Ionizing Radiation: Particle Interaction

Question 1

What are some examples of charged and uncharged particles?

Answer 1

Alpha particles (α2+ or He2+), protons (p+), beta particles (β−), positrons (β+), and electrons (e−) are some examples of charged particles. Photons (ie, X-ray and gamma-ray), neutrons, and neutrinos are some examples of uncharged particles.

Question 2

What are the three types of interactions that a charged particle can have with the matter?

Answer 2

There are excitation, ionization, and bremsstrahlung. Excitation and ionization are interactions of the charged particle with the orbital electrons. Bremsstrahlung is an interaction of the charged particle with the nucleus.

Question 3

What is the difference between excitation and ionization?

Answer 3

The main difference between excitation and ionization is whether an orbital electron is ejected from the atom. In excitation, the transferred energy does not exceed the binding energy of the electron, so the electron is raised to a higher energy level without actual ejection. In ionization, the transferred energy exceeds the binding energy of the electron, so the electron is ejected from the atom.

Question 4

What are some examples of ionizing and nonionizing radiations?

Answer 4

X-rays, gamma-rays, electrons, protons, alpha particles, and neutrons are some examples of ionizing radiation. Microwaves, radio waves, and optical photons are some examples of nonionizing radiation.

Question 5

What are the differences between directly and indirectly ionizing radiation?

Answer 5

Directly ionizing radiation comes from charged particles (eg, protons, electrons, and alpha particles) and indirectly ionizing radiation from uncharged particles (eg, photons and neutrons).

Question 6

What are delta rays?

Answer 6

Delta rays are secondary electrons with sufficient energy to travel a significant distance away from the primary radiation beam and produce further ionization (ie, secondary ionization).

Question 7

What is specific ionization (SI)?

Answer 7

The average number of primary and secondary ion pairs produced per unit length of the charged particle’s path is called the SI. SI is often expressed in units of ion pairs per mm (IP/mm).

Question 8

What is the Bragg peak?

Answer 8

As a charged particle travels through matter, it loses velocity causing its specific ionization (SI) to increase to a maximum (called the Bragg peak) when it stops. The SI drops off rapidly after the proton has deposited its energy.

Question 9

What is the difference between the path length and range of a particle?

Answer 9

The path length of a particle is the distance that the particle travels, while the range of a particle is the depth of penetration of the particle in matter. The path length of an individual electron almost always exceeds its range, while the path length of a heavy charged particle (eg, alpha particles) is essentially equal to its range.

Question 10

What is the linear energy transfer (LET)?

Answer 10

the average amount of energy deposited locally in matter per unit path length. LET is often expressed in units of keV per μm (keV/μm).

Question 11

What is the dependence of linear energy transfer (LET) on the charge (Q) and kinetic energy (Ek) of the incident charged particle?

Answer 11

The LET of a charged particle increases with Q^2 and decreases with E . Thus, LET ∝ Q^2/E .

Question 12

What are some examples of high linear energy transfer (LET) and low LET radiation?

Answer 12

Alpha particles, protons, and neutrons are examples of high LET radiation. Electrons, X-rays, and gamma-rays are examples of low LET.

Question 13

What is a bremsstrahlung X-ray?

Answer 13

As an electron interacts with an atomic nucleus, it is deflected and decelerated by the positively charged nucleus, with a loss of kinetic energy as emission of bremsstrahlung X-rays. Bremsstrahlung is a German word meaning “braking radiation.”

Question 14

What is the dependence of bremsstrahlung on the atomic number (Z) of the absorber and the mass (m) of the incident particle?

Answer 14

Total bremsstrahlung emission per atom increases with Z^2 and decreases with m^2. Thus, bremsstrahlung ∝ Z^2/m^2.

Question 15

What is positron annihilation?

Answer 15

A positron interacts with an electron at the end of its range, resulting in the annihilation of the electron–positron pair and the conversion of their rest mass to energy in the form of two oppositely directed 0.511 MeV photons.

Question 16

What is the stopping power of a charged particle?

Answer 16

The stopping power of a charged particle is the energy loss per unit of path length in a medium, usually given in units of MeV/m or joule (J/m).

Question 17

What is the mass stopping power of a charged particle?

Answer 17

The mass stopping power of a charged particle is the stopping power divided by the density of the medium, usually given in units of MeV m2/kg or J m2/kg.

Question 18

What are the two types of stopping power, according to how the energy is lost by the charged particle?

Answer 18

According to the fate of the energy lost by the charged particle, the stopping power may be divided into “collisional stopping power” from excitation and ionization, and “radiative stopping power” from bremsstrahlung.

Question 19

What are the relative speeds of alpha particles (α2+), protons (p+), and electrons (e−) with the same energy from the slowest to the fastest?

Answer 19

α2+ < p+ < e−. Kinetic energy depends on mass and speed squared. For the same energy, the lightest particle will be fastest, so the rank follows the mass of the particles.

Question 20

What are the relative ranges of alpha particles (α2+), protons (p+), and electrons (e−) with the same energy from the longest to the shortest?

Answer 20

e− > p+ > α2+. Range is proportional to mass and inversely proportional to charge squared.

Question 21

Which key feature allows proton and heavier charged particle beams to concentrate dose inside the tumor target while minimizing dose to the surrounding normal tissues?

Answer 21

As charged particles slow down, they release more energy to the medium, this property causes the Bragg peak, a large deposit of energy at the end of the particle’s range.

Question 22

What are two main types of neutron interactions with matter?

Answer 22

The two main types of neutron interactions with matter are scattering and absorption. Neutrons may interact with nuclei via scattering in “billiard ball”-like collisions, producing recoil nuclei that deposit their energy via excitation and ionization. Neutrons may also be absorbed by nuclei and cause a variety of emissions, such as gamma-rays, charged particles, neutrons, or fission fragments.

Question 23

Do neutrons directly cause excitation and ionization?

Answer 23

No, neutrons do not directly cause excitation and ionization. Neutrons are uncharged particles, so they do not interact with electrons via excitation and ionization.

Question 24

What is the Cerenkov effect?

Answer 24

The Cerenkov effect occurs when a charged particle travels in a medium at a speed greater than the speed of light in that medium (no massive particle can travel faster than light in a vacuum). Under this condition, the charged particle creates an electromagnetic “shock wave,” similar to the acoustic shock wave when an airplane travels faster than the sound speed. The electromagnetic shock wave appears as a burst of visible radiation, referred to as Cerenkov radiation. Potentially this could be used to measure the position and intensity of deposited dose.

Question 25

How frequently does the Cerenkov effect occur?

Answer 25

The probability of the Cerenkov effect occurring is very small (much less than 1%), but some patients receiving electron treatment near their eyes may describe seeing bluish rings during treatment.