical &magnetic field.
EM rays range from 10-7 M(radiowaves to 10 -13 (ultra high energy xray)
2)Particulate radiation

5. 2)Particulate radiation
-Refers to energy propagated by traveling corpuscles
which have definite rest mass
-elementary atomic particles eletron,protons,Neutron
-positron ,neutriono are subatomic particles

6. Nature of matter

8. RADIATION INTERACTION

9. Photon Beam may undergo the following 4
process
ATTENUATION
ABSORPTION
SCATTERING
TRANSMITTED

10. ATTENUATION
• Attenuation is the product of absorption and scattering. It is the
removal of photons from the beam due to absorption and scattering.
• it is given by the relation
I is the number of transmitted photons,
I0 is the number of incident photons,
e is the base of natural logarithm and
µ is the linear attenuation coefficient of the absorber material

12. Linear Attenuation Coefficient
• The linear attenuation coefficient is defined as the reduction in the
radiation intensity per unit path length and its unit is cm–1.
• It refers the fractional reduction produced in any mono enegetic
photon-beam is constant for any given material per unit thickness.
• This constant is know as the linear attenuation coefficient .
• linear attenuation coefficient express of the probality of the photon
being removed by a given material.
• linear attenuation coefficient is related to the half value layer by
following expressing µ=0.693/HVL or

13. Mass Attenuation Coefficient
•The mass attenuation coefficient is obtained by dividing
the linear attenuation coefficient by the density P
•has the symbol µ/p and unit cm2/g
•The mass attenuation coefficient is independent of
density

14. The half value layer (HVL)
• The half value layer (HVL) is the thickness required to reduce the
beam intensity to half of its original value
• The linear attenuation coefficient is related to the term half value
layer as follows: HVL = 0.693/µ
• HVL is an indirect measure of photon energies.
• Varies material used for measurement of HVL
Generator energy <30KV 30-150KV 120-600KV 500KV-2MV
material cellophane aluminium copper lead

15. X-RAY AND GAMMA-RAY INTERACTIONS
or photon beam attenuation
•When traversing matter, photons will penetrate
without interaction, scatter, or be absorbed
•There are four major types of interactions of x-ray and
γ ray photons with matter, the first three of which play
a role in diagnostic radiology and nuclear medicine

16. THERE ARE FIVE BASIC WAYS THAT AN X-RAY
PHOTON CAN INTERACT WITH MATTER
1. Coherent scattering
2. Photoelectric effect
3. Compton scattering
4. Pair production
5. Photodisintegration

17. COHERENT SCATTERING
• "coherent scattering" is given to those interactions in which radiation un
dergoes a change in direction without a change in wavelength.
• For this reason, term "unmodified scattering" is sometimes used.
• There are two types
1) Thomson scattering-Thomson scattering a single electron
is involved in the interaction
2) Rayleigh scattering-Ray leigh scattering results from a cooperative
interaction with all the electrons of an atom
Both types of coherent scattering may be described in terms of a wave-
particle interaction, and are therefore some times called "classical
scattering.

18. • Low-energy radiation encounters the electrons of an atom and sets them into
vibration at the frequency of the radiation
• This is the only type of interaction between x rays and matter that does not cause
ionization
• no ionization occurs with coherent scattering. Its only effect is to change the
direction of the incident radiation
• coherent scattering occurs
throughout the diagnostic
energy range, but it never plays a
major role
• it produces scattered radiation
• which contributes to film fog,
the total quantity is too small to
be important in diagnostic radiology.

19. Coharent scattering properties

20. PHOTOELECTRIC EFFECT

23. COMPTON SCATTERING

27. PAIR PRODUCTION
• When a photon having energy > 1.02 MeV, passes near the nucleus of
an atom, will be subjected to strong nuclear field.
• The photon may suddenly disappear and become a positron and
electron pair
• r. For each particle 0.511 MeV energy is required and the excess
energy > 1.02 MeV, would be shared between the positron and
electron as kinetic energy