Interactions of X rays with matter

INTERACTIONS OF
X RAYS WITH
MATTER

INTRODUCTION
When x ray photon pass through matter ,three
scenarios are possible.
A photon can be deflected from its path and be
scattered.
A photon can pass straight trough and be
transmitted.
A photon can lose all its energy to an atom and
be absorbed.
The reduction in no of photons as radiation
passes through matter is called attenuation.

Interactions
Atoms are bounded into molecules by
electrons in the outermost shell.
X ray photons may interact either with orbital
electrons or with the nucleus of atoms.
In the diagnostic energy range, the interactions
are always with orbital electrons.
The most important factor is the atomic
makeup of a tissue not its molecular structure.

Absorbed and Scattered radiation
When the photons are absorbed,they are
completely removed from x ray beam and
cease to exist.
When photons are scattered ,they are deflected
into random course and no longer carry useful
information.
When they scatter,they cannot portray an
image , only thing they produce on film is
blackness.

This scatter radiation adds noise to the
system.
This noise is refered as Filmfog.
When an x ray film is badly fogged, the image
may be completely obscured.

About 1% of radiation that strike a patient s
body emerge from the body to produce the
final image .The radiographic image is formed
on radiographic plate that is similar to the film
of a camera.
Remaining 99% of the x rays are
Scattered/Absorbed.

There are 5 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

Two forms of X ray interactions
important in diagnostic study
Photoelectric effect
Compton radiation

COHERENT SCATTERING
The term is used when a radiation undergoes
a change in direction without a change in
wavelength.
It is also known as Unmodified scattering or
Classical scattering.
Further it is divided into
1.Thomson scattering
2.Rayleigh scattering

Both types of coherent scattering are
described in terms of waves and particles.
Thomson Scattering is single electron
involved in the interaction.
Rayleigh scattering results from cooperative
interaction with all electrons of an atom.

Rayleigh scattering
Low energy radiation encounters electrons
Electrons are set into vibration
Vibrating electron, emits radiation.
Atom returns to its undisturbed state

This type of interaction between x rays and
matter does not cause ionization. To produce an
ion pair energy must be transferred to the atom.
As no energy is transferred no ionization.
The only effect is change in direction.
The percentage of radiation that undergoes
coherent scattering is small,in general 5%.
Even though some coherent scattering
occurs,quantity is too small to be important in
diagnostic radiology.

PHOTOELECTRIC EFFECT
In the structure of atom.positively charged
nucleus holds the negatively charged electrons
in circular orbits having different energy levels
termed as K,L,M,N…etc
The shell closer to nucleus is tightly held by
nucleus.
The electrons in outermost shell are loosely
bound and are called free electrons.

The energy value of electronic shells is
determined by atomic no.
K shell electrons are more tightly bound in
elements with high atomic no than they are
in elements with low atomic no.
The electrons in K shell are at lower energy
level than electrons in L shell.
The energy debt is greatest when they are
close to nucleus with a element of high
atomic no.

Photo electric effect
An incident PHOTON encounters a K shell electron and ejects it from the
orbit
The photon disappears, giving up ( nearly) all its energy to the
electron
The electron ( now free of its energy debt) flies off into space as a
photoelectron carrying the excess energy as kinetic energy and becomes
negative ion.
The K shell electron void filled immediately by another electron from
adjacent shell
As electron drops into K shell it gives up energy in the form of x ray
photon.
This energy produced is characteristic of each element called as
CHARACTERISTIC RADIATION.( Atom remains positive ion)

Thus the Photoelectric effect yields three
end products :
 Characteristic radiation
 A -ve ion (photoelectron)
 A +ve ion (atom deficient in one
electron)

Probability of Occurance
1. The incident photon must have sufficient energy
to overcome the binding energy of electron.
2. A photoelectric effect is most likely to occur when
the photon energy and electron binding energy
are nearly same.
1
Photo electric effect ~
(energy)³
3. The tighter an electron is bound in orbit,the more
likely it is to be involved in a photo electric effect.
Photoelectric effect  (atomic no.)³

In summary, photoelectric reactions most likely
occur with low energy photons and elements
with high atomic numbers provided the
photons have sufficient energy to overcome
the binding forces of the electron.

Characteristic radiation
After the electron has been ejected, the atom is left
with a void in the K shell & an excess of energy
equivalent to the binding energy.
This state of the atom is highly unstable & to
achieve a low energy stable state ( as all physical
systems seek the lowest possible energy state ) an
electron immediately drops in to fill the void.
 As the electron drops into the K shell, it gives up its
excess energy in the form of an x-ray photon. The
amount of energy released is characteristic of each
element & hence the radiation produced is called

Characteristic radiation generated by the
photoelectric effect is exactly the same as in the x
ray tube.
The only difference is the modality used to eject the
inner shell electron.
 In x ray tube a high speed electron ejects the bound
electron,
while
 In photoelectric effect an X ray photon ejects the
bound electron.

In both cases
 the atom is left with an excess of energy = the
binding energy of an ejected electron.
Usually referred to as Secondary Radiation to
differentiate
it from scatter radiation……
End result is same for both,
“A Photon that is deflected from its original path”

K-shell electron binding energies of
elements important in diagnostic
radiology
Atomic no Atom K shell Binding Energy(k
ev)
Calcium 20 4.04
Iodine 53 33.2
Barium 56 37.4
Tungsten 74 69.5
Lead 82 88.0

Calcium which has highest atomic no of any
element in the body in significant quantities
emits a 4 K ev maximal energy,characteristic
photon, which is little energy in x ray standards.
It is absorbed within few millimeters of its site of
origin.
The contrast agents iodine and barium are the
only elements encountered in diagnostic
radiology that emit characteristic radiation
energetic enough to leave the patient and fog an
x ray film.

Characteristic radiation from iodine

Applications of photoelectric effect
to Diagnostic Radiology
ADVANTAGES: It produces radiographic images of
excellent quality.
As,
It does not produce scatter radiation.
It enhances natural tissue contrast.
Depends on 3rd power of the atomic no., so it
magnifies the difference in tissues composed of
different elements, such as bone & soft tissue

DISADVANTAGES: Patients receive more
radiation from photoelectric interaction.
All the energy of the incident photon is absorbed
by the patient.

The importance of photoelectric effect can be
mimimized by using high energy (k.vp)
techniques.
We should use radiation of the highest energy
consistent with that of diagnostic quality x ray
films to minimize the patient exposure.

Summary
The photoelectric interaction depends on two
factors.
The atomic no of the absorber.
The energy of the radiation.
The reactions are most common with low
energy photons and absorbers with high
atomic number.

COMPTON SCATTERING
 It occurs due to the interaction of the X-ray with
the outermost (and hence loosely
bound) valence electron at the atomic level.
 The resultant incident photon gets scattered
(changes direction) as well as ejects the electron
(recoil electron),producing an ion pair
The photon always retains part of its original
energy.

The reaction produces an ion pair
A +ve atom
A –ve electron ( recoil electron )
Almost all the scatter radiation that we
encounter in diagnostic radiology comes from
Compton Scattering

High energy incident photon strikes a free
outer shell electron
Ejection of electron Photon is deflected by
from its orbit occurs electron so travels in
and it travels new direction as
scatter radiation.

The energy of incident photon is distributed in two
ways.
Part of it goes to recoil Rest of energy is retained
electron as kinetic energy. By deflected photon

Interactions of X rays with matter

 Unlike a photoelectric reaction in which most of a
photon s energy is expended freeing the electron
from its bond,in compton reaction no energy is
needed for this purpose,as recoil electron is
already free.
Two factors determine the amount of energy
photon retains.
1. Its initial energy
2. Its angle of deflection.

 1.Initial energy :- Higher the energy more
difficult to deflect.
High energy : Travel straight retaining
most of the energy.
Low energy : Most scatter back at
angle of 180º, transferring more energy.

2. Angle of deflection :- Greater the
deflection angle, more energy is lost.
A zero angle deflection results in no energy
loss.
A 180 degrees more energy is transferred to
the secondary electron.
 Photons scattered back towards incident x
ray beam are called Backscatter Radiation.

In the diagnostic energy range upto 150
Kev,the photon retains most of its energy ,and
a very little is transferred to the recoil electron.
At narrow angles of deflection,scattered
photons retains almost all their original
energy.

Energy of Compton scattered
photon

Disadvantages of Compton reaction :
Scatter radiation : Almost all the scatter radiation that we
encounter in diagnostic Radiology comes from Compton
scattering.
In the diagnostic energy range, the photon retains most
of its original energy.
This creates a serious problem, because photons that
are scattered at narrow angles have an excellent chance
of reaching an x-ray film & producing fog.
Exceedingly difficult to remove –
► cannot be removed by filters because they are
too energetic.
► cannot be removed by grids because of narrow
angles of deflection.

It is also a major safety hazard. Even after 90˚
deflection most of its original energy is
retained.
Scatter radiation as energetic as the primary
radiation.
Safety hazard for the radiologist, personnel and
the patient.

Probability of occurrence :
It depends on :-
 Total number of electrons : It further
depends on density and number of electrons
per gram of the absorber.
All elements contain approx. the same no. of
electrons per gram, regardless of their atomic
no. Therefore the no. of Compton reactions is
independent of the atomic no. of the absorber.
 Energy of the radiation : The no. of
reactions gradually diminishes as photon
energy increases, so that a high energy
photon is more likely to pass through the body
than a low energy photon

PAIR PRODUCTION
A high energy photon interacts with nucleus of the
atom, the photon disappears and its energy is
converted to matter in the form of two particles.
One is ordinary electron and other is positron, a
particle with same mass as electron but positive
charge.
 Mass of one electron is 0.51 MeV.
2 electron masses are produced, so the interaction
cannot take place with photon energy less than
1.02 MeV.

PHOTODISINTEGRATION
In photo disintegration,part of the nucleus of
atom is ejected by a high energy photon.
The ejected portion may be a neutron, a
proton, an alpha particle or a cluster of
particles.
The photon must have sufficient energy to
overcome nuclear binding energies of the
order 7 to 15 Mev.

Because pair production does not occur
with photon energies less than 1.02 Mev,
and photodisintegration does not occur with
energies less than 7 Mev, neither of these
interactions is any of importance in
diagnostic radiology, where we rarely use
energies above 150 kev.

Relative frequency of basic
interactions :

In the above interactions, the toatal no of
reactions is always 100%.
Thus if Coherent scattering accounts for 5% of
interactions, Compton scattering for 20%, and
Photoelectric effect for 75%.total is 100%.
Water is used to illustrate the behavior of tissues
with low atomic no s such as air, fat and muscle.
In water compton scattering is dominant .
 Bone is intermediate between water & the
contrast agents.

The contrast agents because of their high
atomic no are involved exclusively in
photoelectric reactions
At low energies, Photoelectric reactions are
more common, while at high energies,
Compton scattering is dominant.

SUMMARY
Only two interactions are important in diagnostic
radiology, the Photoelectric effect & Compton
scattering.
The Photoelectric effect is the predominant
interaction with low energy radiation & high
atomic no. absorbers.
It generates no significant scatter radiation &
produces high contrast in the x-ray image.
 But, unfortunately it exposes the patient
to a great deal of radiation

Compton scattering
 is the most common interaction at
higher diagnostic energies.
 responsible for almost all scatter
radiation.
radiographic image contrast is less
compared to photoelectric effect.
Coherent scattering is numerically
unimportant.
Pair production & Photodisintegration
occur at energies above the useful
energy range.

Interactions of X rays with matter

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