Historical Background, etc.

1. Roent Lec 1
NATURE OF THE ATOM
Historical Background - Matter is a physical manifestation possessing mass
(occupies space and has weight) and having form or
- X-rays were discovered in Nov. 8, 1895 by William
shape.
Konrad Roentgen (pronounced as “rentken”)
- Elements are simple substances that cannot be
- He pioneered 3 keys areas of x-ray imaging:
decomposed by ordinary means.
1. An x-ray photograph of his closed wooden box
- There are 105 known elements at present.
of weights clearly revealed its contents, thus
- The atom is the smallest particle of an element that
presaging the security application found at
has characteristic properties of that element.
every airport check-in.
- Each atom consists of a small nucleus, which has a
2. An x-ray image of his hunting rifle revealed a
positive charge and a number of lighter particles
flaw inside the metal of the gun. This was the
with negative charges called electrons, which move
first time a hidden structural flaw had been
around the nucleus in definite orbits.
exposed without destroying the object.
- The atom is said to be neutral when the net number
3.
of positive charges of the nucleus (protons) equals
4. He took a permanent x-ray photograph of his
the negative charges of the orbital electrons.
wife Bertha’s left hand, revealing the bones and
- The closer of the electrons to the nucleus, the higher
the rings that she was wearing.
energy level.
- The electrons are kept in their orbits by the balance
between:
1. The electrostatic attraction of unlike charges.
2. The centrifugal forces of the fast moving
electrons.
ATOMIC NUMBER
- The atomic number, or Z number, is the no. of
…(taken: January 1896 by Roentgen)
protons in the nucleus or the no. of electrons
outside the nucleus.
- Z number range from 1 for the simplest atom
-The hand of the cadaver, they injected (Hydrogen) to 105 for the most complex atom yet
mercury that reveals the architecture of discovered (Hahnium).
the hand.
Orbital Electrons
- Electrons are very small particles carrying 1 unit of
Amputated negative charge.
hand after
- They revolve around the nucleus in well-defined
exposure to
shells that exist at varying distances from the
too much
radiation nucleus.
for 5 years.- - A maximum no. of seven potential electron-
containing orbits or shells are designated as
K,L,M,N,O,P and Q in order of increasing distance
from the nucleus.
- Roentgen was awarded the first Nobel Prize for Physics in
- Electrons in the
1901.
most outermost
- Roentgen announced to the word the discovery of the
shell are termed
new kind of ray in 1895; he called it the “X-ray” after the
valence electrons
algebraic symbol for unknown.
and determine the
- Otto Walkhoff, DDS, MD completed the first dental
chemical
radiograph ever med in January 14, 1896.
properties of the
- Dr. Otto Walkhoff- first dental radiograph 25minute
atom.
exposure (< 0.5 seconds today).

2. electron has a negative charge (-1) thus, an ion pair
ELECTRON ORBITS OR SHELLS has been formed.
- The electrons in an atom do not spontaneously fly
off from the nucleus by centrifugal force or, on the IONIZING RADIATION
other hand, drop into the nucleus by electrostatic
Types of Ionizing Radiation
attraction (unlike charges attract), because in the
-Corpuscular or Particulate radiation
normal atom there is a balance between centrifugal
- Electromagnetic radiation
and electrostatic force.
- This balance results in a definite electron path or
PARTICULATE RADIATION
orbit for each electron around the nucleus.
- Particulate radiations are actually minute particles of
- Each shell has a different energy level which is
matter that travel in straight lines at high speeds
dependent on the atomic no. of the atom and
from their sources.
distances the electron from the nucleus.
- Although incredibly small, they possess mass.
- The attractions force is greater when the electron
- All are charge electrically, except neutrons, and they
shell is nearer the nucleus.
all move extremely fast- sometimes as fast as light.
- Thus, it would require more work (energy) to
- Alpha Particles are composed of a combination of
remove an electron from the K shell and out of range
two protons and two neutrons.
of the nuclear electric field than to remove an
- It is the helium nucleus without orbital electrons.
electron from one of the outer shells.
- Alpha particles are emitted only from the nuclei of
BINDING ENERGIES heavy metals.
- The energy required to remove an electron from a - Compared with the other particles, the alpha particle
particular shell is designated as binding energy of is enormous and exerts a large electrostatic
that shell. attraction.
- The binding energy is characteristic of a given - They have little ability to penetrate tissues and give
element and shell. up their large energies within a very short distance in
- This is the kinetic energy of an electron accelerated air (5cm) and in soft tissue (100 um).
through a potential difference of 1 volt. - Beta Particles (Negatrons) are emitted from the
- Larger multiple units of the electron volt are nucleus of radioactive atoms and possess 1 unit of
frequently used: kcV for 1000 or kiloelectron volts, negative charge.
and McV for 1 million or megaelectron volts. - They have very small atomic masses.
- Beta particles more penetrating that alpha particles
IONIZATION and may penetrate 10- 100 cm of air and
- Ionization is the process by which a neutral atom or approximately 1-2 cm of the soft tissue.
molecule acquires either a positive or a negative - Cathode Rays (Electrons) are streams of electrons
charge. passing from the hot filament of the cathode to the
- When an atom losses or gains an electron, it is said target of the anode in an x-ray tube.
to be ionized. - Beta particles come from the nucleus of radioactive
- An ionized atom (called an ion) is not electrically atoms, whereas the cathode rays originated from
neutral but carries a charge equal to the difference the orbital electrons of the atoms of the filament
between the number of protons and electrons. material for an x-ray tube.
- An atom that is not electrically balanced is called an - Protons are accelerated hydrogen nuclei.
ion. - Because protons are heavy, charged particles, the
- In any ionization process, ion pairs are formed; it is lose kinetic energy as they penetrate matter.
this process that elicits chemical changes in matter. - Neutrons carry no electron- charge and have nearly
- When an x-ray transfers its energy to an orbital the same mass as a proton.
electron, it ejects it from the atom, and an ion pair is - The characteristic if being electrically neutral has
formed. proved of great importance in nuclear physics
- The atom becomes a positive ion (=1 charges because such a particle can penetrate into the
because it has lost an electron and the ejected nucleus of an atom without being subjected to e

3. enormous forces of a positively charged particle. 9. They cause fluorescence of certain crystals.
10. They affect photographic film (producing a latent
image which can be developed chemically).
ELECTROMAGNETIC RADIATION 11. They produce chemical and biologic changes (this is
- X-rays and gamma rays belong to a group of essential biologic cell damage and treatment of
radiation called electromagnetic radiations. malignancies, these biologic changes depend o n
- Electromagnetic radiation is the propagation of ionizations).
energy through space accompanied by electric and
magnetic force fields. WAVE CONCPET OF ELECTROMAGNETIC RADIATION
- X-rays and gamma rays belong to a group of
radiation called electromagnetic radiation. - Electromagnetic radiation is the propagation of
- X-rays are produced outside the nucleus in the wave-like energy through space or mass at the speed
4
electron orbital system. of light (186, 000 miles per second or 3 x 10 m/sec).
- Gamma rays are emitted from the nucleus of a - It is called electromagnetic radiation because the
radionuclide. energy that is radiated is accompanied by oscillating
electric and magnetic fields.
PARTICULATE and ELECTROMAGNETIC RADIATION
EXAMPLES OF ELECTROMAGNETIC RADIATION
- The radio wave that we hear.
- The light waves we see.
- The infrared waves that can take pictures in the dark.
- The ultraviolet rays that cause sunburn.
- The x-rays.
- The gamma rays of the atomic bombs.
- The cosmic rays that hinder travel in space.
NATURE OF X-RAYS Electromagnetic waves need no such medium as
- The wave concept of electromagnetic radiation they can be propagated within and transmitted
explains why it may be reflected, refracted, through a vacuum.
diffracted and polarized. All waves have an associated wavelength and
- The particle concept is used to describe the frequency.
interaction between radiation and matter. The wavelength of a wave is the distance between
two successive crests or valley, and is given the
X-RAY PROPERTIES
symbol λ (the Greek letter lambda, the symbol for
1. They are weightless packages of pure energy
length).
(photons) without an electrical charge which travel
in waves with specific frequency at speed of 186,
- The
000 miles per second (speed of light).
no. of waves
2. Invisible, highly penetrating waves (electromagnetic
passing a
waves).
particular
3. Electrically neutral (cannot be deflected by electrical
point during
or magnetic fields).
a specific
4. Wide useful range of wavelengths in medicine and
period is
dentistry = 0.01 is equal to 1 billionth of a meter).
called the
5. Most beams of x-rays are heterogeneous (consisting
frequency
of rays of many different wavelengths).
and I given
6. They emerge from the tube in straight lines,
the symbol λ
diverging form the focus of the target of the anode.
(the Greek
7. They are capable of ionizing gases because of their
letter nu, the symbol for number).
ability to remove orbital electrons from atoms.
- It is usually identified as oscillations per second or
8. They travel as the same speed as light (1866, 000
cycles per second.
miles per second in a vacuum).
- The unit of frequency measurement is the hertz (Hz).

4. - One hertz equals 1 cycle/ sec and 80 Hz is the long waves are in the radio region of the
standard 60 cycle/ sec. electromagnetic spectrum.
Dangers:
THE ELECTROMAGNETIC SPECTRUM - Large doses of radio wavesare believed to cause
cancer, leukaemia and other disorders.
What are electromagnetic waves?
- Some people claim that the very low frequency field
- All forms of electromagnetic radiation are grouped
from overhead power cables near their homes has
according to their wavelengths in what is called the
affected their health.
electromagnetic spectrum.
- All have no mass, no electrical charge and travel with
MICROWAVES
wave motion.
- Microwaves are basically extremely high frequency
- Electricity can be static like what a balloon to the wall or
radio waves, and are made by various types of
makes your hair stand on end. Magnetism can also be
transmitter in a mobile phone, they’re made by
static like a refrigerator magnet. But when they change
or move together, they make waves electromagnetic a transmitter chip and an antenna, in a
waves. microwave oven they’re made by a
- Electromagnetic waves are formed when an electric “magnetron”. Their wavelength is usually a
field (blue arrows) couples with a magnetic field (red couple of centimetres. Stars also give off
arrows). Magnetic and electric fields of an microwaves.
electromagnetic wave are perpendicular to each other Uses:
and to the direction of the wave.
- Microwaves cause water and fat molecules to
vibrate which makes the substances hot. Thus we
can use microwaves to cook many types of food.
- Microwaves have wavelengths that can be measured
in centimetres. The longer microwaves, those closer
to a foot in length, are the waves which heat our
food in a microwave oven.
- Microwaves are good for transmitting information
from one place to another because microwave
energy can penetrate haze, light rain and snow,
THE ELECTROMAGNETIC SPECTRUM clouds and smoke.
Rabbits Radio - Shorter microwaves are used in remote sensing.
Mate Microwaves These microwaves are used for radar like the
In Infra-Red doppler radar used in weather forecasts.
Very Visible light - Microwaves, used for radar, are just a few inches
Unusual Ultra-violet long. This microwave tower can transmit information
eXpensive X-rays like telephone and computer data from one city to
Gardens Gamma rays another.
- Because microwaves can penetrate haze, light rain,
RADIO WAVES snow, clouds and smoke, these waves are good for
- Radio waves are made by various types of viewing the earth from space.
transmitter depending on the wavelength. They are - Mobile phones use microwaves as they can be
also given off by stars sparks and lightning which is generated by asmall antenna which means that the
why you hear interference on your radio in a phone doesn’t need to be very big. The drawback is
thunderstorm. that, being small, they can’t put out much power and
- Objects in space, such as planets and comets, giant they also need a line of sight to the transmitter. Thus
clouds of gas and dust, and stars and galaxies, emit means that mobile phone companies need to have
light at many different wavelengths. many …
- Some of the light they emit has very large
wavelengths (sometimes as long as a mile). These

5. Dangers: - Each color has a different wavelength.
- Prolonged exposure to microwaves is known to - Red has the longest wavelength and violet has the
cause “cataracts” in your eyes which is a clouding of shortest wavelength.
lens preventing you from seeing clearly (if at all!). So - When all the waves are seen together they make
don’t make a habit of pressing your face against the white light.
microwave oven door to see if your food’s ready! - When the white light shines through a prism or
- People work on aircraft carnet decks wears special through water vapour like the rainbow, the white
suits which reflect microwaves to avoid being light is broken apart into the colors of the visible
“cooked” by the powerful radar units in modern light spectrum.
military planes. - The light which our eyes can detect as part of the
visible spectrum (small).
INFRA-RED - There is a lot of radiation around us which is
- Infra-red waves are just below visible red light in the “invisible” in our eyes but can be detected by other
electromagnetic spectrum (“intra” means “below”). remote sensing? instruments and used to our
- You probably think of infra-red waves as heat advantage.
because they’re given off by hot objects and you can - The visible wavelengths cover a range from
feel them as warmth in your skin. approximately 0.4 to 0.77 mm (violet to red).
- Infra-red waves are also given off by stars, lamps,
flames and anything else that’s warm- including you.
Uses:
-Infra-red waves are called IR for short. They are used for
many tasks, for example, remote controls for TVs, and
video recorders and physiotherapists use heat lamps to
help heal sports injuries.
- Shorter, near infrared waves are not hot at all- in fact
you cannot even feel them. These shorter wavelengths RADIATION
are the ones used in your TV’s remote control.
Dangers:
The warmer the object, the more infrared radiation it - Too much light can damage the retina in your eye.
emits. - This can happen when you look at something very
bright, such as the sun.
- Because every object gives off IR waves, we can use
- Although the damage can heal, if it’s too bad, it’ll be
them “to see in the dark. Night sights for weapons
permanent.
sometimes use a sensitive IR detector.
- Gas infrared waves are thermal. In other words, we ULTRAVIOLET
experience the type of infrared radiation every day
in the form of heat! The heat that we feel from the -Ultra-violet
sunlight, a fire, a warm sidewalk are infrared. light is made by
- You’ve probably seen TV programmes in which the special lamps,
police helicopter track criminals at night using for example, on
cameras which can see in the dark. These cameras sun beds.
use IR instead of “ordinary light” which is why -It is also given
people look bright in pictures. off by the sun
in large
Dangers; quantities.
- The danger from too much infra-red radiation is very - We call it UV.
simple, overheating.
VISIBLE LIGHT Uses:
- Sun tan
- Visible light waves are the only electromagnetic - Detecting fake money
waves we can see. - Light sure composite
- We see these waves as the colors of the rainbow.

6. - UV in discos (glow in the dark) - The amount of energy carried by each quantum photon
- Sterilization depends on the frequency of the radiation.
- Sunlight (vitamin D) - If the frequency (vibrations per second) is doubled, the
The OZONE layer protects the earth from the heat of energy of the photon is doubled.
the sun. - The unit used to measure the energy of photons is the
Filters about 95% of heat. electron-volt (eV).
Only 5% heat UV reaches earth’s surface.
WHAT ARE X-RAYS?
Dangers: - X-rays are weightless packages of pure energy
- Sunburn- basal cell CA (photon) that have no electrical charge and travel in
- Cataract- blindness waves with specific frequency at a speed of 3 x 108
m/sec.
X-RAYS
- Their energies depend on the frequency of their
Uses:
wavelengths.
- MRI, CT-scan
- The greater the frequency of their wavelength, the
- Intra-oral x-rays
greater the energy of the photon.
- A drink of barium sulphate. This will absorb x-rays,
- The greater the energy of the photon, the more
and so the patient’s intestines will show up clearly
readily it will penetrate matter.
on an x-ray image
PROPERTIES OF X-RAYS
GAMMA RAYS
- X-rays are invisible and weightless; they cannot be
- They have the smallest wavelengths and the most
seen, felt or smelled.
energy of any other wave in the electromagnetic
- X-rays travel in straight lines, they can be deflected
spectrum.
from their original direction but the new trajectory is
- These waves are generated by radioactive atoms and
linear.
in nuclear explosions. 8.
- X-rays travel at the speed to light (3 x 10 ? m/sec).
- Gamma-rays can kill living cells, a fact which
- X-rays have a wide range of wavelengths, 0.01- 0.05
medicine uses to its advantage using gamma rays to
nm in length.
kills cancerous cells.
- X-rays cannot be focused to a point over distance
Radioactivity is particularly damaging to rapidly
the beam diverges much like a beam of light.
dividing cells, such as cancer cells.
- Because of their extremely hot wavelengths, they
This also explains why damage is done by
are able to penetrate materials that absorb and
radiotherapy to other rapidly dividing cells in
reflect visible light.
the body such as the stomach lining (hence
- X-rays cause certain substances to fluoresce, that is
nausea), hair follicles (hair tends to fall out) and
to emit radiation of longer wavelength (it is this
a growing fetus (not because of mutations, but
property that makes it possible to use intensifying
simply major damage to the baby’s rapidly
screens in radiography.
dividing cells).
- X-rays produce biological changes that are valuable
Dangers: in radiation therapy but necessitate caution when
- Gamma rays cause cell damage and can cause a used for diagnostic purposes.
variety of cancers. - X-rays can ionize gases that is remove electrons from
- Cause mutations in growing foetuses, so unborn atoms to form ions, which can be used for measuring
babies are especially vulnerable. the controlling exposure (ionization chambers).
PARTICLE CONCEPT OF ELECTROMAGNETIC RADIATION
-Short electromagnetic waves, such as x-rays, may react with
matter as if they were particles rather than waves.
- These particles are actually discrete bundles of energy
having no mass, and each of these bundles of energy is called
quantum or photon.
- These photon travel at the speed of light.