chapter 1: atom and luminescence - xraykamarul thomson atom 1.1.4 bohr atom 1.6 radioactivity ......
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PHYSICS FOR RADIOGRAPHERS 2
CHAPTER 1:
Atom and LuminescencePREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
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CHAPTER 1: ATOM AND LUMINESCENCE
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LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Define what is atomic structure and its theory.
Differentiate between mass number and atomic number.
Explain the fluorescence, phosphorescence and thermo luminescence
in medical imaging.
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CHAPTER 1: ATOM AND LUMINESCENCE
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TOPIC OUTLINES
INTRODUCTION
1.1 Centuries of Discovery 1.4 Atomic Nomenclature
1.1.1 Greek Atom
1.1.2 Dalton Atom 1.5 Combinations of Atoms
1.1.3 Thomson Atom
1.1.4 Bohr Atom 1.6 Radioactivity
1.6.1 Radioisotopes
1.2 Fundamental Particles 1.6.2 Radioactive Half Life
1.3 Atomic Structure 1.7 Types of Ionizing Radiation
1.3.1 Electron Arrangement
1.3.2 Electron Binding Energy 1.8 Luminescence
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1.1 Centuries of Discovery
1.1.1 Greek Atom
It states all matter composed of four
substances: earth, water, air, and
fire.
All matter is a combination of these
four substances in various
proportions with modification of
wet, dry, hot, and cold.
The Greeks used the term atom
(indivisible) to describe the smallest
part of the four substances.
Figure 1 The ancient Greek
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1.1 Centuries of Discovery
Figure 2: Symbolic
representation of the
substances and essences of
matter as viewed by the
ancient Greek.
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1.1 Centuries of Discovery
1.1.2 Dalton Atom
In 1808, John Dalton (English school
teacher) published that the
elements could be classified
according to integral values of
atomic mass.
An element was composed of
identical atoms that reacted the
same way chemically.
E.g. all O2 atoms were alike but very
different from atoms of any other
element.
Figure 3 John Dalton
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1.1 Centuries of Discovery
1.1.2 Dalton Atom (Continued)
The physical combination was
visualized as being an eye-and hook
affair.
The size and number were different
for each other.
The Dalton’s work has triggered a
Russian scholar (Dmitri Mendeleev)
to arrange the elements in order
that resulted in the first periodic
table of elements.
Figure 4 The difference between
Greek and Dalton.
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1.1 Centuries of Discovery
1.1.3 Thomson Atom
In the late 1890s, J.J. Thomson
concluded that electrons were an
integral part of all atoms.
He described the atom as a plum
pudding, where the plums
represented negative electric
charges (electrons) and the pudding
was a shapeless mass of uniform
positive electrification.
Figure 5 J.J. Thomson
Figure 6 The model of Thomson
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1.1 Centuries of Discovery
1.1.3 Thomson Atom (Continued)
The number of electrons and
positive charges are equal as atom
was known as neutral.
However, in 1911, Ernest
Rutherford disproved Thomson’s
model and he introduced the
nuclear model as atom contains a
small, dense, positively charged
center surrounded by electrons. The
center is known as nucleus.
Figure 7 Ernest Rutherford
Figure 8 Rutherford’s Atomic Model
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CHAPTER 1: ATOM AND LUMINESCENCE
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1.1 Centuries of Discovery
1.1.4 Bohr Atom
In 1913, Niels Bohr improved
Rutherford’s model.
Bohr’s model was a miniature solar
system in which the electrons
revolved about the nucleus in orbits
(energy levels).
As similar to the nuclear model, it
has electrons that revolve in fixed
and well defined orbits about the
nucleus.
Figure 9 Niels Bohr
Figure 10 Bohr Model
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1.1 Centuries of Discovery
Figure 11: Through the years,
the atom has been
represented by many symbols.
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1.2 Fundamental Particles
The fundamental particles of an atom are the electron, the proton, and the
neutron.
The atom can be viewed as miniature solar system.
Electrons carries one unit of negative electric charge. (mass 9.1 x 10-31 kg).
Because atomic particle is extremely small, its mass is expressed in atomic
mass unit (amu) for convenience.
1 amu = 1 ½ of mass a carbon -12 atom. [electron (amu) = 0.000549 amu]
Nucleus contains nucleons (protons and neutrons).
Mass of proton is 1.673 x 10-27 kg and the neutron is 1.675 x 10-27 kg.
Proton carries one unit of positive charge while Neutron carries no charge
(neutral).
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1.2 Fundamental Particles
Figure 12: An atom. Figure 13: The proton, electron and
neutron.
Figure 14: The THREE elements in an atom.
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1.3 Atomic Structure
The atom is essentially empty space.
The number of protons determines the chemical elements and the neutrons
are neutral charge.
If the atoms have the same number of protons but differ in the number
neutrons are called isotopes.
Electrons can exist in only in certain shells which represent different electron
binding energies or energy levels.
Electron orbit shells have been identified as K, L, M, N, and so forth to show
the different energy levels from the closest to the farthest to the nucleus.
The total number of electrons in the orbital shells is equal to the number of
protons in the nucleus. If it has extra, it will be removed of ionization.
Ionization is the removal of an orbital electron from an atom.
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1.3 Atomic Structure
Figure 15: The nucleus consists of
protons and neutrons, which are
made of quarks bound together by
gluons.
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1.3 Atomic Structure
1.3.1 Electron Arrangement
The number of electrons that can exist in each shells increases with the
distance of the shell from nucleus.
The maximum number of electrons per shell can be calculated using this
formula [2n2], where n is the shell number.
The number of electrons in the outermost shell of an atom is always limited to
eight electrons.
No outer shell can contain more than eight electrons.
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1.3 Atomic Structure
1.3.2 Electron Binding Energy (Eb)
It is the strength of attachment of an
electron to the nucleus.
The closer the an electron is to the
nucleus, the more tightly it is bound.
K-shell electrons have higher binding
energies than L, M, N and so forth.
The greater the total number of
electrons in an atom, the more
tightly each is bound.
The larger and more complex the
atom, the higher is the Eb for
electrons in any given shell.
Figure 16: An example of binding
energy for atom of Tungsten.
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1.4 Atomic Nomenclature
Element
It is indicated with chemical symbols. e.g. Ca, H, Be
The chemical properties of an element are determined by the number
and arrangement of electrons.
Atomic Number
(Z)The number of protons.
Atomic Mass
Number (A)The number of protons and neutrons
IsotopesAtoms that have the same atomic number but different atomic mass
numbers.
IsobarAtomic nuclei that have the same atomic mass number but different
atomic numbers.
IsotoneAtoms that have the same numbers of neutrons but different numbers
of protons.
Isomer The same atomic number and atomic mass number.
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1.5 Combinations of Atoms
Molecule = combination of atoms of various elements.
Example 1: Four atoms of Hydrogen (H2) and two atoms of oxygen (O2)
can combine to form two molecules of water.
2H2 + O2 2H2O
Example 2: An atom of sodium (Na) can combine with an atom of chlorine
(Cl) to form a molecule of sodium chloride (NaCl),
Na + Cl NaCl
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1.5 Combinations of Atoms
Compound = A chemical compound is any quantity of one type of molecule.
Example: Sodium, Hydrogen, Carbon, and Oxygen atoms can combine to
form a molecule of sodium bicarbonate (NaHCO3).
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1.6 Radioactivity
Radioactivity is the emission of particles and energy in order to become
stable.
Figure 17: The emission of particles or energy by an
unstable element.
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1.6 Radioactivity
1.6.1 Radioisotopes
Many factors affect nuclear stability.
When the nucleus contains too few or too many neutrons, the atom can
disintegrate radioactively, bringing the number of neutrons and protons into a
stable and proper ratio.
Radioisotopes are the isotopes that have radioactivity. It can be artificially
produced in machines such as particle accelerators or nuclear reactors.
Also, a few elements have naturally occurring radioisotopes as well.
.
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1.6 Radioactivity
Figure 18: Radioisotopes can decay and result in emission of alpha,
beta particles or gamma rays.
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1.6 Radioactivity
1.6.2 Radioactive Half-Life
Radioactive matter is not here one day and gone the next.
Rather, radioisotopes disintegrate into stable isotopes of different elements at
a decreasing rate, so that the quantity of radioactive matter never reaches
zero.
Radioactive material is measured in curies (Ci). {1 Ci is equal to 3.7 x 1010 Bq}
Half Life (T1/2) of radioisotopes is the time required for a quantity of
radioactivity to be reduced to one-half of its original value.
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1.7 Types of Ionizing Radiation
It can be classified into TWO categories:
a) Particulate radiation
b) Electromagnetic radiation
Although of ionizing radiation acts on biological tissue in the same manner,
there are fundamental differences between different types of radiation
according to the mass, energy, velocity, charge, and origin.
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1.7 Types of Ionizing Radiation
1.7.1 Particulate Radiation
There are TWO types of particulate radiation which are alpha particles and
beta particles.
Both are associated with radioactive decay.
1.7.1.1 Alpha particle
Is a helium nucleus that contains two protons and two neutrons.
Its mass is 4 amu and carries 2 units of +ve electric charge.
Travels with high velocity through matter but in short range.
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1.7 Types of Ionizing Radiation
1.7.1.2 Beta particle
Is an electron emitted from nucleus of a radioactive atom.
Light particles with atomic mass number is zero.
Carry 1 unit of –ve or +ve charge.
Originate in the nuclei of radioactive atoms.
Positive beta particles are positrons.
Travels in longer range than alpha particle.
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1.7 Types of Ionizing Radiation
1.7.2 Electromagnetic Radiation
X-rays and gamma rays are forms of electromagnetic ionizing radiation.
Often called photons which no mass and no charge.
They travel at the speed of light (c = 3 x 108 m/s).
Gamma rays emitted from the nucleus of radioisotopes and are usually
associated with alpha and beta particles.
X-rays are produced outside the nucleus in the electron shells.
Both have unlimited of travel range in matter.
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1.8 Luminescence
1.8.1 Introduction
Any material that emits light in response to some outside stimulation is
called a luminescent material or phosphor.
The emitted visible light is called luminescence.
A number of stimuli, including electric current (fluorescent light), biochemical
reactions (the lightning bug), and x-rays (a radiographic intensifying screen),
cause luminescence in materials.
In radiography, the intensifying screen, absorption of a single x-ray causes
emission of thousands of light photons.
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1.8 Luminescence
1.8.2 Principle
When a luminescent material is stimulated, the outer shell electrons are
raised to excited energy levels.
Then, it creates a hole in the outer-shell electron, which is an unstable
condition of atom.
The hole is filled when the excited electron returns to its normal state.
This transition is accompanied by the emission of a visible light photon.
Luminescent materials emit light of a characteristic color.
Three types of luminescence have been identified in medical imaging
modalities: fluorescence, phosphorescence, and thermoluminescence.
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1.8 Luminescence
1.8.2.1 Fluorescence
Emission of electromagnetic radiation.
Emitted only while the phosphor is stimulated.
Usually visible light, caused by excitation of atoms in a material, which then
re-emit almost immediately (within about 10−8 seconds).
i.e. It ceases as soon as the exciting source is removed.
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1.8 Luminescence
1.8.2.2 Phosphorescence
Emission of light from a substance exposed to radiation.
Persisting as an afterglow after the exciting radiation has been removed.
The phosphor continues to emit light after stimulation.
Requires additional excitation to produce radiation and may last from about
10-3 second to days or years, depending on the circumstances.
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1.8 Luminescence
1.8.2.3 Thermo luminescence
It is phosphorescence triggered by temperatures above a certain threshold.
Heat is not the primary source of the energy, only the trigger for the release
of energy that originally came from another source.
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1.8 Luminescence
Luminescence
Fluorescence
Immediate
Phosphorescence
Short Time Long Time
Thermo luminescence
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1.8 References
No. REFERENCES
1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for
radiographers. Blackwell.
2 Bushong, S. C. (2008). Radiologic science for technologists. Canada:
Elsevier.
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CHAPTER 1: ATOM AND LUMINESCENCE
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Activity
Define or otherwise identify the following:
a) Photon Answer
b) The Rutherford atom Answer
c) Positron Answer
d) Nucleons Answer
e) Radioactive Half –Life Answer
f) Alpha Particle Answer
g) Beta particle Answer
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Activity
Who developed the concept of the atom as a miniature solar system?
Answer
List the fundamental particles within an atom.
Answer
Describe the difference between alpha and beta emission.
Answer
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Activity
Define luminescence
Answer
What are types of luminescence?
Answer
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SUMMARY
As a miniature solar system, the Bohr atom set the stage for the modern interpretation
of the structure of matter.
Atom is the smallest part of an element, and molecule is the smallest part of a
compound.
Three fundamental particles: proton, electron, neutron.
Some atoms have the same number of protons and electrons but different number of
neutrons, different atomic mass. These are isotopes.
Radioactivity : some atoms contain too many or too few neutrons in the nucleus that
can disintegrate.
Two types of particulate radiation: alpha and beta particles.
Half-life: time required of radioactivity to be reduced to one-half its original value.
Electromagnetic radiation: x-rays and gamma rays.
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NEXT SESSION PREVIEW
CHAPTER 2: ELECTROMAGNETIC RADIATION
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APPENDIX
FIGURE SOURCE
Figure 1 http://www.universetoday.com/wp-content/uploads/2009/12/Democritus.jpg
Figure 2
Figure 3 http://3.bp.blogspot.com/_nW9kILlRDjU/THYkY9gTwnI/AAAAAAAAAAM/YFmQh
2QLfw4/s1600/John+Dalton.jpg
Figure 4 http://abyss.uoregon.edu/~js/images/atom_prop.gif
Figure 5 http://upload.wikimedia.org/wikipedia/commons/c/c1/J.J_Thomson.jpg
Figure 6 http://2011period6group4.wikispaces.com/file/view/Thomson's_Model.gif/1684
83477/Thomson's_Model.gif
Figure 7 http://www.vias.org/physics/img/rutherford.jpg
Figure 8 http://i54.tinypic.com/n2l3r9.png
Figure 9 http://abyss.uoregon.edu/~js/images/nbohr.gif
Figure 10 http://cdn.timerime.com/cdn-33/users/13890/media/Atom_diagram.jpg
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APPENDIX
FIGURE SOURCE
Figure 11
Figure 12 http://1.bp.blogspot.com/_SmG3fxIEtUk/TD7yf3eF7XI/AAAAAAAADKQ/uziSUELf
Bwc/s1600/proton.jpg
Figure 13 http://www.cartage.org.lb/en/themes/sciences/chemistry/generalchemistry/a
tomic/BasicStructure/atmparts.gif
Figure 14 http://www.chemistryland.com/ElementarySchool/BuildingBlocks/NeutronProt
onElectronLight.jpg
Figure 15
Figure 16 http://www.medcyclopaedia.com/upload/book%20of%20radiology/chapter03/ni
c_k3_0.jpg
Figure 17 http://www.boluodusmyportfolio.com/Images/radioactivity2.gif
Figure 18 http://www.universetoday.com/wp-content/uploads/2011/04/Radioactive-
Isotopes.jpg