chapter 1: atom and luminescence - xraykamarul thomson atom 1.1.4 bohr atom 1.6 radioactivity ......

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

TOPIC

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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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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Figure 2: Symbolic

representation of the

substances and essences of

matter as viewed by the

ancient Greek.

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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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1.1.2 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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1.1.3 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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1.1.3 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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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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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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Figure 15: The nucleus consists of

protons and neutrons, which are

made of quarks bound together by

gluons.

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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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1.3.2 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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1.7.1 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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1.7.1.2 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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1.7.2 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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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

chapter 1: atom and luminescence - xraykamarul thomson atom 1.1.4 bohr atom 1.6 radioactivity ......

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