Matter and Radiation Flashcards
1
Q
What is the mass of an electron?
A
1/2000 atomic mass unit (amu)
2
Q
What is the charge of an electron?
A
a negative charge of 1 electronic charge (e)
3
Q
What defines the size of an atom?
A
The diameter of the outermost shell defines the size of the atom, which is much larger (thousands of times larger) than the size of the nucleus.
4
Q
What defines the chemical properties of an atom?
A
The electrons and their configuration in shells, determine the chemical properties of the atom.
5
Q
What are nucleons?
A
Protons and neutrons can be known collectively as nucleons.
6
Q
What is the mass and charge of a proton?
A
Protons have a mass of approximately 1 amu, and a positive charge of 1 electronic charge.
7
Q
What is the mass and charge of a neutron?
A
Neutrons have a mass of approximately 1 amu, and a zero electric charge.
8
Q
What keeps the nucleus stable?
A
A short-range force between nucleons keeps the nucleus stable. It acts between one proton and another, between one neutron and another, and between a proton and a neutron. This force that keeps the nucleus together is known as the strong nuclear force or the strong interaction
9
Q
How does the force between nucleons vary with distance?
A
a strong force of attraction (negative force) exists for nucleon separation below about 10^-15 m and that this changes to a force of repulsion (positive force) at about 10^-16 m. Therefore the nucleons are kept apart at a distance of about 5 x 10^-16 m.
10
Q
other than the strong interaction, what other force acts within the nucleus?
A
the electrostatic force of repulsion (Coulomb force) between the positively charged protons. At separations of 10^-15 m to 10^-16 m, the attractive strong interaction is much greater than the repulsive electrostatic force.
11
Q
How many electrons are in each shell?
A
The maximum number of electrons that a shell can hold is 2n^2 where n is the quantum number of the shell.
The K-shell holds a maximum of 2 electrons
The L-shell holds a maximum of 8 electrons
The M-shell holds a maximum of 18 electrons
12
Q
How does the energy differ between bound electrons in different shells?
A
Electrons in different shells have different energies and, since the electrons are held or bound within the atom, these energies are negative. The outermost shells have the greatest (i.e. least negative) energy.
13
Q
What force binds electrons in their shells?
A
electrostatic force between it and the positively charged nucleus. This attractive force gives the electron negative potential energy.
14
Q
What is the binding energy of an electron?
A
Electrons attached to atoms need to be given energy to become free, that is to cancel out their negative energy and achieve zero energy. The energy needed to free an electron from an atom is called the binding energy of the electron. Binding energy is a positive quantity; its magnitude is equal to that of the actual (negative) energy of a bound electron
15
Q
Are the binding energies higher or lower with greater atomic number?
A
The binding energies increase with atomic number i.e. the number of protons on the nucleus. At higher atomic numbers, the electrons feel a greater force from the greater number of protons, which represent a greater positive charge.
16
Q
Where is the positive binding energy greatest?
A
The positive binding energy is greater for inner shells than for outer shells, while the reverse is true for the actual (negative) energy. The inner shells are closer to the nucleus and so that the electrons feel a greater attractive force.
17
Q
What are the units for electron binding energies?
A
Both actual electron energies and binding energies are expressed in electron volts (eV) or, more usually, kilo electron volts (keV), where 1 eV = 1.6022 x 10-19 joule (J).
18
Q
What is electron excitation?
A
At a certain distance from the nucleus there is a shell with zero energy - this is the boundary of the atom. A large number of unoccupied closely spaced shells exist between the outermost one occupied by electrons and this boundary. Electrons may be raised to these unoccupied shells by a process known as excitation.
19
Q
What is ionisation?
A
An electron not attached to an atom is called a free electron. The process of releasing a bound electron from an atom is called ionisation; the neutral atom is left as a positive ion.
20
Q
What is the equation for kinetic energy?
A
Kinetic energy = (mass x velocity^2)/2
21
Q
What is a nuclide?
A
Atoms whose nuclei have the same atomic number and the same mass number constitute a particular nuclear species or nuclide.
22
Q
What determines atomic number?
A
The atomic number is the number of protons in the nucleus.
23
Q
What determines mass number?
A
The mass number is the total number of protons and neutrons in the nucleus
24
Q
True or false - Electron energy levels are equally spaced
A
False. The separation of the energy levels decreases (i.e. they get closer together) as the distance from the nucleus increases.
25
True or false - The electron energy levels of tungsten are the same as those of copper
False. The atomic number of tungsten (74) is greater than that of copper (29) and so for each electron shell, the binding energy for tungsten is greater than that for the same shell of copper.
26
True or false - Electrons in the K-shell have less kinetic energy than L-shell electrons
False. The closer an electron shell is to the nucleus, the greater is the electron velocity and hence kinetic energy.
27
True or false - Electrons in the K-shell have a greater negative value of potential energy than L-shell electrons
True. The closer an electron shell is to the nucleus, the greater is the attractive force of the nucleus on the electrons and hence the greater is the magnitude of the negative potential energy. This is always larger than the magnitude of the positive kinetic energy of the electrons and so the total energy is negative.
28
True or false - The electron has zero mass
False. The electron's mass is 0.00055 amu or 9.109 x 10-31 kg.
29
What atomic property is associated with radioactive decay?
nuclear instability - Radioactive decay is caused by nuclear instability due to a combination of protons and neutrons that does not produce a balance between repulsive and attractive forces. It is usually the case that particles are emitted from (or an electron captured by) the radioactive parent nucleus during its transformation or decay to a daughter nucleus (which might itself be stable or radioactive).
30
What are unstable nuclides called?
radionuclides
31
What is radioactivity?
the process whereby radionuclides are transformed into other nuclides, which may be stable or unstable.
32
How is the activity of a radionuclide measured?
number of nuclear transformations per second.
33
What is an isotone?
Different nuclides with the same value of neutrons are called isotones; for example, the nuclides boron-12 and carbon-13 both contain 7 neutrons and so they are isotones
34
What does the term metastable mean?
Normally the nucleus of an atom is in its ground (lowest energy) state. However some radionuclides can exist for a significant period of time with the nucleus in an excited (higher energy) state before falling to the ground state. Such radionuclides are called metastable. The best known example in medicine is 99mTc (used in many gamma camera imaging procedures). The 'm' stands for metastable
35
What is an isotope?
isotopes are nuclides of the same chemical element (i.e. they have the same atomic number Z) with different neutron number N and mass number A, for example, 123I, 127I and 131I are isotopes of iodine
36
how does the balance between protons and neutrons change as Z increases in stable nuclides?
Low atomic number nuclides have similar numbers of neutrons and protons. As Z increases a greater number of neutrons is required for nuclear stability.
The plot of stable nuclides thus approximates to a straight line for lighter elements near the origin, but becomes progressively more curved for heavier elements
37
If a nucleus has Too many protons and neutrons how will it decay?
Alpha (α) decay
38
If a nucleus has too many neutrons how will it decay?
Beta minus (ß-) decay
39
If a nucleus has too few neutrons how will it decay?
Electron capture (EC) and positron emission (β+) decay
40
What is the decay constant?
λ is known as the decay constant. It is characteristic of the parent radionuclide and a measure of its instability.
λ is related to the half-life (t½) by the formula: λ = ln2/t1/2 = 0.693/t1/2
41
how can you define half-life?
The half-life of the parent, t½ is the time taken for half of its atoms to decay. Alternatively, half-life may be defined as the time taken for the activity to reduce to half of its initial value.
42
How is radioactivity changed over time?
activity of a radioactive sample decays exponentially with time in exactly the same way as the number of radioactive atoms in that sample i.e. with the same decay constant and half-life. After one half-life (t½) the activity is reduced to one half of its original value
43
What units are used to measure radioactivity?
The S.I. unit of activity is the becquerel (Bq).
1 Bq = 1 transformation/second
in practice radioactivity is usually measured in units of kilobecquerel (kBq), megabecquerel (MBq) or gigabecquerel (GBq):
1 kBq = 10^3 Bq
1 MBq = 10^6 Bq
1 GBq = 10^9 Bq
44
What makes up alpha radioactivity?
Alpha particles consist of two protons and two neutrons is emitted from the nucleus. The daughter nucleus is thus a different element from the parent, with atomic number reduced by two and mass number reduced by four They are thus helium ions, and are positively charged. They are relatively heavy particles, with a short range
45
What makes up Beta radioactivity?
In beta decay a neutron is converted into a proton and an electron. The electron is emitted from the nucleus, in the form of a beta particle.
They are much smaller and lighter than alpha particles. They carry negative charge.
It is convenient to sub-divide beta particles into negative electrons (β-) and their anti-matter equivalents, positrons (β+), which carry a positive charge. β- particles are sometimes known as negatrons.
The daughter nuclide thus has the same mass number as the parent, with atomic number increased by one.
46
What makes up gamma radioactivity?
Gamma rays are photons, at the high energy end of the electromagnetic spectrum. They are identical to x-rays of the same energy apart from their origin. (Gamma rays are emitted from nuclei of radioactive atoms; x-rays are a result of electron energy changes.)
Following radioactive decay, the nucleus of the daughter nuclide may be left in an excited state. Gamma rays carry away the energy that is present in the excited daughter nucleus as it de-excites in one or more stages to the ground state. Gamma radiation is the result of transitions between nuclear energy levels and gamma photon energies correspond to differences between these levels
47
What makes up positron emission radioactivity?
Neutron-deficient radionuclides may also decay by positron emission. In this process a proton is converted into a neutron and a positron, which is emitted from the nucleus: The atomic number thus reduces by one and the mass number stays the same
p → n + β+
48
What is Auger electron radioactivity?
Auger electrons are electrons which are ejected from electron shells as a result of some radioactive decay processes which create electron shell vacancies. The emission of Auger electrons is a process which competes with the emission of characteristic x-rays.
49
What is electron capture radioactivity?
Radionuclides with too few neutrons for stability decay via either electron capture (EC) or positron emission. In the process of electron capture an inner shell electron, normally a K-shell electron, is captured by the nucleus. This causes a proton to be converted into a neutron:
p + e- → n
The mass number thus stays the same and the atomic number decreases by one.
50
What is isomeric transition?
A radionuclide in metastable excited state decays to the ground state by isomeric transition (IT).
emits one or more gamma photons as it falls to the ground state, either directly or in stages (via intermediate excited states). However, if the excited state is metastable, the daughter nucleus exists in this state for a longer period before decaying to the ground state.
During isomeric decay, the energy difference between excited and ground nuclear states is emitted as gamma radiation.
Mass number and atomic number are unchanged by an isomeric transition (and by internal conversion).
51
What is the energy from within a positron -electron pair?
the mass of a positron has an energy equivalence of 511 keV. However it is not possible to conjure up a positron on its own without producing a corresponding electron. A positron-electron pair has energy equivalence of 1.02 MeV and this is the threshold energy difference between parent and daughter ground state nuclear energy levels for radionuclides to be able to decay by positron emission.
A positron exists for only a very short time before interacting with an electron. The matter-antimatter particles cannot coexist and both disappear, with their mass being converted to energy in the form of two 511 keV gamma rays travelling in opposite directions. This 511 keV gamma radiation, sometimes known as annihilation radiation, is the basis of image formation in positron emission tomography (PET).
52
What is internal conversion?
Internal conversion (IC) competes with isomeric decay. In IC, the energy difference is used to release a bound electron from an electron shell; any energy in excess of the electron binding energy appears as kinetic energy of the internal conversion electron.
53
what is ionising radiation?
radiation that has sufficient energy to overcome the binding energy of the most tightly bound electrons in atoms
54
what can charged particles be deflected by?
magnetic and electrical fields
55
What is 1 quanta of charge?
The quantum of charge is 1.9 x 10^-19 coulomb (C).
56
what is the difference bewteen x-rays and gamma rays?
X-rays are only distinguishable from gamma rays by their source. They are produced in an x-ray tube, which means that x-ray production may be stopped rapidly by switching off the supply of electrical energy to the tube. Gamma rays are produced by decay processes within unstable radionuclides. It is not possible to switch on or off the production of gamma radiation.
57
what is the photon energy range for diagnostic radiography?
The photon energies used for diagnostic radiography are in the range 10-150 kilo electron volts (keV).
58
What is the equation for photon energy?
Ep = hf (where h is Planck's constant).
59
what is the equation for xray wavelength?
c = fλ therefore λ = c/f (c=speed of light)
60
What is the typical range of electromagnetic radiation wavelength (λ) for x-rays used in diagnostic radiology?
an x-ray photon energy range of 10-150 keV corresponds to a wavelength range of about 10-120 picometres
61
How do you calculate the kinetic energy of an object?
The KE in joules (J) of a moving object is given by half the product of its mass m (kg) and the square of its velocity V (ms-1)
62
Why is kinetic energy important in the production of xrays?
In an x-ray tube it is the KE of electrons (accelerated across the x-ray tube by the large potential difference) that is converted into x-rays and heat by the law of conservation of energy.
63
How can attenuation be thought about with an equation?
Attenuation = absorption + scatter
64
What leads to total xray photon attenuation?
the photoelectric effect. The x-ray photon transfers all its energy to a bound electron. Some of it is used to overcome the binding energy and the remainder appears as KE of the electron, which is now free (of the atom) and called a photoelectron.
65
What leads to xray photon scattering?
Photon scattering with partial absorption is called Compton scattering. The absorbed energy appears as KE of an electron, which is called the Compton recoil electron.
66
what happens to photoelectrons and compton recoil electrons in tissue?
Of course, both photoelectrons and Compton recoil electrons are charged particles, and they lose their KE in multiple interactions with the surrounding tissue
67
How can the penetrating power of an xray beam be described?
by its Half-value thickness
68
What is half value thickness?
As an x-ray beam passes through a material, its intensity will decrease exponentially (in the case of a monoenergetic beam) as the thickness of the material increases. The HVT is the thickness of a known attenuating material required to reduce the intensity of an incident radiation beam to one-half of its original value
69
What is the equation for HVT?
ln2/µ. µ = the linear attenuation coefficient of the attenuating material and ln2 is the natural logarithm (logarithm to the base e) of 2 (which is equal to 0.693).
70
What is the linear attenuation coefficient?
The linear attenuation coefficient is defined as the fractional reduction in intensity per unit increase in thickness; a related quantity is the mass attenuation coefficient, defined as the linear coefficient divided by the physical density of the material.
71
What is HVT normally measured in?
HVT is expressed in mm of aluminium (Al) when characterising an x-ray beam in terms of its penetrating power. In radiation protection, the HVT is often expressed in mm of lead (Pb) or cm of concrete since these materials are used as radiation barriers (shielding).
72
What is the linear attenuation coefficient dependent on?
The linear attenuation coefficient (μ) depends on the photon energy (Ep) and on certain properties of the material with which the photon interacts. These properties are the physical density (mass per unit volume), the electron density (number of electrons per unit mass) and the average or effective atomic number.
73
what does a loss of KE in an intercation with a charged particle produce?
emission of electromagnetic radiation
74
What does an interaction with an outer shell electron produce?
A charged particle may interact with a bound electron in the outer shell of a target atom. Loss of KE may occur through ionisation or excitation. Such interactions result in the emission of low-energy electromagnetic radiation (e.g. infra-red, visible, ultraviolet or soft x radiation) which is absorbed in the target and converted into heat.
75
What does an interaction with an inner shell electron produce?
Energy loss through ionisation may occur if the KE of the incident particle is greater than the binding energy of the inner shell electron. This results in the emission of high-energy characteristic radiation (characteristic x-rays).
76
What does an interaction with the nucleus electric field produce?
Loss of electron KE gives bremsstrahlung radiation. The probability of this type of interaction is significant only for particles with low mass and high KE (e.g. fast electrons).
77
What is meant by elastic and inelastic interactions?
An elastic interaction is one in which the KE of the particles is conserved. An inelastic interaction is one in which a portion of the particles' KE is converted to another form (e.g. photons of electromagnetic radiation).
78
What is an elastic collision?
Elastic collisions between charged particles and bound electrons in an atom are likely to occur only for free electrons with KE of about 10 eV or less. They involve a change in the particle's direction of travel (scattering) but little or no energy transfer, which means that they are relatively unimportant. Therefore, elastic collisions with bound electrons are unlikely for electrons that have been accelerated across an x-ray tube such that they have high KE of many tens of keV, and they play no part in the emission of x-rays from the tube.
79
What is an inelastic collision and how are they different when they are with electrons and nuclei?
Those that involve bound electrons and cause the ionisation (or excitation) of atoms are called directly ionising collisions, and the loss of particle KE is called collisional loss. This type of collision mainly results in the production of heat, although this may be accompanied by the emission of photons at specific energies that are characteristic of the atoms of the material in which the collisions happen. The KE loss that occurs as a result of inelastic collisions with the nucleus is called radiative loss, because the lost energy appears as electromagnetic radiation with a wide range of energy; this is called bremsstrahlung.
80
How is electromagnetic radioation produced with ionisation/excitation with inelastic collisions?
"Inelastic interactions of charged particles with bound electrons in their shells may cause excitation or ionisation of the target atoms. Excitation and ionisation of outer shell electrons leads to the production of low energy electromagnetic radiations - These radiations are absorbed by the material thus depositing energy within it. This energy eventually manifests itself as heat
These are emitted as an electron falls to fill the hole or vacancy created by the original excitation or ionisation. If the vacancy is created by excitation, this process is known as de-excitation."
81
What is collisional stopping power?
Collisional energy loss causes excitation and ionisation. The collisional stopping power is:
Directly proportional to the square of the particle charge, Directly proportional to the electron density (number of electrons per unit mass) of the absorber, Inversely proportional to the square of the particle velocity
The collisional stopping power is relatively large for slow particles of high charge travelling in an absorber of high density and high atomic number (e.g. slow α-particles in a material). In practice, however, collisional loss is the dominant mechanism for the loss of charged particle KE in a wide range of situations. This applies to free electrons as well as heavy particles.
82
What is the difference bewteen particle tracks for alpha particles and electrons?
heavy charged particles undergo no significant deflection at each collision and so their tracks in an absorber are straight. This means that the path (the total length of the track) is nearly equal to the range. The range is the depth of penetration i.e. the straight line distance between the particle’s starting point and its final position when it has lost all its KE.
The mass of a free electron or β-particle, on the other hand, is equal to that of a bound electron and so the track is tortuous, which means that the path is usually much greater than the range.
83
What is the bragg curve?
graph of the number of collisions (or ionisations) per unit distance against distance travelled in the absorber. For heavy charged particles, a prominent feature of this curve is a sharp peak (the Bragg peak) near the maximum depth of penetration into the absorber. A large fraction of the KE is deposited in the region of the Bragg peak and this is the basis of particle beam (e.g. proton beam) radiotherapy.
84
What are the steps in characteristic xray production?
Steps in the production of K characteristic radiation: An accelerated electron interacts with an inner K-shell bound electron, The K-shell electron is ejected, An L- (or M-) shell electron fills the hole, Lost energy is emitted as a photon of characteristic Kα (or Kβ) radiation. A Kα-characteristic photon is emitted when a K-shell vacancy is filled by an electron from the adjacent L-shell, while a Kβ-characteristic photon is emitted when a K-shell vacancy is filled by an electron from a non-adjacent shell (M or N)
85
What is the inherent attenuation of the tube?
attenuated only by the target itself and the glass envelope that contains the vacuum
86
What is an auger electron/auger effect?
process that competes with the emission of characteristic x-rays following inner shell ionisation. The vacancy created by the charged particle interaction is again filled by a bound electron from a shell further from the nucleus. However, the energy released is used not to create a characteristic photon but to release another bound electron i.e. to cause another ionisation of the atom. The released electron is called an Auger electron.
87
What is brehmstrahlung?
the radiation that is emitted when an energetic charged particle is suddenly deflected and slowed by the electric field of the nucleus of an atom (which has a high positive charge). This is radiative loss of KE. In practice, this type of inelastic collision with the nucleus only occurs to any significant extent with electrons and is the mechanism for most of the x-ray production in an x-ray tube.
88
What is radiative stopping power and what is it proportional to?
Radiative loss is due to bremsstrahlung. The radiative stopping power is: Directly proportional to the square of the atomic number of the absorber, Directly proportional to the particle kinetic energy, Inversely proportional to the square of the particle mass. Radiative stopping power is relatively large for light particles of high energy travelling in an absorber of high atomic number (e.g. high-energy electrons in tungsten). Conversely, it is very unlikely for heavy charged particles such as protons and α-particles.
89
What effects the energy of brehmstrahlung?
The closer the interaction of the incoming electron with the nucleus, the greater is its deflection (change in direction of travel), the greater is the loss of electron KE and the greater is the energy of the emitted bremsstrahlung photon . The maximum energy of a bremsstrahlung photon from an x-ray tube occurs when an incoming electron passes into the target without losing any KE through ionisation or excitation prior to being completely stopped by a close interaction with the nucleus. In this case, all of the KE of the electron is converted to a single bremsstrahlung x-ray photon. The energy of this x-ray will be equal to of the original electron KE and, when expressed in keV, numerically equal to the kV of the x-ray tube. However, this chain of events is very unlikely, so the x-ray emission intensity at the maximum photon energy is very low.
90
What does the theoretical bremsstrahlung spectrum look like?
continuous linear spectrum with well-defined maximum photon energy that is equal to the KE of the accelerated electrons.
91
Why does the actual brehmstrahlung spectrum look different to the theoretical?
it is attenuated by the tube itself as it escapes to form an x-ray beam. This means that, like characteristic radiation, it is filtered by the materials of the tube, principally the target itself and the glass envelope. This inherent filtration reduces the intensity of the lowest energy photons to an extremely small value (near zero) and determines the shape of the low energy region of the emergent bremsstrahlung spectrum that is obtained in practice.
92
What is absorbed dose?
the energy per unit mass deposited by ionising radiation in a material (or the energy absorbed by the material from the radiation). It is usually what is meant by the term 'radiation dose' and its unit is the gray (Gy) where 1 Gy = 1 Jkg-1. Absorbed dose is a purely physical quantity and takes no account of the biological effects of radiation.
93
What is kerma?
Kerma' is an acronym that stands for 'kinetic energy released to matter'. It is a quantity that is related to absorbed dose but only applies to photons (and neutrons). It has the same units as absorbed dose. Kerma is concerned with only the first stage of energy transfer, for example from photons to electrons. In practice, it is equal to absorbed dose for photons in the diagnostic radiology energy range because the photoelectron and Compton recoil electron range is small, which means that the energy released is also deposited in close proximity to the initial interaction.
94
Is it possible to differentiate between an x-ray and a γ-ray of identical energy without knowledge of their origin?
No
95
The reduction in x-ray intensity through a uniform material may be approximately described by which relationship? A. Linear
B. Exponential
C. Quadratic
D. Polynomial
exponential
96
In a diagnostic x-ray tube what is the typical conversion efficiency of KE (from the accelerated electrons) to x-rays?
1%
97
Is it very likely that an electron will lose 100% of its incident KE through a single interaction with an atomic nucleus?
no
98
T or F - Bremsstrahlung occurs when a high energy electron is suddenly slowed (or stopped) by the electric field of the atomic nucleus
TRUE
99
T or F - The maximum energy of a bremsstrahlung photon is numerically equal to the kVp applied across the x-ray tube
TRUE
100
T or F - The atomic nucleus has a strong negative charge and the incoming electron is repelled as it approaches
FALSE
101
T or F - All of the x-ray photons produced by bremsstrahlung may be seen in the output spectra of the x-ray tube
False
102
attenuation can be described by what exponential equation?
transmitted intensity = incident intensity x e^-µx. µ = linear attenuation coefficient (LAC), x = thickness of attenuating material
103
assuming a monoenergetic beam, how does thickness of a material relate to attenuation?
Equal increases in the thickness of the material (in the direction of the radiation beam) produce equal fractional or percentage decreases in intensity - characteristic of exponential decrease
104
What is the mass attenuation coefficient?
In order to explore the dependence of the attenuation coefficient on other factors, it is helpful to remove the effect of density. This is done through introducing the mass attenuation coefficient (MAC), which is the LAC divided by density. Since LAC is directly proportional to density, MAC is a constant for a particular substance (at a particular photon energy). For example, the density, and therefore the LAC, of the familiar substance H2O varies with its physical state (solid ice, liquid water or water vapour). However, the MAC of H2O is the same irrespective of its physical state
105
What does the MAC due to photoelectric absorption depend on?
photon energy (E) and the atomic number (Z) of the material. It is approximately proportional to the cube of Z and inversely proportional to the cube of E. The same applies to the component of the LAC, except that it is also directly proportional to density (ρ). In a polyenergetic beam - E is the average photon energy and for a chemical compound or complex material, Z is the average atomic number.
106
What is an absorption edge on a graph?
If the incident photon energy is just greater than the binding energy of a particular shell, there is a sudden increase in the number of photoelectric interactions occurring. This increase is partly due to the extra number of electrons available and partly due to an increased probability of the interaction occurring at photon energies close to the binding energy. This sudden increase in attenuation coefficient is called an absorption edge. On either side of an absorption edge, the probability of a photoelectric interaction (and thus the LAC and MAC) still decreases rapidly with increasing photon energy (E) (i.e. in inverse proportionality to E3).
107
What is compton scatter?
This is an interaction between a gamma or x-ray photon and a free electron. However, if the binding energy of a bound electron is much less than the energy of the incoming photon, the electron is effectively 'free' and can take part in Compton scattering. Thus Compton scattering is most likely to occur with weakly bound outer atomic electrons. he electron takes some of the energy of the photon (in the form of kinetic energy) and travels away from the site of interaction as a Compton recoil electron. The photon is scattered (deflected from its path) and has reduced energy compared with that of the incoming photon
108
Does a glancing hit or a direct hit give the incident photon the most kinetic energy in compton interactions?
Glancing hit - If the scattered photon goes almost straight forward (θ ≈ 0°), the direction of the recoil electron is about 90° to the direction of the incident photon (Φ ≈ 90°). If the scattered photon travels backwards (θ ≈ 180°), the recoil electron travels straight forwards (Φ ≈ 0°). This is the scattering angle at which the scattered photon has minimum energy and the recoil electron receives maximum kinetic energy.
109
What is the change in energy and wavelength of the incident scattered electron in compton scatter dependent on?
the photon scattering angle θ. This is the angle between the direction of the incident photon and the direction of the scattered photon.
110
How does the distribution of compton scatter change with increasing incident photon energy?
at higher incident photon energies, the scattered radiation tends to travel in the forward direction whereas at lower incident photon energies, there is a more uniform distribution of scattered radiation in all directions.
111
In practise where does more of the compton scatter escape from the patient?
In practice, the distribution of Compton scattered radiation escaping from a patient during radiographic imaging will also depend on its attenuation in tissue. At diagnostic x-ray energies, more scatter emerges from the patient in the backward direction, since those scatted photons travelling in the forward direction are more likely to be attenuated by tissue.
112
What does the component of the MAC due to Compton scattering depend on?
the photon energy E and the electron density of the material. It is directly proportional to the electron density and approximately inversely proportional to E.
113
How does MAC change with increasing photon energy?
After an initial increase, they decrease slowly as photon energy increases
114
What is the difference in MAC of Compton effect between bone and soft tissue?
At a particular value of photon energy, the MAC of bone is about 0.9 times the MAC of soft tissue because the electron density of bone is only 0.9 times that of soft tissue
115
What is the difference in LAC of Compton effect between bone and soft tissue?
The LAC of bone is about 1.7 times greater than the LAC of soft tissue because of the additional factor of about 2 (the ratio of the densities). This means that if only Compton scattering is considered, bone is about 1.7 times more attenuating than the same thickness of soft tissue
116
Why does soft tissue have a mass attenuation coefficient due to Compton scattering greater than that of bone?
the nucleus of ordinary hydrogen consists of only one proton and so Z/A = 1. This means that the electron density of hydrogen is about twice that of other elements and materials that contain a relatively large amount of hydrogen have a relatively high electron density. Because of its water content, soft tissue has a higher electron density than bone
117
What is avogadro's constant?
Avogadro’s number 6 x 10^23
118
what is the difference in compton interactions at high and low energy?
The total Compton attenuation coefficient and therefore the number of Compton interactions decreases with increasing photon energy. At low photon energies the total coefficient is almost entirely due to energy scatter but as photon energy increases, so does the relative amount of energy absorption.
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what is total attenuation coefficient?
combination of that due to photoelectric absorption and Compton scattering
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What is the total attenuation coefficient dependent on?
The relative contribution due to each type of interaction depends on the atomic number and electron density of the material and on the incident photon energy.
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What type of interaction dominates at low energy?
photoelectric
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what type of interaction dominates at higher energy?
compton
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what type of interaction dominates for most of the diagnostic radiography window?
compton (as the point at which equal numbers of photoelectric and Compton interactions occur; its value is about 27 keV in soft tissue + 50 for bone)
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What is elastic scattering?
In elastic scattering, the photon is deflected from its path but loses no energy. This means that it may be described as scattering alone with no absorption (of energy by the material) and therefore no ionisation of atoms of the material. Elastic scattering is also known as classical, coherent or Rayleigh scattering.
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When can pair production occur?
At very high photon energies -at least 1.022 MeV
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What is pair production?
a photon interacts with the electric field of an atomic nucleus. The photon disappears and its energy E is transformed into mass m according to Einstein’s famous equation E = mc2, in which c is the speed of light. The mass takes the form of a pair of charged particles: an ordinary negative electron (sometimes called a negatron) and a positive electron (called a positron).
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What is the effect of pair production?
The interaction itself does not cause ionisation but the charged particles lose their kinetic energy through multiple excitations and ionisations of atoms of the material. When the positron has lost all its kinetic energy, it annihilates with an electron. Both particles disappear and their mass is converted into energy in the form of two photons each of energy 511 keV (annihilation radiation) travelling in opposite directions. Pair production does not contribute to attenuation in diagnostic radiology or radionuclide imaging.
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T or F - In relation to photoelectric absorption:
A. It occurs only with electrons in the K-shell
B. It can result in the production of characteristic x-rays
C. It occurs with free electrons
D. It converts all the photon’s energy into kinetic energy
E. Is less likely as the incident photon energy increases
A. False - The effect also occurs with electrons in other shells but for low atomic number materials it occurs mostly for K-shell electrons.
B. True - When the vacancy left by the photoelectron is filled by an electron from a higher orbit, a photon of characteristic radiation is emitted. The energy of the characteristic photon is equal to the difference in electron binding energies and may be in the x-ray region of the electromagnetic spectrum if the difference is sufficiently large.
C. False - Photoelectric absorption is the interaction of a photon in which a bound electron is released from an atom. It is most likely for tightly bound electrons, provided that the photon energy is greater than the electron binding energy.
D. False - Some (sometimes most) of the photon energy is used in overcoming the electron binding energy.
E. True - The component of the attenuation coefficient due to photoelectric absorption is proportional to 1/E^3 (or E^-3) where E is the photon energy. This means that the likelihood (probability) of the interaction decreases rapidly as photon energy increases.
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T or F- Concerning the Compton effect:
A. Compton interactions make no contribution to subject contrast in a planar radiographic or computed tomography (CT) image
B. Compton interactions give rise to radiation outside the main x-ray beam
C. In Compton scatter, the energy lost by the photon depends only on the angle through which it is scattered
D. The wavelength shift that the photon undergoes depends only on the angle of scatter
E. Compton interactions are more likely in materials of higher atomic number
A. False. Compton interactions contribute to the reduction in intensity (attenuation) of an x-ray beam by the body and therefore to subject contrast. In situations where the interactions are almost entirely Compton scatter events, subject contrast may be small since it depends only on thickness, density and electron density variations in tissue. However, if photoelectric absorption makes a significant contribution to attenuation, subject contrast is likely to be greater because differences in the average atomic number of tissue also play a role.
B.True. In radiography and CT, scattered photons from the patient are a source of radiation dose to members of staff even though they may be standing well away from the main x-ray beam.
C. False. The energy change between the incident and Compton scattered photon depends on the energy of the incident photon as well as the angle of scatter.
D. True. The wavelength shift depends only on the scattering angle i.e. the angle between the direction of the incident and Compton scattered photon.
E. False. The probability of a Compton interaction depends on the ratio Z/A where Z is the atomic number and A is the mass number. This ratio is proportional to electron density (number of electrons per unit mass), which is nearly the same for all elements other than hydrogen.
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when does luminescence occur?
when a material absorbs energy in some form and re-emits all or part of that energy as visible light.
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What is photoluminescence?
Photoluminescence results from the interaction of x and gamma photons (any EM radiation) with solid materials and subsequent electron transitions between different energy states. the emitted photon energy is always of lower than the incident photon energy
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What is fluorescence?
The light emission starts as soon as the irradiation starts and it stops as soon as the irradiation stops (or very soon afterwards, within 10^-9-10^-6 s)
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What is phosphorescence?
The emission of light may persist even after the irradiation has stopped (for a period ranging from about 10^-4 s to many seconds)
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What are luminescent material often called?
phosphors, even if the light is emitted by fluorescence
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What can a material that is mainly fluorescent also be called?
A phosphor that emits light predominantly by fluorescence may be called a fluorescent material or, in some contexts, a scintillator.
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What is an energy band?
similar to energy levels due to bound electrons but in compunds these discrete levels become bands as electron energies are affected by the surrounding atoms; this applies in particular to the electrons that are furthest from atomic nuclei. The inner energy levels (e.g. K and L) remain discrete but the outer levels are broadened into energy bands, sometimes to the extent that the bands themselves overlap so that the association with discrete levels is lost
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What is the conduction band and valence band?
The band that is furthest from the nucleus and has a full complement of electrons is known as the valence band and the band immediately above is known as the conduction band
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How do the valence and conduction bands effect the properties of a material?
It is the valence and conduction bands that determine the electrical properties of the material. If the conduction band contains some electrons, they are able to move throughout the material (under the influence of an applied potential difference for example) and so the material is an electrical conductor. On the other hand, if there are no electrons in the conduction band, the material is an insulator.
139
What is the normal structure of a luminescent material?
Luminescent substances are crystalline materials that behave as electrical insulators with a relatively wide energy gap (about 10 eV) between the valence and conduction bands.
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What happens to electrons to make luminescent materials work?
Under normal circumstances, there are no electrons in the conduction band but the energy deposited by ionising radiation can raise or excite electrons across the forbidden zone from the valence band to the conduction band; this leaves electron vacancies or holes in the valence band. Light may be emitted as the electrons lose energy in returning to the valence band to fill the holes. Typically, the energy of the incident particle or photon is much greater than the band gap and so each interaction causes many thousands of valence band electrons to be excited.
141
What is added to manufactered luminescent materials and why?
defects or impurities in the crystal can promote luminescence, i.e. they increase the number of visible photons emitted. a controlled amount of a different compound (the impurity) is added in very small concentration to the molten material during manufacture. This process is sometimes known as activation (or doping) and the impurity is called the activator (or dopant). Pure materials are not very efficient at converting ionising radiation energy into visible light energy.
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Why does impurity in a luminescent material work to increase luminescence?
The impurity atoms create localised energy levels that are different to those of the host crystal and lie within the forbidden zone. These act as luminescence centres and increase the probability of a light photon being emitted when an excited electron returns to fill a hole in the valence band.
143
Describe the main steps in scintillation/fluorescence
An x or gamma photon releases an energetic secondary electron via the photoelectric effect or Compton scattering.
The photoelectron or Compton recoil electron excites many electrons from the valence band to the conduction band through inelastic collisions, leaving holes in the valence band; this continues until the secondary electron has lost all its kinetic energy.
Electrons in the conduction band and holes in the valence band move towards luminescence centres.
When a hole reaches a luminescence centre it is filled by an electron from the lower energy level (ground state) of the activator leaving a hole in the lower level.
This allows an electron in the valence band to fall into the upper (excited) state of the activator and then into the hole in the lower level.
As it does so, a photon of visible light is emitted; its energy is equal to the energy difference between the upper and lower activator levels
144
What is an electron trap?
energy levels associated with a single impurity atom are sometimes described as an electron trap. This may be considered as a single localised energy level lying within the forbidden zone just below the bottom of the conduction band.
145
How does the properties of an electron trap change the properties of a luminescent material?
whether the traps contain or do not contain electrons under normal circumstances. For example, the presence or absence of electrons in the traps determines whether the material produces fluorescence (each trap normally contains an electron) or phosphorescence (the electron traps are empty) when irradiated by ionising radiation.
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How do the steps in phosphorescence differ with those in fluorescence?
in phosphorescence the incident photon allows the electron traps to be filled. These electrons then have a delay while they collect energy (thermal). This allows it to be excited back to conduction band and then fall back down to valence band which emits light.
147
What are thermoluminescent materials?
Thermoluminescent materials are similar to phosphorescent materials but they have very deep electron traps that are empty under normal circumstances which require heat to excite the electrons in the traps enough to be excited back to the conduction band.
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How can thermoluminescent materials be used in dosimetry?
Thermoluminescent materials can be used to measure ionising radiation dose for both personnel (staff) and patients. Following radiation exposure, the total amount of emitted light is measured as the material is heated in an oven; the amount of light is proportional to dose. When used in this way, the material is known as a thermoluminescent dosemeter (TLD).
149
What is a photostimulable luminescent material?
similar to a thermoluminescent material in that it has deep electron traps that empty under normal circumstances but the material is irradiated with visible light rather than heat to rexcite the electrons.
150
What are the 2 main uses for photostimulable luminescent materials?
Computed radiography (Red laser light is used for stimulation and the emitted light is blue.) and Optically stimulated luminescent dosimetry (This method is an alternative to thermoluminescent dosimetry (TLD).)
151
What is farraday's law?
a time-varying magnetic field induces an electric field (and vice versa), and therefore when either field is changing with time, the other kind is induced in adjacent regions of space.
152
What sort of waves are EM radiation?
transverse - the electric and magnetic field components are at right angles to each other and to the direction of the wave, hence they are transverse waves.
153
What is the amplitude of a wave?
The wave oscillation has a particular maximum value called its amplitude (A), and this is the peak field strength.
154
What is wavelength of a wave?
The wavelength (λ) is the distance between successive peaks (also known as crests). The wavelength is measured in metres (m).
155
What is the period of a wave?
If the sinusoidal variation is with time, the interval between two successive peaks is called the period (T) of the wave. The period is measured in seconds (s).
156
What is the frequency of a wave and what're the units?
The frequency of the wave (f) is the number of wave peaks passing a given point in a second. Frequency is the inverse of period i.e. f = 1/T. As the unit for the period is the second (s), frequency is expressed as s-1. This is given the special name of hertz (Hz).
157
What is the velocity of a wave?
The velocity of a wave (v) is the distance travelled by a peak in one second. It is equal to the number of peaks passing per second (frequency) multiplied by the distance between one peak and the next (wavelength). This means that velocity = frequency multiplied by wavelength (v = fλ).
158
What is the equation for photon energy?
E = hf. where f is the frequency of the wave and h is Planck's constant (6.6 x 10-34 joule second (Js)). Thus h is the constant of proportionality between photon energy E and wave frequency f.
159
What are the normal units for photon energy and why?
The value of E for a photon in terms of joules is inconveniently small so the unit of electron volt is used, where 1 eV = 1.6 x 10-19 J. The multiples kilo electron volt (keV = 1000 eV) and mega electron volt (MeV = 1 000 000 eV) are often used in radiological physics.
160
What is photon fluence rate?
photon fluence over the time t is the number of photons passing through unit area of the beam over a period of time
161
What is photon fluence?
If a source emits a beam of electromagnetic radiation such that N photons pass through an area A the photon fluence is given by N/A.
162
What is energy fluence?
The total energy passing through a unit cross-sectional area in a given time. The energy fluence is the photon fluence multiplied by the energy (or average energy) of each photon.
163
What is energy fluence rate?
The total energy per unit area per unit time in a radiation beam is the energy fluence rate.
164
How do we measure energy fluence in practice?
In practice, we cannot easily measure the energy fluence and intensity and so we measure them indirectly as 'air kerma' or 'air kerma rate'. Here kerma stands for kinetic energy released in matter.
165
What is the inverse square law?
As the rays diverge outwards, the intensity decreases with distance from the source. The beam intensity is inversely proportional to the area of the beam, which is proportional to the square of the distance from the point source.
166
T or F -
A. Photons travel at the speed of light
B. EM waves are self-propagating and need a medium through which they can travel
C. EM radiation propagates as longitudinal waves
D. Visible light has a shorter wavelength than infrared
E. Planck’s constant has the same units as frequency
A. True. The velocity of a photon is the same as that of an EM wave. B. False. EM waves are self-propagating, but they can travel through a vacuum. C. False. EM waves are transverse. D. True. In the EM spectrum, visible radiation is nearer to x and gamma radiation (characterised by high frequency and short wavelength) than infrared radiation. E. False. Since E = hf, the units of h are Js.
167
T or F - A radiation beam may be described in terms of fluence or fluence rate:
A. Fluence is defined as the number of photons passing through an area A in 1 minute
B. Energy fluence rate is the sum of the energies of all the photons passing through unit area per second
C. Radiation intensity is proportional to the square of the amplitude of the wave
D. Radiation intensity is related to kerma rate
E. Fluence is the number of photons per unit area (number of photons passing through area A, divided by A) in a given time (such as the duration of a radiographic exposure)
A. False. Fluence is the number of photons per unit area (number of photons passing through area A divided by A) in a given time (such as the duration of a radiographic exposure). B. True. C. True. D. True. E. True.
168
T or F - Regarding the inverse square law:
A. Radiation radiates in a given direction only
B. Intensity decreases with distance
C. Intensity is inversely proportional to the beam area
D. Doubling the distance from a source reduces the intensity to a quarter
A. False. From a point source, radiation radiates equally in all directions. However, it can be collimated (shaped) to produce a directional beam as is done for radiographic x-ray imaging. B. True. C. True. D. True.
