Session 3: Atomic Structure and
Ionizing Radiation (cont’d)
Lecture 3
CLRS 321
Nuclear Medicine Physics and
Instrumentation 1
Lecture 2 Objectives
(Adapted from your Textbook)
• Describe the interactions of charged particles with matter.
• Discuss the processes of excitation and ionization.
• Describe the processes of photoelectric effect, Compton scattering,
and pair production.
• Discuss the production of characteristic X-rays.
• Discuss the process that produces Auger electrons.
• Write the general form of the attenuation equation for gamma
photons.
• Calculate the reduction of gamma radiation using the general
attenuation equation.
• State the relationship between the linear attenuation coefficient and
the half-value layer.
Interactions
• Excitation
– Charged particle or electromagnetic radiation
supplies energy to outer shell electrons
• The “excited” electron moves to a higher shell or
subshell
• Electron spontaneously returns to a less excited
state giving up electromagnetic radiation
• Ionization
– Charged particle or electromagnetic radiation
completely removes electron from atom
• Results in an ion pair
Interactions: Excitation
Paul Early, D. Bruce Sodee, Principles and
Practice of Nuclear Medicine, 2nd Ed.,
(St. Louis: Mosby 1995), pg. 13.
Interactions: Ionization
If an electron has a binding energy of 70 keV, then it would require 70 keV of energy to kick
that electron out of its shell.
.
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis:
Mosby 1995), pg. 11.
Interactions: Alpha & Beta
• Alpha
– Typically have energies between 3 & 8 MeV
• Requires about 34 keV to strip an electron from an
atom
– Thus alphas can create hundreds of thousands of ion
pairs in less than a mm of tissues
• Beta
– Can create Bremsstrahlung radiation when
near high Z materials
• With pure beta emitters, plastic is better shielding
than lead to avoid Bremsstrahlung radiation
Interactions: Photons
• Represent electromagnetic radiation
– Visible light
• Reflected or absorbed
– X-rays and gamma rays
• One of three (really, maybe four) possibilities
– No interaction (pass through)
– Scatter (partially absorbed)
– Completely absorbed
» And also may become matter and thus absorbed
• Rate of absorption increases exponentially with
distance travelled through matter
Interactions: Photoelectric Effect
Total absorption of
a gamma photon
at the expense
of an electron
Photon energy
must be equal or
greater than
electron binding
energy
Electron falls from
outer shell and
emits
characteristic Xray photon
Paul Christian, Donald Bernier, James Langan, Nuclear
Medicine and Pet: Technology and Techniques, 5th Ed.
(St. Louis: Mosby 2004) p 52.
E  Ebinding  Ekinetic
Interactions: Compton Scattering
Gamma Photons don’t just
disappear when they
confront matter—their
energy has to be accounted
for
Compton is a type of scatter in
which an electron is ejected
and the gamma photon
continues at a deflected
angle
The amount of energy that the
photon is reduced is
dependent upon the angle
at which it is scattered
when it ejects the electron
Paul Christian, Donald Bernier, James Langan, Nuclear
Medicine and Pet: Technology and Techniques, 5th Ed.
(St. Louis: Mosby 2004) p 53.
The more the photon is
deflected (greater angle),
the less its energy it retains
Interactions: Compton Scatter
Compton events tend to
increase with higher Z
material
The incident photon energy is
equivalent to the binding
energy of the electron and
its kinetic energy of its
recoil, plus the deflected
energy of the photon
The deflected energy of the
photon can be calculated
based on its deflected
angle (θ)
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/imgqua/compton2.gif
ESc 
EO
EO


1 
 1  cos  
0.511
MeV


Interactions: Compton Scatter
NOTE: YOUR BOOK IS WRONG!
The minimum amount of
energy of a backscattered
(180◦) Compton Scatter
photon can be calculated
as:
Emin 
E0
2 E0
1
0.511MeV
The maximum amount of backscatter energy transferred to
the recoil electron in a
backscatter event can be
calculated as:
Emax
Paul Christian, Donald Bernier, James Langan, Nuclear
Medicine and Pet: Technology and Techniques, 5th Ed.
(St. Louis: Mosby 2004) p 53.
E02

( E0  0.2555MeV )
Interactions: Compton Scatter
An example for calculating the minimum amount of energy a
Tc-99m backscattered 140 keV photon can have:
Emin
0.140 MeV

 0.090MeV
2(0.140 MeV )
1
0.511MeV
An example for calculating the maximum energy a recoil
electron can have from a maximum backscattered Tc-99m
photon:
(0.140MeV )2
Emax 
 0.049MeV
(0.140MeV  0.2555MeV )
Interactions: Compton Scatter
• What does all this mean???
– The minimal energy of a backscattered
photon will form something called the
“Backscattered peak” on the energy spectrum
(we’ll cover that later).
– Emin of the backscatter photon and Emax of the
recoil electron is energy-dependent and the
difference between the two increases with
incident photon energy
Interactions: Compton Scatter
Radionuclide
Photon E
Emin of
Backscattered
Photon
Emax of Recoil
Electron
I-125
27.5 keV
24.8 keV
3.3 keV
Xe-133
81 keV
62 keV
19 keV
Tc-99m
140 keV
91 keV
49 keV
I-131
364 keV
150 keV
214 keV
Annihilation
511 keV
170 keV
341 keV
Co-60
1330 keV
214 keV
1116 keV
--
To infinity
255.5
To infinity
Since the energy imparted to the recoil electron must exceed the binding
energy of the electron, this means that Compton Scatter is more likely to
occur at higher incident photon energies (to a point—we will soon see).
From Table 6-2, p. 78, Physics in Nuclear Medicine, 3rd Ed., by Simon Cherry, James Sorenson, and Michael
Phelps, Saunders: Philadelphia, 2003.
Interactions: Pair Production
Requires gamma photon
of at least 1.022 MeV
to pass near a highelectrical field of a
nucleus
Energy is converted to
matter (m=E/c2)
A positron and electron
are created, each
with a mass
equivalent of 511keV
Paul Christian, Donald Bernier, James Langan, Nuclear
Medicine and Pet: Technology and Techniques, 5th Ed.
(St. Louis: Mosby 2004) p 53.
Extra Nuclear Release:
Bremsstrahlung
Important consideration when using
beta emitters
German for “breaking radiation”
Beta decelerating in vicinity of high
density (high Z) nucleus
dissipates energy in the form of
x-ray photons
Best to use plastic or lucite syringe
shields with beta emitters to
avoid the Bremsstrahlung effect
as the beta particles penetrate
lead shielding
Paul Christian, Donald Bernier, James Langan, Nuclear
Medicine and Pet: Technology and Techniques, 5th Ed.
(St. Louis: Mosby 2004) p 54.
Can get poor quality nuclear
medicine images using
Bremsstrahlung (ex: Sr-89)
Extra Nuclear Release:
(Energy States of Electrons)
This picture from the
Sodee text represents the
electron energy states as
different speed limits
around the nucleus of an
atom.
In order for a car at 70
mph to go down to the 65
mph speed limit, it must
lose a “quantum” of 5
mph. For electrons, this
quantum is in the form of
a specific wavelength of
electromagnetic radiation.
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis:
Mosby 1995), pg. 11.
Extra Nuclear Release:
Characteristic X-rays
Paul Christian,
Donald Bernier,
James Langan,
Nuclear Medicine
and Pet:
Technology and
Techniques, 5th
Ed. (St. Louis:
Mosby 2004) p
54.
This figure from your textbook shows what happens when an electron loses energy to
move from the L shell to the K shell.
Again Electromagnetic radiation is emitted, but it is of a higher energy (shorter
wavelength/higher frequency) than visible light and is in the form of an X-ray photon.
Such an emission is called a “characteristic X-ray” and its “character” is dependent upon
and equal to the specific difference in energy states between the L and K shells of the
atom.
Extra Nuclear Release:
Auger Electrons
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), Fig 2-25, p 57.
Attenuation and Transmission of
Photons
This is how all the gamma radiation eventually succumbs to matter
It is absorbed or attenuated. This is how it relates to instrumentation
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St.
Louis: Mosby 2004) p 52.
Attenuation and Transmission of
Photons
• Attenuation
Combined effects of
attenuation is expressed
by the linear attenuation
coefficient (μ), which is in
the units 1/distance(cm-1).
The attenuation of incident
radiation (I) can be
expressed as follows:
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet:
Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52.
X is the distance through which the incident
radiation travels through the attenuating material.
Paul Christian & Kristen M. Waterstram-Rich,
Nuclear Medicine and Pet/CT: Technology
and Techniques, 6th Ed. (St. Louis: Mosby
2004), p 57.
Attenuation and Transmission of
Photons
• Half-Value Layer (HVL)
– Similar concept to T1/2
– Layer of attenuating material that will absorb
½ the incident radiation
– Specific for type of material and energy of
incident radiation
– Is related to μ according to the following:
Paul Christian & Kristen M. Waterstram-Rich,
Nuclear Medicine and Pet/CT: Technology
and Techniques, 6th Ed. (St. Louis: Mosby
2004), p 57.
Where have we
seen this
before???
Attenuation and Transmission of
Photons
• Substituting the previous for μ, our
attenuation equation now looks like…
Paul Christian & Kristen M. Waterstram-Rich,
Nuclear Medicine and Pet/CT: Technology
and Techniques, 6th Ed. (St. Louis: Mosby
2004), p 57.
Attenuation and Transmission of
Photons
• An example (from book)
– I-131
• (364 keV principle gamma photon E)
– Lead is the shielding
• HVL is 0.3 cm for 364 keV photons
• Thickness of the lead is 0.9cm
– Incident radiation field is 5mR/hr
0.693

 2.31cm 1
0.3cm
Paul Christian & Kristen M.
Waterstram-Rich, Nuclear
Medicine and Pet/CT:
Technology and Techniques,
6th Ed. (St. Louis: Mosby
2004), p 57.
Attenuation and Transmission of
Photons
• Mass Attenuation Coefficient
– Based on material density
• Is related to the linear attenuation coefficient
– Physicists can break this down so that they
can measure attenuation according to
Compton scatter, photoelectric effect, and pair
production
Paul Christian & Kristen M.
Waterstram-Rich, Nuclear
Medicine and Pet/CT:
Technology and Techniques,
6th Ed. (St. Louis: Mosby
2004), p 57.
Next time:
Basic Electrical Concepts
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EVERSION=1241027409629