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EN250: Lecture 2Radiation Physics
X-ray Production
Disclaimer: These slides are a compilation of pictures obtained from the WWW and various books, etc, and none is original. For example the flash demos are obtained from http://learntech.uwe.ac.uk/radscience/xray_prod/production_of_xrays03_files/frame.htm
Overview
• Review of the structure of the atom
• Electromagnetic Radiation
• Production of X-rays
• Interaction of X-ray with matter
• Properties of X-rays
• Formation of an X-ray image
• Potential for 3D?
History of the Development of Atomic Models
1. Democritus (400 B.C.)– matter is made of atomos.2. John Dalton (1800’s) – proof for atoms. 3. J.J. Thomson (1904) – discovered electrons in
atoms; his model was of a positive sphere with e- embedded in it.
4. Ernest Rutherford (1911) – discovered nucleus; proposed that e- orbited a nucleus that had almost all the mass.
5. Niels Bohr (1913) – said e- orbited in fixed paths.6. James Chadwick (1932) – nucleus contained
protons & neutrons. 7. Erwin Schrodinger (1926) – electron “cloud”
model.
Review of the Structure of the Atom
• Atom is mostly empty space
• Mass is concentrated in its Nucleus
• Nucleus consists of Protons and Neutrons, the Neutrons, the NucleonsNucleons
• Electrons orbiting around the nucleus
ELECTRONS PROTONSNEUTRONS
ELECTRONS PROTONS NEUTRONS
Negative charge Positive charge Zero or neutral charge
# of electrons determines the charge of the atom
# of protons determines type of element the atom is
# of neutrons determines type of isotope the atom is
Much much smaller than protons
Much much bigger than electron
Much much bigger than electron
Contributes little to mass or weight of atom-hardly anything
Contributes significantly to mass or weight of atom
Contributes significantly to mass or weight of atom
Atomic Number
• Atomic Number A: Number of Nucleons
• Mass number Z: number of protons
Characterizing Atoms
• A Nuclide is characterized by A and Z
ORBITAL BASICS
(1) A shell is sometimes called an orbital or energy level.(2) Shells are areas that surround the center of an atom.(3) The center of the atom is called the nucleus.(4) Electrons live in something called shells.
SHELLS ONLY HOLD SOME ELECTRONS
• Not all shells hold the same number of electrons. For the first 18 elements, there are some rules. – The k-shell only holds two electrons. – The l-shell only holds eight electrons. – The m-shell only holds eight electrons (for the first 18
elements). The m-shell can actually hold up to 18 electrons as you move further along the periodic table.
• YOU CAN'T KNOW WHERE AN ELECTRON IS• Niels Bohr came up with all these ideas in
1913.
1IA1A
18
VIIIA8A
11H
1.008
2IIA2A
13IIIA3A
14IVA4A
15VA5A
16VIA6A
17VIIA7A
2He4.003
23Li
6.941
4Be 9.012
5B
10.81
6C
12.01
7N
14.01
8O
16.00
9F
19.00
10Ne20.18
311Na 22.99
12Mg 24.31
3IIIB3B
4IVB4B
5VB5B
6VIB6B
7VIIB7B
8 9 10 11IB1B
12IIB2B
13Al
26.98
14Si
28.09
15P
30.97
16S
32.07
17Cl
35.45
18Ar
39.95------- VIII -------------- 8 -------
419K
39.10
20Ca40.08
21Sc
44.96
22Ti
47.88
23V
50.94
24Cr
52.00
25Mn54.94
26Fe
55.85
27Co58.47
28Ni
58.69
29Cu63.55
30Zn65.39
31Ga69.72
32Ge72.59
33As74.92
34Se78.96
35Br
79.90
36Kr
83.80
537Rb85.47
38Sr
87.62
39Y
88.91
40
Zr91.22
41
Nb92.91
42
Mo95.94
43
Tc(98)
44
Ru101.1
45
Rh102.9
46
Pd106.4
47
Ag107.9
48
Cd112.4
49
In114.8
50
Sn118.7
51
Sb121.8
52
Te127.6
53
I126.9
54
Xe131.3
655Cs132.9
56Ba137.3
57La*138.9
72
Hf178.5
73
Ta180.9
74
W183.9
75
Re186.2
76
Os190.2
77
Ir190.2
78
Pt195.1
79
Au197.0
80
Hg200.5
81
Tl204.4
82
Pb207.2
83
Bi209.0
84
Po(210)
85
At(210)
86
Rn(222)
787Fr
(223)
88Ra(226)
89Ac~
(227)
104
Rf(257)
105
Db(260)
106
Sg(263)
107
Bh(262)
108
Hs(265)
109
Mt(266)
110
---()
111
---()
112
---()
114
---()
116
---()
118
---()
Lanthanide Series*
58
Ce140.1
59
Pr140.9
60
Nd144.2
61
Pm(147)
62
Sm150.4
63
Eu152.0
64
Gd157.3
65
Tb158.9
66
Dy162.5
67
Ho164.9
68
Er167.3
69
Tm 168.9
70
Yb173.0
71
Lu175.0
Actinide Series~
90
Th232.0
91
Pa(231)
92
U(238)
93
Np(237)
94
Pu(242)
95
Am(243)
96
Cm(247)
97
Bk(247)
98
Cf(249)
99
Es(254)
100
Fm(253)
101
Md(256)
102
No(254)
103
Lr(257)
Valence Shell• The outermost shell is the valence shell• Valence Shell determines chemical, thermal,
optical, and electrical properties of the element• X-rays involves inner shells• Radioactivity and Gamma rays involve the
nucleus• Valence shells have at most 8 electrons• Metals have 1, 2, or 3, one of which can be easily
detached, the “free” electron, responsible for the excellent conduction of heat and electricity
Binding Energy• An atom is ionized when one of is electrons is completely
removed• The Binding Energy E is the energy required to remove this
electron• Usually measured in electronvolts (eV)• Example: Tungsten (W; Z=74)
– EK = 70, EL = 11, EM = 2, All in Kev
• Binding Energy EK for various atoms (usually < 100 kev)– W Z = 74 70 Kev– I Z = 53 33 Kev– Mo Z = 42 20 Kev– Cu Z = 29 9 Kev
• eV is the amount of energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt
Exciting an Atom
• Atom is called excited when an electron is raised from one shell to another further out, as a result of expenditure of energy
• When the electron falls back it emits this energy in a packet of energy, a photon of light (visible or ultraviolet)
• A Photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation– Developed by Einstein (1905-1917) to explain the non-wave
properties of EM waves
Electromagnetic Radiation
• X-rays and Gamma rays are EM waves• Quantum aspects: Travel in a straight line,
photons: packets or quanta of energy• Wave aspects: Electric and Magnetic Fields at
right angle to each other and to the direction of wave travel of the wave; field strength sinusoidal: frequency f, period T, wavelength
Electromagnetic Specturm• Radio Waves and X-Ray/Gamma rays are
the only E&M Waves that can penetrate the body
Combined View• E = h f, h Plancks constant• E (in Kev)= 1.24 / Lambda (in nm)
– Blue Light lambda = 400nm E = 3ev– X-ray lambda = 0.1 nm E = 140Kev
• Rays travel from source in all directions• A beam is a collimated set of rays• Energy flux: Total energy (total number of
photons ) per unit area• Intensity is inversely proportional to
distance from source
Discovery of X-ray: Wilhelm Conrad
Roentgen (1845-1923)
Physical Institute of the University of Wurzburg
Roentgen’s early tube design
Roentgen’s Laboratory
Nov 1895: Penetrating solids
First X-Ray Picture: 22 Dec 1895
Modern X-ray Tubes
Production of X-raysProduction of X-rays• X-Rays are produced when fast moving
electrons are suddenly stopped by impact on a metal target
• The kinetic energy of the electrons is converted into heat (99%) and X-rays (1%)
• Structure of an X-ray Tube– A Negative Electrode, Cathode, fine Tungsten coil
or filament, heated to incandescence (2200 C) which gives off electrons (thermionic emission)
– A Positive Electrode, Anode, smooth flat metal target, usually Tungsten, collects these electrons which hit it with about half the speed of light
• Filament heating voltage about 10V and using about 10A
• The accelerating voltage in the range 30-150 kV and current of 0.5-1000 mA
Processes underlying X-ray formation– Elastic collision with Atoms– Inelastic collision with electrons in the outer shell
of an atom:• Excitation • Ionization
– Inelastic collision with electrons in the inner shell of an atom (Characteristic Radiation)
– Inelastic collision with the nuclei of the atom (Bremsstrahlung)
Elastic collision with target
The electron is deflected but looses very little kinetic energy because its mass is negligible, and continues in tortuous path because of successive interactions
Interaction between filament Interaction between filament electron and outer shell electron (1)electron and outer shell electron (1)
• Excitation
• i) This results in an outer shell electron gaining energy & being raised to a higher level.
• ii) Heat is produced as the electron falls back into its original path.
• No contribution to x-ray production
Interaction between filament Interaction between filament electron & outer shell electron (2)electron & outer shell electron (2)
• Outer shell electron ejected from target atom results in an outer shell electron being completely removed from the target atom.
• Both the filament & the ejected electrons may interact in either the first or second of these interaction processes with other target atoms
• Ultimately, this type of interaction may cause the target material to heat up
Illustration (2)
Inelastic collisions with electrons in the inner Inelastic collisions with electrons in the inner shell of an atom (characteristic radiation)shell of an atom (characteristic radiation)
• The incoming electron transfers sufficient energy to remove an The incoming electron transfers sufficient energy to remove an inner shell electron from its atom in the target. inner shell electron from its atom in the target.
• In order for this to occur the electron must possess energy at In order for this to occur the electron must possess energy at least as least as greatgreat as the as the binding energybinding energy of the inner shell. of the inner shell.
• Any surplus energy appears as additional kinetic energy in the Any surplus energy appears as additional kinetic energy in the ejected electron ejected electron
• The inner shell vacancy is quickly filled by an electron falling The inner shell vacancy is quickly filled by an electron falling inwards from a shell further out from the nucleus inwards from a shell further out from the nucleus
• This transition is accompanied by a burst of electromagnetic This transition is accompanied by a burst of electromagnetic radiation with energy equal to the difference in binding energies radiation with energy equal to the difference in binding energies of the two shells. of the two shells.
• This type of x-ray production is termed This type of x-ray production is termed characteristiccharacteristic because because the exact photon energy is characteristic of the element of the exact photon energy is characteristic of the element of which the target is made.which the target is made.
Illustration (3)
Energy levelsEnergy levels
• The difference in the binding energies of the K and L shells in tungsten is 70 keV - 11 keV = 59 keV.So a characteristic photon of 59 keV is emitted.
• The difference in the binding energies of the M and K shells in tungsten is 70keV - 2 keV = 68 keV.
• These two electron transitions are the most likely to occur, producing x-ray photons of 68 and 59 keV.
• Characteristic x-rays contribute less than 10% of an x-ray beam. The majority of x-ray production results from inelastic collisions of incoming electrons with the nuclei of the target atoms
Inelastic collisions with the nuclei of the atom Inelastic collisions with the nuclei of the atom (Bremsstrahlung Radiation)(Bremsstrahlung Radiation)
• The incoming electron passes very close to the nucleus of a The incoming electron passes very close to the nucleus of a target atom (1). target atom (1).
• The attraction causes the electron to deviate in its course (2) The attraction causes the electron to deviate in its course (2) • The sudden change of direction stimulates the electron to The sudden change of direction stimulates the electron to
release energy in the form of a photon of electromagnetic release energy in the form of a photon of electromagnetic radiation (3) radiation (3)
• The emission of radiation results in a reduction in the electrons The emission of radiation results in a reduction in the electrons kinetic energy causing it to slow down. kinetic energy causing it to slow down.
• The energy of the radiation depends on the degree of The energy of the radiation depends on the degree of deviation the electron suffers. deviation the electron suffers.
• In an extreme case the electron may actually be brought to In an extreme case the electron may actually be brought to rest. Thus the photon energy can be of any value from rest. Thus the photon energy can be of any value from zerozero up up to a maximum equal to the to a maximum equal to the initial initial kinetic energy of the kinetic energy of the incoming electron. incoming electron.
• This gives rise to a continuous spectrum of x-radiation and is This gives rise to a continuous spectrum of x-radiation and is known as braking (known as braking (BremsstrahlungBremsstrahlung) radiation.) radiation.
Illustration (4)
Interaction of X-Ray with matter
Example X-ray images
End of Lecture 2
Additional Slides
• Collimated vs non-collimated x-ray
• Dose Limits• Occupational Whole Body5,000 mrem per
yearExtremity and Skin of Whole Body 50,000 mrem per year Lens of the Eye 15,000 mrem per year Any single organ (e.g., Thyroid) 50,000 mrem per year Fetus of Declared Pregnant Worker500 mrem per term of pregnancyGeneral Public100 mrem per yearFor additional information see Section 6 of the University of Washington Radiation Safety Manual
