Interaction Of Radiation With Matter

Question 1

Energetic charged particles interact with matter by electrical (i.e. coulombic) forces and lose of kinetic energy via ______,______, and ________.

Answer 1

Excitation, ionization, and radiative losses

Question 2

These occur when charged particles lose energy by interacting with orbital electrons in the medium.

Answer 2

Excitation and ionization

Question 3

These refer to the coulombic forces exerted on charged particles when they pass in proximity to the electric field generated by the atom’s electrons and protons.

Answer 3

Interactional, or collisional, losses

Question 4

This is the transfer of some of the incident particles’ energy to electrons in the absorbing material, promoting them to electron orbits farther from the nucleus (i.e., higher energy level).

Answer 4

Excitation

In excitation, the energy transferred to an electron does not exceed its binding energy

Question 5

Following excitation, the electron will return to a lower energy level, with the emission of the excitation energy in the form of electromagnetic radiation or Auger electrons.

This process is referred to as what?

Answer 5

De-excitation

Question 6

What is the result of ionization?

This consist of an ejected electron and positively charged atom.

Answer 6

Ion pair

Question 7

Sometimes, the ejected electron possess sufficient energy to produce further ionization called secondary ionization.

These electrons are called what?

Answer 7

Delta rays

Question 8

Approximately 70% of energy deposition of energetic electrons in soft tissue occurs via what process?

Answer 8

Ionization

Question 9

The average number of primary and secondary ion pairs produced per unit length of the charged particle’s path is called the what?

It is expressed in ion pairs (IP)/mm.

Answer 9

Specific ionization

Question 10

When the alpha particle slows, the specific ionization increases to a maximum. Which is called what?

Answer 10

Bragg peak

Beyond of which it decreases rapidly as the alpha particle acquires electrons and becomes electrically neutral, thus losing its capacity for further ionization

Question 11

This is defined as the distance the particle travels.

Answer 11

Path length of a particle

Question 12

This is defined as the depth of penetration of the particle matter.

Answer 12

Range of a particle

Question 13

This is a measure of the average amount of energy deposited locally (near the incident particle track) in the absorber per unit path length.

Answer 13

Linear energy transfer

Question 14

This refers to an interaction that deflects a particle or photon from its original trajectory.

Answer 14

Scattering

Question 15

A scattering event in which the total kinetic energy of the colliding particles is unchanged is called what?

Answer 15

Elastic

Billiard ball collisions, for example, are elastic (disregarding frictional losses)

Question 16

When scattering occurs with a loss of kinetic energy (i.e., the total kinetic energy of the scattered particles is less than that of the particles before the interaction), the interaction is said to be what?

Answer 16

Inelastic

Question 17

While most electron interaction with the atomic nuclei are elastic, electrons can undergo inelastic interactions in which the path of the electron is deflected by the positively charged nucleus, with loss of kinetic energy.

The energy is instantaneously emitted as what?

Answer 17

Electromagnetic radiation

Question 18

The radiation emission accompanying electron deceleration is called what?

It is a German word meaning “breaking radiation”.

Answer 18

Bremsstrahlung

Question 19

Neutrons often interact with atomic nuclei of light elements by scattering in what type of collisions?

Answer 19

“Billiard-ball”- like collisions

Producing recoil nuclei that lose their energy via excitation and ionization.

Question 20

What are the four major types of interactions of x-ray and gamma-ray photons with matter?

Answer 20

Rayleigh scattering
Compton scattering
Photoelectric absorption
Pair productions

Question 21

In what kind of scattering will the incident photon interacts with and excites the total atom?

As opposed to individual electrons as in Compton scattering or the photoelectric effect

Answer 21

Rayleigh scattering

This interaction occurs mainly with very low energy x-rays, such as mammography (15 to 30 keV).

In this interaction, electrons are not ejected, and thus, ionization does not occur.

Question 22

Rayleigh interactions are also referred to as what?

Answer 22

“Coherent” or “classical” scattering

Question 23

It is the predominant interaction of x-ray and gamma-ray photons in the diagnostic energy range above 26 keV in soft tissue but predominate well beyond diagnostic energies to approximately 30 MeV.

Answer 23

Compton scattering

This interaction is most likely to occur between photons and outer (“valence”)-shell electrons.

The electron is ejected from the atom, and the scattered photon is emitted with some reduction in energy relative to the incident photon.

Question 24

What kind of scattering results in the ionization of the atom and a division of the incident photon’s energy between the scattered photon and the ejected electron?

Answer 24

Compton scattering

The Compton scattered photon may traverse the medium without interaction or may undergo subsequent interactions such as Compton scattering, photoelectric absorption, or Rayleigh scattering.

Question 25

In this interaction, all of the incident photon energy is transferred to an electron, which is ejected from the atom.

Answer 25

Photoelectric effect

In order for photoelectric absorption to occur, the incident photon energy must be greater than or equal to the binding energy of the electron that is ejected.

Question 26

Although the probability of the photoelectric effect decreases, in general, with increasing photon energy, there is an exception.

For every element, the probability of the photoelectric effect, as a function of photon energy, exhibits sharp discontinuities called what?

Answer 26

Absorption edges

The probability of interaction for photons of energy just above an absorption edge is much greater than that of photons of energy slightly below the edge.

Question 27

This interaction predominates when lower energy photons interact with high Z materials.

Answer 27

Photoelectric process

In fact, photoelectric absorption is the primary mode of interaction of diagnostic x-rays with image receptors, radiographic contrast materials, and radiation shielding, and all of which have higher atomic numbers than soft tissue.

Question 28

This interaction can only occur when the energies of x-rays and gamma rays exceed 1.02 MeV.

Answer 28

Pair production

In pair production, an x-ray or gamma-ray interacts with the electric field of the nucleus of an atom.

The photon’s energy is transformed into an electron-positron pair.

Question 29

The fate of positrons is unlike that of negatively charged electrons that ultimately become __________.

Answer 29

Bound to atoms

When a positron (a form of antimatter) reaches the end of its range, it interacts with a negatively charged electron, resulting in the annihilation of the electron-positron pair and the complete conversion of their rest mass to energy in the form of two oppositely directed 0.511-MeV annihilation photons.

Question 30

Imaging of the distribution of radionuclide decay by positron-emitting radiopharmaceuticals in patients is accomplished by the detection of the annihilation photon pairs during what procedure/modality

Answer 30

Positron emission tomography (PET)

Question 31

This does not occur in diagnostic x-ray imaging because the threshold photon energy is well beyond even the highest energies used in medical imaging.

Answer 31

Pair production

Question 32

This is the removal of photons from a beam of x-rays or gamma rays as it passes through a matter.

Answer 32

Attenuation

Attenuation is caused by both absorption and scattering of the primary photons

Question 33

At low photon energies (less than 26 keV), the ___________ dominates the attenuation processes in soft tissue.

Answer 33

Photoelectric effect

Question 34

The fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness of material is called what?

Typically expressed in units of inverse centimeters.

Answer 34

Linear attenuation coefficient

In the diagnostic energy range, the linear attenuation coefficient decreases with increasing energy except at absorption edges.

Question 35

For a given material and thickness, the probability of interaction is proportional to the number of atoms per volume.

This dependency can be overcome by normalizing the linear attenuation coefficient for the density of the material.

The linear attenuation coefficient, normalized to unit density, is called _____________.

Answer 35

Mass attenuation coefficient

Question 36

This is defined as the thickness of material required to reduce the intensity of an x-ray or gamma-ray beam to one half of its initial value.

Answer 36

Half-value layer (HVL)

Question 37

The half-value layer of a beam is an indirect measure of the photon energies (also referred to as quality) of a beam, when measured under conditions of what geometry?

Answer 37

Narrow-beam geometry

This refers to an experimental configuration that is designed to exclude scattered photons from being measured by the detector.

Question 38

In this geometry, the beam is sufficiently wide that a substantial fraction of scattered photons remain in the beam.

These scattered photons reaching the detector results in an underestimation of the attenuation coefficient (i.e., an overestimated HVL)

Answer 38

Broad-beam geometry

Most practical applications of attenuation occur under broad-beam conditions.

Question 39

This is analogous to the HVL, except that it is the thickness of the material that is necessary to reduce the intensity of the beam to a tenth of its initial value.

Answer 39

Tenth-value layer (TVL)

The TVL is often used in x-ray room shielding design calculations.

Question 40

X-ray beams in radiology are polyenergetic.

Which means what?

Answer 40

It means that they are composed of a spectrum of x-rays energies

Question 41

The half value layer, usually measured in millimetres of aluminum (mm Al) in diagnostic radiology, can be converted to a quantity called the what?

Answer 41

Effective energy

The effective energy of a polyenergetic x-ray beam is an estimate of the penetration power of the x-ray beam, expressed as the energy of a monoenergetic beam that would exhibit the same “effective” penetrability.

Question 42

The lower energy photons of polyenergetic x-ray beam will preferentially be removed from the beam while passing through matter.

The shift of the x-ray spectrum to higher effective energies as the beam traverses matter is called what?

Answer 42

Beam hardening

Question 43

This is the ratio of the first to the second HVL and describes the polyenergetic character of the beam.

Answer 43

Homogeneity coefficient

The first HVL is the thickness that reduces the incident intensity to 50%, and the second HVL reduces it to 25% of its original intensity.

Question 44

The number of photons or particles passing through a unit cross-sectional area is referred to as what?

Answer 44

Fluence

Question 45

The fluence rate (e.g., the rate at which photons or particles pass through a unit area per unit time) is called what?

Answer 45

Flux

The flux is useful in situations in which the photon beam is on for extended periods of time, such as in fluoroscopy.

Question 46

The amount of energy passing through a unit cross-sectional area is referred to as what?

Answer 46

Energy fluence

Question 47

As beam of indirect (unchanged) ionizing radiation (e.g. , x-rays or gamma rays or neutrons) passes through a medium, it deposits energy in the medium in a two-step process:

Answer 47

Step 1. Energy carried by photons (or other indirectly ionizing radiation) is transformed into kinetic energy of charged particles (such as electrons)

Step 2. The directly ionizing (charged) particles deposit their energy in the medium by excitation and ionization.

Question 48

This is defined at the kinetic energy level transferred to charged particles by indirectly ionizing radiation per unit mass.

Answer 48

Kerma

Kerma is an acronym for kinetic energy released in matter.

Question 49

This is the mass attenuation coefficient multiplied by the fraction of the energy of the interacting photons that is transferred to charged particles as kinetic energy.

Answer 49

Mass energy transfer coefficient

Question 50

This is defined for all types of ionizing radiation (i.e., directly and indirectly ionizing)

Answer 50

Absorbed dose

The SI unit of absorbed dose and kerma, is the same (gray), where 1 Gy = J kg-1.

The older unit of absorbed dose is the rad (an acronym for radiation absorbed dose).

Question 51

The amount of electrical charge produced by ionizing electromagnetic radiation per mass of air is called what?

Answer 51

Exposure

Exposure is expressed in the units of charge per mass, that is coulombs per kg.

Radiation beams are often expressed as an exposure rate (R/h or mR/min)

Question 52

This is a useful quantity because ionization can be directly measured with air-filled radiation detectors, and the effective atomic numbers of air and soft tissue are approximately the same.

Answer 52

Exposure

Question 53

The total amount of energy deposited in matter is called what?

It is the product of the dose and the mass over which energy is imparted.

Answer 53

Imparted energy

The unit of imparted energy is the joule.

Question 54

A measure of absorbed dose weighted for the biological effectiveness of the type(s) of radiation (relative to low LET photons and electrons) to produce stochastic health effects in humans

Answer 54

Equivalent dose

Question 55

A measure of dose equivalent, weighted for the biological sensitivity of the exposed tissues and organs (relative to whole body exposure) to stochastic effects in human.

Answer 55

Effective dose equivalent

Question 56

The sum of the products of the equivalent dose to each organ or tissue irradiated and the corresponding weight factor for the organ or tissue is called what?

Answer 56

Effective dose

A measure of equivalent dose, weighted for the biological sensitivity of the exposed tissues and organs (relative to whole body exposure) to stochastic health effects in humans.