07.07.2023_-_Unit_4_-_Interaction_of_Radiation_with_Matter_061254.pdf

Interaction of radiation with matter

1. Unit 4.0 –Interaction of Radiation
with matter
Barbara C M’ule
Radiation Sciences –RAD 2310
UNZA

2. Unit 4.0 Interactions of Radiation with Matter
qOUTLINE
ØGeneral interactions of radiation with matter
ØInteractions with the atom
ØInteractions at bone and soft-tissue interface
ØRadiation dose

3. Objectives
At the end of the lesson the student should be able
to:
1.Write an equation for attenuation of photons in a given
thickness of a material
2.Describe each of the five (5) x-ray interactions with matter.

4. Discussion will refer to X-rays, however it this
discussion applies to gamma rays.

5. There are three (3) possiblr
outcomes when x-ray
travel through matter
Transmitted:pass
through unaffected, or
with a lower energy as
primary (initial beam) or
direct radiation.
Absorbed:the photon
or x-ray gives all of
energy to the matter
after that it no longer
exists (does not pass
through)

6. Scattered: diverted in a
new direction with or
without loss of energy
transferring to the
matter, and so may
leave the material as
scattered or secondary
radiation.

7. X-ray absorption and scattering processes are
stochastic (probabilistic in nature) processes
governed by the statistical law of chance.
It is impossible to predict which of the individual
photons in a beam will be transmitted by a material,
but it is possible to predict quite precisely the
fraction that will be.

8. The x-ray image formation is by the transmitted photons.
Those that are absorbedor scatteredrepresent attenuation by matter.
Attenuated x-rays are those that are absorbed, transmitted
with a lower energy or scattered.
An understanding of how the properties of x-rays and the
materials through which they travel affect the relative
amount of attenuation and transmission gives an
understanding of how the x-ray image is formed

9. Attenuation
Attenuation –refers to the fact that they are fewer
photons in the emerging beam than in the beam
entering the material.
It is an exponential process and therefore, the beam
intensity never reaches zero
It is represented by the photons that are absorbed,
transmitted with a lower energy and those that are
scattered.
Attenuation can be represented numerically by:
Half value layer
Linarattenuation coefficient and
Mass attenuation coefficient

10. Half Value Layer – (HVL)
qThe HVL is the measure of the penetrating power of the x-ray
beam and is the amount of matter require to attenuate the beam to half its energy value
qIt differs for different materials and strengths of beams
qTo calculate the factor of reduction us 2HVL
ØFor example – if the HVL of a beam is 2 mm by what factor is
the beam attenuated if it passes through 8 mm of material?
Ø8 mm = 4 HVLs
Ø24 = 16
ØThe beam is attenuated by a factor of 16

11. qFirstly let us consider a simple
case of attenuation
§A narrow, mono-energetic
beam of X-rays
The fundamental law of x-ray
attenuation states that, for a
mono-energetic beam, equal
thickness of an absorber
transmits equal fractions
(percentages) of radiation
entering them,
This is shown by the figure
below where each sheet
reduces the beam by 20%

12. The half-value layer (HVL) is the thickness of stated
that will reduce the intensity of a narrow beam of X-
radiation to one-half of its original value.
From the figure above (b) two HVLs reduce the
intensity of the beam by a factor 22=4 from 1024 to
256. 10 HVLs would reduce the intensity of the
beam by a factor 210=1024 to 1 for this example.

13. qThe HVL is a measure of the penetrating power or effective energy of
the beam. It is useful to have a parameter that quantifies the
attenuating properties of the material.
§This is the linear attenuation coefficient (μ), which is inversely proportional to
the HVL
§!=!.#$%
&'(

14. Linear Attenuation Coefficient
qMore precisely, the linear attenuation coefficient measure the
probability that a photon interacts (i.e. is absorbed or scattered) per unit length of the path it travels in a specified
material.
§The linear attenuation coefficient (LAC) is the probability of the
material to attenuate the beam
qIt may also be expressed as the amount of energy transferred
to the material per unit of track length of the particle
qThe LAC (μ)
§!=!.#$%
&'(§Unit of μ is cm-1

15. However, the linear attenuation coefficient applies
only to narrow mono-energetic beams.
The HVL can be used for beams that are not mono-
energetic but applies only to narrow beams.

16. Mass attenuation coefficient
qThe MAC is a measure of the rate of energy loss by a photon beam as
it travels through an area of material.
qBy dividing the LAC by the density of the material the effect of
density is removed
qThe MAC is thus independent of the density and depends only on the
atomic number of the material and the photon energy.
§#$%=)
*

17. As HVL decreases the linear attenuation coefficient
increases, as:
§The density of the material increases
§The atomic number of the material increases
§The photon energy of the radiation decreases
For example lead is more effective than either
aluminium or tissue at absorbing X-rays because of
its higher density and atomic number

18. X-rays of 140 keV are more penetrating and are said
to be ‘harder’ than those of 20 keV.
The mass attenuation coefficient (µ/ρ)is obtained
by dividing the linear attenuation coefficient by the
density of the material. It is therefore independent
of the density and depends only on the atomic
number of the material and the photon energy.

19. However thick the
absorber, it is never
possible to absorb an
X-ray beam completely

21. If as in the figure above the
percentage transmission is
plotted on a logarithmic scale,
it results in a linear graph,
making it easier to read off the
HVL and to calculate µ

22. In order to measure HVL
and determine the
attenuation factor a
narrow beam arrangement
needs to be employed
The beam is restricted by
means of a lead diaphragm
or collimator such that the
aperture is enough to
cover a small detector.

23. B and the sheets of
absorbing material are
positioned halfway
between the source A and
the detector D.
This arrangement
minimises the amount of
scattered radiation S
entering the detector.

24. However for amount of
scatter produced and detected in a broader
beam is much greater.
Thus in the broad beam
the measured HVL would
be increased.

25. However for amount of
scatter produced and detected in a broader
beam is much greater.
Thus in the broad beam
the measured HVL would
be increased.

26. Attenuation of a heterogeneous
beam
The beams produced by X-ray tubes is heterogeneous
(polyenergetic) –comprise of photons of a wide range of energies as shown in the figure
At beams travel through an attenuating material, the lower-
energy photons are attenuated proportionally more than the
higher-energy photons.
The exponential law does not therefore apply exactly. It is
still correct to refer to the HVL of the beam.
The HVL of a typical diagnostic beam is 30 mm in tissue and
12 mm in bone and 0.15 mm in lead

27. As the beam penetrates the material it becomes
progressively more homogenous. The proportion of
higher-energy photons in the beam increases, a
process described as beam hardening.
The average energy of the photons increases –the
beam becomes harder or more penetrating. The
second HVL, which would reduce the beam intensity
from 50% to 25% is thus greater than the first HVL
which reduces the intensity from 100% to 50%.

28. The X-ray beam used in practice are usually both
wide and heterogeneous and the exponential law of
absorption does not strictly apply.
However it is still possible to use the exponential law
of X-ray attenuation in approximate calculations
together with an effective attenuation coefficient.

29. “The only real wisdom is knowing
you know nothing” -Socrates