Title: EN250: Lecture 2 Radiation Physics
1EN250 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
2Overview
- 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?
3History of the Development of Atomic Models
- 1. Democritus (400 B.C.) matter is made of
atomos. - 2. John Dalton (1800s) 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.
4Review of the Structure of the Atom
- Atom is mostly empty space
- Mass is concentrated in its Nucleus
- Nucleus consists of Protons and Neutrons, the
Nucleons - Electrons orbiting around the nucleus
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6ELECTRONS PROTONS NEUTRONS
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
7Atomic Number
- Atomic Number A Number of Nucleons
- Mass number Z number of protons
8Characterizing Atoms
- A Nuclide is characterized by A and Z
9ORBITAL 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.
10SHELLS 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.
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13Valence 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
14Binding 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 Z74)
- EK 70, EL 11, EM 2, All in Kev
- Binding Energy EK for various atoms (usually lt
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
15Exciting 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
16Electromagnetic 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
17Electromagnetic Specturm
- Radio Waves and X-Ray/Gamma rays are the only EM
Waves that can penetrate the body
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20Combined 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
21Discovery of X-ray Wilhelm Conrad Roentgen
(1845-1923)Physical Institute of the University
of Wurzburg
22Roentgens early tube design
23Roentgens Laboratory
24Nov 1895 Penetrating solids
25First X-Ray Picture 22 Dec 1895
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32Modern X-ray Tubes
33Production 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
34- 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
35Processes 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)
36Elastic 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
37Interaction between filament 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
38Interaction between filament 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
39Illustration (2)
40Inelastic collisions with electrons in the inner
shell of an atom (characteristic radiation)
- The incoming electron transfers sufficient energy
to remove an inner shell electron from its atom
in the target. - In order for this to occur the electron must
possess energy at least as great as the binding
energy of the inner shell. - Any surplus energy appears as additional kinetic
energy in the ejected electron - The inner shell vacancy is quickly filled by an
electron falling inwards from a shell further out
from the nucleus - This transition is accompanied by a burst of
electromagnetic radiation with energy equal to
the difference in binding energies of the two
shells. - This type of x-ray production is termed
characteristic because the exact photon energy is
characteristic of the element of which the target
is made.
41Illustration (3)
42Energy 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
43Inelastic collisions with the nuclei of the atom
(Bremsstrahlung Radiation)
- The incoming electron passes very close to the
nucleus of a target atom (1). - The attraction causes the electron to deviate in
its course (2) - The sudden change of direction stimulates the
electron to release energy in the form of a
photon of electromagnetic radiation (3) - The emission of radiation results in a reduction
in the electrons kinetic energy causing it to
slow down. - The energy of the radiation depends on the degree
of deviation the electron suffers. - In an extreme case the electron may actually be
brought to rest. Thus the photon energy can be of
any value from zero up to a maximum equal to the
initial kinetic energy of the incoming electron. - This gives rise to a continuous spectrum of
x-radiation and is known as braking
(Bremsstrahlung) radiation.
44Illustration (4)
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47Interaction of X-Ray with matter
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50? Example X-ray images ?
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54End of Lecture 2
55Additional Slides
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58- Collimated vs non-collimated x-ray
59- 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
60http//www.amptek.com/xrf.html
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