1. CLRS 321 Nuclear Medicine Physics and Instrumentation 1
Lecture 5: Interactions of Radiation with Matter Unit 1—the Physics Of Nuclear Medicine
CLRS 321
Nuclear Medicine Physics and Instrumentation 1

2. Lecture 5 Objectives (Adapted from your Textbook)
Discuss the processes of excitation and ionization.
Describe the interactions of charged particles with matter.
Describe the processes of photoelectric effect, coherent & incoherent scattering, and pair production.
Lecture 5 Objectives (Adapted from your Textbook)

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
Interactions

4. Charged Particle Interactions with Matter
Alpha (+2 charge)
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 (minus)
Can create Bremsstrahlung radiation (X-rays) when near high Z materials
With pure beta emitters, plastic is better shielding than lead to avoid Bremsstrahlung radiation
Positron (Beta plus)
Tend to quickly undergo an annihilation reaction with an electron
Charged Particle Interactions with Matter

5. Bremsstrahlung Radiation
Copyright © 2017 Elsevier Inc. All rights reserved.

6. Annihilation Photons
Copyright © 2017 Elsevier Inc. All rights reserved.

7. Photon Interactions with matter
Represent electromagnetic radiation
Visible light
Reflected or absorbed
X-rays, gamma rays, annihilation photons
One of three (really, maybe four) possibilities
No interaction (pass through)
Scatter (and is usually partially absorbed)
Completely absorbed
And also may become matter and thus absorbed
Rate of absorption increases exponentially with distance travelled through matter
Photon Interactions with matter

8. Total absorption of a gamma photon at the expense of an electron
Photon energy must be equal or greater than electron binding energy
“Photoelectron” is ejected
Electron falls from outer shell and emits characteristic X-ray photon
Photoelectric Effect
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52.

9. Coherent (Rayleigh) Scattering
Probability of Coherent Scatter
(More likely to happen with low energy photons and high Atomic number atoms)
Coherent (Rayleigh) Scattering
Copyright © 2017 Elsevier Inc. All rights reserved.

10. Incoherent (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
Incoherent (Compton) Scattering
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

11. 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)
Interactions: Compton Scatter

12. Interactions: Compton Scatter
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:
Interactions: Compton Scatter
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

13. 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:
Interactions: Compton Scatter

14. 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

15. 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).
Interactions: Compton Scatter
From Table 6-2, p. 78, Physics in Nuclear Medicine, 3rd Ed., by Simon Cherry, James Sorenson, and Michael Phelps, Saunders: Philadelphia, 2003.

16. 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
Interactions: Pair Production
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

17. Next time:
Attenuation and Transmission of Photons