Physics and Chemistry of Radiation Absorption

Chapter 1

In the beginning…

Radiation is Everywhere

The Humble Discovery

Wilhelm Conrad Roentgen

Gas discharge tube

X-rays1895

Ionizing Radiation

– Alpha (), beta () – Gamma (), X-ray – Protons, neutrons

• Ionizing vs. Excitation

Ionization Excitation

Ionizing Radiation

• Particulate Radiation– Alpha, beta, protons,

neutrons, heavy charged ions, etc.

• Electromagnetic Radiation– (UV), X-rays and

Gamma rays

X-ray/γ-ray Absorption

• Photoelectric effect– Low energy range• Electron emitted• X-ray emitted

• Compton Scattering– High energy range• Electron emitted• Scattered photon

Direct vs Indirect Action

• Particulate radiation = HIGH LET – Direct Action– Damages DNA directly by breaking bonds

• Electromagnetic radiation = LOW LET– Indirect Action (~two thirds of time)– Damages DNA indirectly via radicals and reactive

molecules

Direct vs Indirect Action

Direct Action

Cannot be modified by sensitizers or protectors

Indirect ActionH2O

H2O+

e-H

+

H2O2

OHo

HO2

OH-

Ho

H2

WATERIncoming Radiation

Production of free radicals within the cell can result in indirect effects

Most abundant molecule within humans: Water

Can be modified by sensitizers or protectors

Indirect Action Summary

Incident X-ray Photon

Fast Electron (e-)

Ion Radical

Free Radical

Chemical Changes for Breakage of Bonds

Biological Effects

Ionizing Radiation

Natural Background82%

Man-made18%

Radiation Exposure

Nuclear Medicine

21%

Consumer Products

16%Occupa-

tional2%Fallout

2%Nuclear

Fuel Cycle1%

Medical X-rays58%

Man-made Radiation

Q & A’s

• 1. What is the difference between X-rays and gamma rays?

a. X-rays are produced extra-nuclearly whereas gamma rays are produced in nuclear decays.

b. X-rays have higher energies than gamma rays. c. X-rays and gamma rays interact with matter

differently.d. A and B are correct.

Q & A’s

• 2. What is the main source of natural background radiation?

a. Electrons. b. X-rays. c. Neutrons. d. Alpha-particles. e. Gamma rays

Q & A’s

• 3. Which photon processes are dominant in the context of diagnostic radiology?

a. Compton scattering and photoelectric effect.

b. Photoelectric effect and pair production. c. Compton scattering and excitation. d. Photoelectric effect and ionization.

Q & A’s

• 4. Direct action of radiation is the dominant process for

a. X-rays. b. Neutrons and alpha particles. c. Electrons. d. Gamma rays.

Q & A’s

• 5. Which of the following statements is true?

a. Indirect action can be modified by protectors or sensitizers.

b. Direct action can be modified by protectors or sensitizers.

Q & A’s

• 6. A free radical is:

a. Any charged particle. b. An atom or molecule with an unpaired

electron in the outer shell. c. An atom with an even number of

electrons. d. A chemically stable atom.

Q & A’s

• 7. All types of radiation can induce biologic effects by direct or indirect action.

a. True.

b. False.

Q & A’s

• 8. What is the main reason for the difference in biologic effects caused by neutrons and photons?

a. The energy of the radiation. b. The fact that the charged particles produced by

neutrons are, for the most part, positively charged.

c. The density of ionization. d. The fact that neutrons can interact with nuclei.

Q & A’s

• 9. What is the correct order from low to high LET?

a. Protons, neutrons, alpha particles, x-rays b. Alpha particles, protons, beta, gamma raysc. X-rays, beta, protons, neutrons d. gamma rays, beta, protons, alpha particles

Q & A’s

• 10. What year did Röntgen discovery x-rays?

a. 1885b. 1886c. 1895d. 1897e. I don’t care.