interaction of x ray with matter

1. M.KARTHIKEYAN
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING
AAA COLLEGE OF ENGINEERING & TECHNOLOGY, SIVAKASI
karthikeyan@aaacet.ac.in
ME8097 NON DESTRUCTIVE
TESTING AND EVALUATION

2. UNIT V RADIOGRAPHY (RT)
1. Principle, interaction of X-Ray with matter,
2. imaging, film and film less techniques,
3. types and use of filters and screens,
4. geometric factors, Inverse square, law,
5. characteristics of films - graininess, density, speed, contrast,
6. characteristic curves, Penetrameters,
7. Exposure charts, Radiographic equivalence.
8. Fluoroscopy- Xero-Radiography,
9. Computed Radiography, Computed Tomography

3.  X-rays in the diagnostic range interact with matter primarily via
two major processes, which are fundamental in understanding
how an image is formed in a radiographic exam.
 These processes are the:
1. photoelectric effect
2. Compton scatter

4. PHOTOELECTRIC EFFECT
 Photoelectric effect or photoelectric absorption is one of the
principal forms of interaction of x-ray and gamma photons with
matter.
 A photon interacts with an inner shell electron in the atom and
removes it from its shell.

5. PROBABILITY OF PHOTOELECTRIC EFFECT
The probability of this effect is maximum when:
 the energy of the incident photon is equal to or just greater
than the binding energy of the electron in its shell (absorption
or k edge) and
 the electron is tightly bound (as in K shell)
 The electron that is removed is then called a photoelectron and
the incident photon is completely absorbed in the process.
 Hence, the photoelectric effect contributes to the attenuation of
the x-ray beam as it passes through matter.
 To stabilize the atom an outer shell electron fills the vacancy in
the inner shell.

6.  The energy which is lost by this electron as it drops to the inner
shell is emitted as characteristic radiation (an x-ray photon) or
as an Auger electron.
The probability of photoelectric absorption occurring is
 proportional to the cube of atomic number of the attenuating
medium (Z), and
 inversely proportional to the cube of the energy of the incident
photon (E), and
 proportional to the physical density of the attenuating medium
(p)

7. • Thus the overall the probability of photoelectric absorption can
be summarized as follows:
Photoelectric absoption ~ p(Z³/E³)
• Therefore if Z doubles, photoelectric absorption will increase by
a factor of 8 (2³ = 8), and if E doubles photoelectric absorption
will reduce by a factor of 8.
• Small changes in Z and E can therefore significantly affect
photoelectric absorption.
• This has practical implications in the field of radiation
protection and is the reason why materials with a high Z such as
lead (Z = 82) are useful shielding materials.
• Photoelectric absorption is also utilized in mammography and
when using contrast agents to improve image contrast.

8. • The dependence of photoelectric absorption on Z and E means
that it is the major contributor to beam attenuation up to
approximately 30 keV when human tissues (Z = 7.4) are
irradiated.
• At beam energies above this, the Compton effect predominates.

9. COMPTON EFFECT
• Compton effect or Compton scatter is one of principle forms
of photon interaction.
• It is the main cause of scattered radiation in a material. It
occurs due to the interaction of the photon (x-ray
or gamma) with free electrons (unattached to atoms) or loosely
bound valence shell (outer shell) electrons.
• The resultant incident photon is scattered (changes direction)
and imparts energy to the electron (recoil electron).
• The scattered photon will have a different wavelength (observed
phenomenon) and thus a different energy (E=hc/λ).
• Energy and momentum are conserved in this process.

10. • The Compton effect is a partial absorption process and as the
original photon has lost energy, known as Compton shift (i.e. a shift
of wavelength/frequency).
• The wavelength change of the scattered photon can be determined
by 0.024 (1- cos θ), where θ is scattered photon angle. Thus, the
energy of the scattered photon decreases with increasing scattered
photon angle.
• Probability of Compton effect
• directly proportional to
– number of outer shell electrons, i.e. the electron density
– physical density of the material
• inversely proportional to
– photon energy
• does not depend on
– atomic number (unlike photoelectric effect and pair production)

11. • In other words, the probability of the Compton effect is
dependent on the number of electrons per gram in the absorbing
material, which for most elements is approximately the same
(approx. 3 x 1023).
• An exception though is the element hydrogen, which has no
neutrons in its nucleus and therefore has an electron density
which is twice that of all other elements (approx. 6 x 1023 ), thus
the Compton effect is independent of the atomic number (Z) of
the absorber.
• The significance of the Compton effect is it becomes the
dominant process when human tissues are irradiated in the 30
keV to 30 MeV energy range which is the diagnostic and
therapeutic radiation range.