RAB: Ch. 3: INteraction Of Radiation With Matter Flashcards
It 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).
The energy transfered to an electron DOES NOT EXCEED its binding energy.
Excitation
How does excitation and ionization occur?
They occur when charged particles lose energy by interacting with orbital electrons in the medium.
Note: Energetic charged particles interact with matter by electrical (i.e. columbic) forces and lose kinetic energy via EXCITATION, IONIZATION and RADIATIVE LOSSES.
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 ____ electrons. This process is referred to as _________.
Auger electrons
de-excitation
If the transfer energy EXCEEDS the binding energy of the electron, _______ occurs, whereby the electron is ejected from the atom.
The result of this is an _____ _____ consisting of the ejected electron and the positively charged atom.
Sometimes, the ejected electrons possess sufficient energy to produce further ionizations called ______ ________. These electrons are called _______ _____.
Ionization
Ion Pair
Secondary ionization; delta rays
It is the average number of primary and secondary ion pairs produced per unit length of the charged particle’s path.
Expressed in ion pairs (IP/mm)
It increase with the square of the electrical charge (Q) of the particle and decreases with the square of the incident particle velocity (v)
Specific ionization
As the alpha particle slows, the specific ionization increases to a maximum (called the _______), beyond which it decreases rapidly as the alpha particle acquires electrons and becomes electrically neutral, thus losing its capacity for further ionization. The large ________ associated with heavy charged particles has applications in radiation therapy
Bragg peak
It is defined as the distance the particle travels
Path length
It is defined as the depth of penetration of the particle in matter
Range of a particle
While specific ionization reflects all energy losses that occur before an ion pair is produced, the ___________ is a measure of the average amount of energy deposited locally (near the incident particle track) in the absorber per unit path length.
It is often expressed in units of keV or eV per um.
The ____ of a charged particle is proportionla to the square of the charge and inversely proportional to the particle’s kinetic energy.
Linear Energy Transfer (LET)
The LET of a particular type of radiation describes the local energy deposition density, which can have a substantial impact on the biologic consequences of radiation exposure. In general, for a given absorbed dose, the dense ionization tracks of “___ LET” radiations (alpha particles, protons, etc.) deposit their energy over a much shorter range and are much more damaging to cells than the spare ionization pattern associated with “___ LET” radiations. Low LET radiation includes energetic electrons (e.g., B- and B+) and ionizing electromagnetic radiation (gamma and x-rays, whose interactions set electrons into motion). By way of perspective the exposure of patients to diagnostic x-rays results in the production of energetic electrons with an average LET of approximately 3 keV/um in soft tissue, whereas the average LET of 5-MeV alpha particles in soft tissue is approximately 100 keV/um.
HIgh LET
Low LET
It refers to an interaction that deflects a particle or photon from its original trajectory
Scattering
It is what you call a scattering event in which the total kinetic energy of the colliding particles is unchanged.
For example: Billiard Ball collisions
Elestic
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 called _____?
Inelastic
For example, the process of ionization can be considered an ______ interaction if the binding energy of the electron is negligible compared to the kinetic energy of the incident electron (i.e., the kinetic energy of the ejected electron is equal to the kinetic energy lost by the incident electron).
If the binding energy that must be overcome to ionize the atom is not insignificant compared to the kinetic energy of the incident electron (i.e., the kinetic energy of the ejected electron is less than the kinetic energy lost by the incident electron), the process is said to be _______.
Eleastic
Inelastic
While most electron interactions 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 a loss of kinetic energy. This energy is instantaneously emitted as electromagnetic radiation (i.e., x-rays). Energy is conserved, as the energy of the radiation is equal to the kinetic energy lost by the electron.
The radiation emission accompanying electron deceleration is called _______, a German word meaning “braking radiation” (Fig. 3-4). The deceleration of the high-speed electrons in an x-ray tube produces the ________ x-rays used in diagnostic imaging
bremsstrahlung
Note:
The energy of a bremsstrahlung x-ray photon can be any value up to and including the entire kinetic energy of the deflected electron. Thus, when many electrons undergo bremsstrahlung interactions, the result is a continuous spectrum of x-ray energies. This radiative energy loss is responsible for the majority of the x-rays produced by x-ray tubes
As mentioned above, all energetic electrons (positively and negatively charged) lose their kinetic energy by excitation, ionization, and radiative interactions.
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 ______.
This process occurs following radionuclide decay by positron emission?
0.511 MeV annihilation photons
Positron Annihilation
Unlike protons and electrons, these cannot cause excitation and ionization via coulombic interactions with orbital electrons.
They can, however, interact with atomic nuclei, sometimes liberating charged particles or nuclear fragments that can directly cause excitation and ionization.
These often interact with atomic nuclei of light elements (e.g., H, C, O) by scattering in “billiard ball”–like collisions, producing recoil nuclei that lose their energy via excitation and ionization.
In tissue, energetic ______ interact primarily with the hydrogen in water, producing recoil protons (hydrogen nuclei). _______ may also be captured by atomic nuclei. Neutron capture results in a large energy release (typically 2 to 7 MeV) due to the large binding energy of the neutron.
Neutrons
Four major types of interactions of x-ray and gamma-ray photons with matter:
[the first three play arole in diagnostic and nuclear medicine)
(a) Rayleigh scattering,
(b) Compton scattering,
(c) photoelectric absorption, and
(d) pair production.
In this type of interaction, the incident photon interacts with and excites the TOTAL ATOM [as opposed to infividual electrons as in Compton scattering or Photoelectric effect.
It occurs mainly with very low energy x-rays, such as in mammography (15 to 30 keV).
Rayleigh Scattering
During this event, the electric field of the incident photon’s electromagnetic wave expends energy, causing all of the electrons in the scattering atom to OSCILLATE in phase.
The atoms electron cloud immediately radiates this energy, emitting a photon of the same energy but in a slightly different direction.
Rayleigh Scattering
In this interaction, electrons are not ejected, and thus, ionization does not occur.
In general, the average scattering angle decreases as the x-ray energy increases. In medical imaging, detection of the scattered x-ray will have a deleterious effect on image quality
However, this type of interaction has a low probability of occurrence in the diagnostic energy range
Rayleigh Scattering
In soft tissue, this type interaction accounts for less than 5% of x-ray interactions above 70 keV and at most only accounts for about 10% of interactions at 30 keV.
These interactions are also referred to as “coherent” or “classical” scattering
Rayleigh scattering
Aka INELASTIC or NONCLASSICAL SCATTERING
It is the predominant interaction of x-ray and gamma-ray photons in the diagnostic energy range ith soft tissue.
It not only predominates in the diagnostic energy range above 26 keV in soft tissue but also continues to predominate well beyond diagnostic energies to approximately 30 MeV.
Compton Scattering
This interaction is most likely to occur between photons and outer (“valence”)-shell electrons
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
Compton Scattering
This type of interaction results in the ionization of the atom and a division of the incident photon’s energy between the scattered photon and the ejected electron. The ejected electron will lose its kinetic energy via excitation and ionization of atoms in the surrounding material
Compton scattering
In this type of interaction, The energy of the scattered photon can be calculated from the energy of the incident photon and the angle (with respect to the incident trajectory) of the scattered photon
Compton Scattering
In this type of interaction, the incident photon energy must be substantially greater than the electron’s binding energy before it could take place.
Thus, the relative probability of this interaction increases, compared to Rayleigh scattering or photoelectric absorption, as the incident photon energy increases. The probability of Compton interaction also depends on the electron density (number of electrons/g x density
Compton scattering
In this type of interaction, all of the incident photon energy is transferred to an electron, which is ejected from the atom.
Photoelectric effect
In order for this to occur, the incident phton energy must be greater than or equal to the binding energy of the electron that is ejected.
The ejected electron is most likely one whose binding energy is closest to but less than the incident phton energy.
Photoelectric effect (photoelectric absorption)
For example, for photons whose energies exceed the K-shell binding energy, photoelectric interactions with K-shell electrons are most probable. Following a _________ interaction, the atom is ionized, with an innershell electron vacancy. This vacancy will be filled by an electron from a shell with a lower binding energy. This creates another vacancy, which, in turn, is filled by an electron from an even lower binding energy shell.
Thus, an __________ from outer to inner shells occurs. The difference in binding energy is released as either characteristic x-rays or Auger electrons
Photoelectric interaction
electron cascade
In photoelectric effect, the probability of characteristic x-ray emission ______ as the atomic number of the absorber decreases, thus, characteristic x-ray emission does not occur frequently for diagnostic energy photon interactions in soft tissue
Decreases
This interaction can and does occur with valence shell electrons such as when light photons strike the high Z materials that comprise the photocathode (e.g., cesium, rubidium and antimony) of a photomultiplier tube. These materials are specially selected to provide weakly bound electrons (i.e., electrons with a low work function), so when illuminated the photocathode readily releases electrons (see Chapter 17). In this case, no inner shell electron cascade occurs and thus no characteristic x-rays are produced
Photoelectric effect
Its benefit in x-ray transmission imaging is that there are no scattered photons to degrade the image
Photoelectric effect.
If the photon energies are doubled, the probability of photoelectric interaction is _____ eightfold: (½)3 5 1/8
Decreased
This process predominates when lower energy photons interact with high Z materials (Fig. 3-11). In fact, photoelectric absorption is the primary mode of interaction of diagnostic x-rays with image receptors, radiographic contrast materials, and radiation shielding, all of which have much higher atomic numbers than soft tissue
Photoelectric effect
Conversely, this predominates at most diagnostic and therapeutic photon energies in materials of lower atomic number such as tissue and air
Compton scattering
At photon energies below 50 keV, these interactions in soft tissue play an important role in medical imaging. The process can be used to amplify differences in attenuation between tissues with slightly different atomic numbers, thereby improving image contrast. This differential absorption is exploited to improve image contrast through the selection of x-ray tube target material and filters in mammography
Photoelectric effect
These interactions can only occur when the energies of x-rays and gamma rays exceed 1.02 MeV
Pair production
In this type of interaction, 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 (Fig. 3-12A). The rest mass energy equivalent of each electron is 0.511 MeV, and this is why the energy threshold for this reaction is 1.02 MeV
Pair production
Photon energy in excess of this threshold is imparted to the electron (also referred to as _________) and positron as kinetic energy.
As discussed previously, when the positron comes to rest, it interacts with a negatively charged electron, resulting in the formation of two oppositely directed 0.511-MeV annihilation photons
Negatron or beta minus particle
It does not occur in diagnostic x-ray imaging because the threshold photon energy is well beyond even the highest energies used in medical imaging. In fact, it does not become significant until the photon energies greatly exceed the 1.02-MeV energy threshold.
Pair production
It is the removal of photons from a beam of x-rays or gamma rays as it passes through matter.
It is caused by both absorption and scattering of the primary photons.
The interaction mechanisms, as previously discussed, in varying degrees cause this process.
Attenuation
When higher energy photons interact with low Z materials (ex. soft tissue), ___________ (type of interaction) predominates.
Compton scattering
It occurs in medical imaging wit low probability (only 10% of interactions in mammography and 5% in chest radiography)
Reyleigh scattering
Only at very high photon energies (greater then 1.02 MeV), beyond the range of diagnostic and nuclear radiology, does pair production contribute to attenuation.
Pair production
This is the fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness of material is , typically expressed in units of inverse centimeters (cm-1).
linear attenuation coefficient (m)
In the diagnostic energy range, the linear attenuation coefficient decreases with increasing energy except at ________ (e.g., K-edge).
Absorption edges
The linear attenuation coefficient, normalized to unit density
mass attenuation coefficient
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.
It is defined as the thickness of material required to reduce the intensity (e.g., air kerma rate) of an x-ray or gamma-ray beam to one half of its initial value.
Half-value layer
It refers to an experimental configuration that is designed to exclude scattered photons from being measured by the detector
Narrow-beam geometry
The HVL of a beam is an indirect measure of the photon energies (also referred to as the quality) of a beam, when measured under conditions of narrow-beam geometry.
An indirect measure of the photon energies (also referred to as the quality) of a beam, when measured under conditions of narrow-beam geometry.
HVL
In this type of geometry, the relationship between the source shield and the detector is such that almost no scattered photons interact with the detector
Narrow-beam geometry
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 (Fig. 3-15B) result in an underestimation of the attenuation coefficient (i.e., an overestimated HVL).
broad-based geometry
___-beam geometry means that the relationship between the source shield and the detector is such that almost no scattered photons interact with the detector.
In ___-beam geometry, scattered photons may reach the detector; thus, the measured attenuation is less compared with narrow-beam conditions.
Narrow;
Broad
Most practical applications of attenuation (e.g., patient imaging) occur under ___-beam conditions.
Broad
The ___ is analogous to the HVL, except that it is the thickness of material that is necessary to reduce the intensity of the beam to a tenth of its initial value.
It is often used in x-ray room shielding design calculations
tenth-value layer (TVL)
The determination of the ___ in dx radiology is a way of characterizing the penetrablity of the x-ray beam.
HVL
X-ray beams in radiology are ___energetic, meaning that they are composed of a spectrum of x-ray energies.
poly
The HVL, usually measured in millimeters of aluminum (mm Al) in diagnostic radiology, can be converted to a quantity called the ___.
effective energy
The ___ 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. The relationship between HVL (in mm Al) and effective energy is given in Table 3-3. The effective energy of an x-ray beam from a typical diagnostic x-ray tube is one third to one half the maximal value.
effective energy
The average distance traveled before interaction of a the photon or the length of the photon beam
mea
mean free path (MFP)
The lower energy photons of the 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 transverses matter is called ___.
Beam Hardening
True or false
Low-energy (soft) x-rays will not penetrate the entire thickness of the body; thus, their removal reduces patient dose without affecting the diagnostic quality of the exam.
X-ray machines remove most of this soft radiation with filters, thin plates of aluminum, copper, or other materials placed in the beam. This added filtration will result in an x-ray beam with a higher effective energy and thus a greater HVL.
The number of photons or particles passing through a unit cross-sectional area and is typically expressed in units of cm-2.
Fluence
The fluence rate (e.g., the rate at which photons or particles pass through a unit area per unit time).
Fluence per unit time.
Flux
It is useful in situations in which the photon beam is on for extended periods of time, such as in fluoroscopy. Flux has the units of cm-2 s-1
Flux
The amount of energy passing through a unit cross-sectional area
energy fluence
As a beam of indirectly (uncharged) 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:
Step 1. Energy carried by the photons (or other indirectly ionizing radiation) is transformed into kinetic energy of charged particles (such as electrons). In the case of x-rays and gamma rays, the energy is transferred by photoelectric absorption, Compton scattering, and, for very high energy photons, pair production.
Step 2. The directly ionizing (charged) particles deposit their energy in the medium by excitation and ionization. In some cases, the range of the charged particles is sufficiently large that energy deposition is some distance away from the initial interactions.
It is an acronym for kinetic energy released in matter and is defined at the kinetic energy transferred to charged particles by indirectly ionizing radiation per unit mass.
its SI unit: joule per kilogram with the special name of the gray (Gy) or milligray (mGy), where 1 Gy 5 1 J kg-1
for xrays and gamma rays, it can be calculated from the mass energy transfer coefficient of the material and energy fluence.
Kerma (K)
It the mass attenuation coefficient multiplied by the fraction of the energy of the interacting photons that is transferred to charged particles as kinetic energy.
The fraction of the mass attenuation coefficient that gives rise to the initial kinetic energy of electrons in a small volume of absorber
Mass energy TRANSFER coefficient
same as the mass energy transfer coefficient when all transferred energy is locally absorbed
Mass Energy Absorption Coefficient
