of Radiation With MatterJohn F. Greskovich, Jr., Mihir Naik, and Marc Apple

INTERACTION OF RADIATION WITH MATTER

JOHN F. GRESKOVICH, JR., MIHIR NAIK, AND MARC APPLE

Question 1

What type of decay does Radium-226 undergo, and what particles are produced in the decay?

Question 2

What are the names of the types of photons within the electromagnetic spectrum from lowest and highest energy?

Question 3

What is the relationship between photon energy, wavelength, and frequency?

Question 4

What is a photon?

Question 1 What type of decay does Radium-226 undergo, and what particles are produced in the decay?

Answer 1

Radium-226 decays by α-decay, creating radon gas (Rn-222) and an alpha particle plus 4.87 MeV of released energy. An alpha particle is a helium nucleus consisting of two protons and two neutrons. The emission of an alpha particle decreases the atomic number by two and the mass number by four.

Hall EJ, Giaccia AJ. Physics and chemistry of radiation absorption. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:3–11.

Question 2 What are the names of the types of photons within the electromagnetic spectrum from lowest and highest energy?

Answer 2

Photons travel as electromagnetic waves, described by the electromagnetic spectrum. This spectrum defines regions based on their energy (and hence wavelength or frequency). From lowest energy (and lowest frequency, highest wavelength) the spectrum starts with radio waves, then microwaves, infrared waves, visible waves, ultraviolet waves, x-rays, and gamma rays.

Question 3 What is the relationship between photon energy, wavelength, and frequency?

Answer 3

The higher the photon energy, the higher the frequency and the smaller the wavelength. Therefore, photon energy is proportional to frequency and inversely proportional to wavelength. This is formalized in the relationship called Planck’s equation: E = hv where E is energy in Joules (J), h is Planck’s constant, 4.13 × 10⁻¹⁸ keV-sec, and v is the frequency in Hertz (Hz, sec⁻¹). Using the electromagnetic wave equation, c = λv we can arrive at the equation E = hc/λ where c is the speed of light, 3.0 × 10⁸ m/sec.

A useful equation is: Energy (keV) = 12.4/λ(angstroms) where angstrom = 10⁻¹⁰ m.

Question 4 What is a photon?

Answer 4

A photon is the fundamental particle of electromagnetic radiation, typically described dually as “packets” of energy (Quantum Theory) and as waves of electrical and magnetic energy (Wave Theory). A photon has no mass or charge.

Question 5

What is the energy range for visible light? x-rays and γ-rays? Why doesn’t visible light cause ionization in tissue?

Question 6

What is ionization? What is excitation? Which photons in the electromagnetic spectrum are ionizing and which are nonionizing?

Question 7

What is the difference between x-rays and γ-rays?

Question 8

Describe the Compton scatter photon interaction.

Question 5 What is the energy range for visible light? x-rays and γ-rays? Why doesn’t visible light cause ionization in tissue?

Answer 5

Using the equation Energy (keV) = 12.4/ λ (angstroms), we can calculate the energy of visible light as 1 to 3 eV since the λ for visible light is 4,000 to 7,000 angstroms. The energy for x-rays and γ-rays are typically in the keV to MeV range since the λ for x-rays and γ-rays are in the 10² to 10⁻⁴ angstrom range. Visible light does not cause ionization in tissue since the average energy for an ionizing event is 34 eV, significantly higher than the 1 to 3 eV energy of visible light.

Hall EJ, Giaccia AJ. Physics and chemistry of radiation absorption. In: Hall EJ, Giaccia, AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:3–11.

Question 6 What is ionization? What is excitation? Which photons in the electromagnetic spectrum are ionizing and which are nonionizing?

Answer 6

Ionization is when enough energy is delivered to eject an electron from an atom or molecule, leading to an electron–ion pair. The average energy dissipated after an ionizing event is 34 eV, enough energy to break a carbon=carbon bond (C=C binding energy is 4.9 eV). Ionizing radiation consists of photons in the ultraviolet, x-ray, and γ-ray range. Nonionizing radiation consists of radio waves, microwaves, infrared (heat) radiation, and visible light. Excitation is the raising of an electron to a higher energy shell without ejection of the electron from the shell.

Question 7 What is the difference between x-rays and γ-rays?

Answer 7

X-rays and γ-rays are two forms of high-energy electromagnetic radiation. They are not different in their physical properties, but their designation reflects the different ways that they are produced. X-rays are produced extranuclearly and γ-rays are produced intranuclearly. For example, x-rays are produced in an electrical device that is used to accelerate electrons and stop them abruptly in a target made of a metal like tungsten or gold (i.e., linear accelerator). However, γ-rays are emitted by radioactive isotopes and represent excess energy that is given off as an unstable nucleus breaks up and decays to reach a more stable form.

Question 8 Describe the Compton scatter photon interaction.

Answer 8

In a Compton scatter photon interaction, the incident x-ray photon interacts with a “free” outer shell electron, ejecting it from the nucleus as a fast electron, and continuing on as a scattered x-ray photon of lower energy. Part of the energy of the incident x-ray photon is imparted to the electron as kinetic energy, and the balance of energy is kept by the scattered x-ray photon. The initial photon may give 0% to 80% of its kinetic energy to the free electron, and this free electron causes ionization of other atoms and produces the chemical and biological effects seen.

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