1. Session 3: Atomic Structure and Ionizing Radiation (cont’d) Lecture 3
CLRS 321
Nuclear Medicine Physics and Instrumentation 1

2. 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.

3. Interactions Excitation Ionization
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

4. Interactions: Excitation
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 13.

5. 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.

6. Interactions: Alpha & Beta
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

7. 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

8. 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 X-ray photon
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52.

9. 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
The more the photon is deflected (greater angle), the less its energy it retains
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

10. Interactions: Compton Scatter
Compton events tend to increase with higher Z material
Compton events tend to decrease with higher photon energy
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)

11. Interactions: Compton Scatter
NOTE: YOUR BOOK IS WRONG!
The minimum amount of energy of a backscattered (180◦) Compton Scatter photon can be calculated as:
The maximum amount of back-scatter energy transferred to the recoil electron in a backscatter event can be calculated as:
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

12. Interactions: Compton Scatter
An example for calculating the minimum amount of energy a Tc-99m backscattered 140 keV photon can have:
An example for calculating the maximum energy a recoil electron can have from a maximum backscattered Tc-99m photon:

13. 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

14. 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
1116 keV --
To infinity
255.5
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.

15. Interactions: Pair Production
Requires gamma photon of at least MeV to pass near a high-electrical 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.

16. 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
Can get poor quality nuclear medicine images using Bremsstrahlung (ex: Sr-89)
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 54.

17. 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.

18. 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.

19. 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.

20. 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.

21. Attenuation and Transmission of Photons
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.
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.
X is the distance through which the incident radiation travels through the attenuating material.

22. 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???

23. 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.

24. 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
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.

25. 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.

26. Basic Electrical Concepts
Next time:
Basic Electrical Concepts