Download for week 5 general science review

RTEC A - WEEK 3
GENERAL SCIENCE REVIEW
&
X-RAY PRODUCTION
IN THE TUBE
Objectives
• General Science review
• Atomic interactions in the tube
Atomic Models
• BOHR model of the
atom.
• Electrons orbit around
a nucleus (center)
Notes from Slide 3
• Bohr’s model of the atom.
• It looks like a miniature solar system in which the
electrons are orbiting around the nucleus (center) at
various energy levels.
• There are many other forms models of the atom, and the
most accurate is the quantum mechanics model. But for
the purposes of radiology the Bohr model better serves
our purpose.
• Atom is the smallest quantity of an element.
ATOM
Notes from Slide 5
• The nucleus is positively charged, small and
dense. In a normal atom the number of electrons
is equal to the number of protons. The primary
particles of the atom are : neutrons, protons and
electrons.
• Neutrons are in the nucleus and is electrically
neutral. Protons are also in the nucleus but they
have a positive charge. Electrons orbit the
nucleus and have a negative charge.
Differences in Binding Energy
Notes from Slide 7
•
•
Difference in energy levels between the shells.
Takes a 70kvp energy to knock out a K shell electron.
•
In this element- which is tungsten. The filament and target material. It has
74 protons.
•
Kev get smaller as the shells go out. This tells you the closer you are to the
nucleus the more binding energy there is. The binding energy is what it will
take to eject an electron from its home. SO in the k shell it would take a
minimum energy (kVp) of 69.5 to remove an electron from a k shell.
•
The P shell needs a much smaller amount. As it has less binding energy.
•
The atoms are very structured. If a k shell electron is empty, it must be filled.
As the holes are filled there is more photon energy released. They are
always moving into the nucleus because they are negative and the nucleus
is positive (law of attraction).
K L M Shells
Notes from Slide 9
• Electrons orbit around nucleus.
• The K,L,M are the shells we are
interested in in radiology. Usually the shell
of main importance is K shell.
Electrostatic Laws
• Repulsion/attraction
– Like charges repel
– Unlike charges attract
• Inverse square relationship
– Electrostatic force is very strong when objects
are close together
– It decreases rapidly as objects separate
Notes from Slide 11
• Like charges repel, unlike charges attract
• Inverse square relationship is much like
the inverse square law of x-ray intensity.
The electrostatic force is directly
proportional to the product of the
electrostatic charges and inversely
proportional to the square of the distance.
How “X-rays” are created
TO PRODUCE X-RAYS
YOU NEED:
• A SOURCE OF ELECTRONS
• A FORCE TO MOVE THEM QUICKLY
• SOMETHING TO STOP THEM SUDDENLY
How “X-rays” are created
• Power is sent to x-ray tube via cables
• mA (milliamperage) is sent to filament on
cathode side.
• Filament heats up – electrons “boil off”
• Negative charge
Notes from Slide 14
• As electron kinetic energy is increased both the intensity
(quantity) and energy (quality) of the x-ray beam are
increased.
• All electrons have mass, electron kinetic energy is
increased by raising the kVp.
• (Kinetic energy is the energy of motion)
• Distance between the filament and the anode target is
only about 1cm.
•
How “X-rays” are created
• Positive voltage (kVp) is applied to ANODE
• Negative electrons = attracted across the tube to
the positive ANODE.
• Electrons “slam into” anode – suddenly
stopped.
• X-RAY PHOTONS ARE CREATED
Notes from Slide 16
• When the projectile electron hit the metal of the
atoms of the x-ray tube target, they transfer their
kinetic energy to the target atoms.
• The electrons can interact with orbital electrons
or the nuclear field of target atoms.
• These interactions result in the conversion of
electron kinetic energy into thermal energy (heat
-99%) or electromagnetic energy (x-rays- 1%).
Electromagnetic Energy Spectrum
• Spectrum
– Continuous range of energy
– Although there are precise ranges defined,
they often overlap
• 3 most important to Radiologic
technology:
– Visible light
– X-radiation
– Radiofrequency
Notes from Slide 18
• EM energy is a continuous range from lower energy AMFM radio to
gamma rays.
• EM travels in the sine wave and oscillates from magnetic to electric
fields
• If you get enough energy it can penetrate matter.These generally
have shorter wavelengths and increased frequency.
• Each can be described as a bundle of energy consisting of various
electric and magnetic fields traveling at the speed of light.
• The photons the EMS differ only in their wavelength and frequency.
Electromagnetic Radiation
• Photon is the smallest quantity of any type
of EM radiation
– It is a small bundle of energy traveling at the
speed of light
– Only visible light is naturally apparent to us
• May be described as wavelike fluctuations
of electric and magnetic fields.
Electromagnetic Radiation
• These bundles of electric and magnetic
fields travel at the same velocity:
– Travel at the speed of light
– 3 x 108 m/s or 186,400 miles per sec
• The Photons of EM radiation differ only in
frequency and wavelength
General Characteristics of EMS
X-ray photons:
• Have no mass or physical form
• Travel in a linear path (until interaction
occurs)
• Dual nature: wave vs. particle
• Unaffected by
– electric or magnetic fields
– gravity
Notes from Slide 22
• Electromagnetic structure.
• Ionizing EM radiation is usually characterized by
the energy of the photon.
• X-ray photons contains considerably more
energy than visible light or radiofrequency.
• The energy of the photon is directly proportional
to is frequency. As frequency increases: energy
increases.
Wavelength and Frequency
• Wavelength is the difference between:
– Crest to Crest
– Valley to Valley
• Frequency is the number of wavelengths
passing a point of observation per second
• Wavelength & frequency are inversely
proportional
– As Wavelength increases frequency decreases
– As wavelength decreases frequency increases
Wavelength and Frequency
• Frequency and wavelength are closely
associated with the relative energy of
electromagnetic radiations.
• More energetic radiations have shorter
wavelengths and higher frequency.
Wavelength
Notes from Slide 26
• A second important characteristic of light waves,
and all electromagnetic energy, is wavelength.
That is the length of one wave measured from
the top of one wave to the top of the next.
The red line may be representative of light, with
a longer wavelength. The blue line has a shorter
wavelength and may be representative of x-rays.
• Courtesy of Mosby’s Radiography Online.
(Elsevier)
Frequency
Notes from Slide 28
• Frequency refers to the number of waves that go by a point in 1
second.
•
Remember that electromagnetic energy waves all travel at the
same speed: the speed of light.
• Imagine two different waves traveling next to each other. You're
timing them with a stopwatch and discover that one wavelength of
the first wave goes by in 1 second. That's a frequency of one wave
per second. The second wave has a wavelength half as long as the
first, and because it is traveling at the same speed, two waves will
go by in the 1 second. It has a frequency of two waves, or cycles,
per second.
• Courtesy of Mosby’s Radiography Online. (Elsevier)
The shorter the wavelength –
the higher the frequency
Notes from Slide 30
• Wavelength is the difference between
crest to crest. Or valley to valley.
• High frequency or short wavelength is
needed to penetrate tissue.
• Frequency how many crests pass per
second.
Notes from Slide 32
• Because all electromagnetic energy waves
move at the same speed, there is a simple
relationship between wavelength and
frequency: the longer the wavelength, the
lower the frequency. The shorter the
wavelength, the higher the frequency
The Electromagnetic Spectrum
• X-rays have wavelengths much shorter than
visible light, but longer than high energy gamma
rays.
Notes from Slide 34
•
As frequency increase the waves getter closer together indicating a higher
energy.
•
Frequency and wavelength are inversely proportional.
•
Right past UV light (sunburn), cannot go all the way through but it does
somewhat. (sunburn)
•
X-ray is called a photon because it has properties of matter and energy. The
scientists are unable to decide if it was matter or energy.
The only difference between x-rays and gamma rays is their origin.
X-rays are artificially simulated- emitted from an e- cloud of an atom.
In electrical imaging systems.
•
•
•
Gamma rays come from inside the nucleus of a radioactive atom.
Spontaneously from radioactive material.
What is Ionization?
Notes from Slide 36
• Removal or addition of an electron. Electrons always
move in a straight line.
• It is removed because another electron moved in and
knocked out and replaced.
• X-rays can ionize atoms.
• In an atoms normal state it is electrically neutral, the
electrical charge is zero. If an atom has an extra electron
or is missing an electron it is said to be ionized.
When an electron is added or
removed from the atom- it is
ionized
Notes from Slide 38
• If we lose on it is positive. If we gain one it
is negative.
• Loss of an negative electron makes it
positive.
Kinetic energy
• Energy of motion
• The electrons KINETIC energy is
converted to electromagnetic or PHOTON
energy
Notes from Slide 40
• Taking kinetic energy and making it into
photon energy.
X-ray production begins
at the atomic level
Energy (photons) are released when
the electron collides with another electron,
or passes close to the nucleus of the atom –
the change in energy of the shells
–produces photons
X-ray
Production
in the TUBE
INTERACTIONS IN THE TUBE
• BREMS (Bremsstrahlung)
• CHARACTERISTIC
• HEAT
Notes from Slide 44
• 3 interactions with the tubes.
Tube Interactions
• Heat = 99%
• X-ray = 1%
• Bremsstrahlung
(Brems) = 80%
• Characteristic =
20%
Notes from Slide 46
• Characteristic is better for contrast but the
patient gets more exposure.
• Brems gets the job done but less exposure
to patient.
Bremsstrahlung Radiation
• Heat & Characteristic produces EM energy
by e- interacting with tungsten atoms e- of
the target material
• Bremsstrahlung is produced by e- passing
by closely with the nucleus of a target
tungsten atom – the change in direction of
the electron – releases a photon of energy
Brem’s Radiation Animation
• http://www.coursewareobjects.com/objects
/mrophysics_v1/mod08/0816a.htm
Heat
• Most kinetic energy of projectile e- is
converted into heat – 99%
• Projectile e- interact with the outer-shell eof the target atoms but do not transfer
enough energy to the outer-shell e- to
ionize
Notes from Slide 49
• Projectile electron from cathode to anode
• Not enough energy to kick a an electron out of its shell…but it
excites the atom.
• Excitation is the release of heat
• 99% of the kinetic energy is converted into heat.
• This occurs when the projectile electron interacts with the outer-shell
electron of the target. But they do not have enough force to ionize
them. Therefore the electrons have an “excitation” phase in which
heat is produced.
• Heat is directly proportional to increasing the x-ray tube current.
Heat also increases when increasing kVp.
Heat
HEAT
8 p+ + 8e- = neutral atom
e
1. Projectile
electrons
from
cathode
2. Pass by the
electrons in
the target
3. Causing the
electrons to
vibrate
(excitation)
e
4. Excitation
produces
small
amounts of
heat
Heat is an excitation
rather than an ionization
Notes from Slide 53
• Not enough energy to ionize….just excite.
Bremsstrahlung
German
word meaning
slowed-down
or braking
radiation
Notes from Slide 55
• This occurs when a projectile electron
loses some of its kinetic energy as a result
of interacting with the nuclear field of the
atom.
• The kinetic energy of the projectile
electron is converted to electromagnetic
energy (x-ray photons).
Notes from Slide 57
•
The projectile electron has great kinetic energy as it approaches the nucleus.
Because the nucleus has a positive charge and the electron has a negative charge,
there is an electrostatic attraction between them. This pulls the electron closer to the
nucleus, even though its momentum continues to carry it forward in a bending line. It
loses kinetic energy due to the acceleration as its path changes. This is the energy
that is emitted as an x-ray photon.
•
Courtesy of Mosby’s Radiography Online. (Elsevier)
•
The projectile electron completely avoids the electrons of the target atom. It is the
close proximity to the positive nuclear field of the nucleus that it interacts with that
creates a braking or slowing down of the electron and a x-ray photon.
•
The closer the projectile e- gets to the nucleus the more it is influenced by the field of
the nucleus.
Notes from Slide 59
• Incoming electron is electrons coming in (straight line).
• Photons when they exit the tube ( wavy).
• Straight line- has to be happening in the tube.
• Wavy line is an x-ray photon in the sine wave form.
• As the projectile e- passes by the nucleus it is slowed down and
changes its course. This change in direction causes a loss of kinetic
energy. This loss of kinetic energy reappears as an x-ray photon.
• The e- can lose all of it’s energy or a fraction of it’s energy
depending how close it is to the nucleus.
Bremsstrahlung Radiation
Notes from Slide 61
• If the projectile electron entering an atom in the metal of the anode
does not strike any of that atom's electrons, it may continue toward
the center of the atom and come near the nucleus. Remember that
the electron has a negative charge and the nucleus has a positive
charge. Therefore the passing projectile electron is attracted to the
nucleus. This attraction slows the electron down as it passes the
nucleus and alters the direction of the electron's path as the nucleus
"pulls" on the electron.
The slowing of the electron means that it loses kinetic energy—and
this energy takes the form of a photon of x-ray energy being
released.
• Courtesy of Mosby’s Radiography Online. (Elsevier)
• Brem’s accounts for 80% of the photons as it can be produced by
any projectile e-. For this reason in diagnostic range, most x-rays
are Brem’s radiation.
X-ray Photons – BREMS
Creates a
polychromatic
spectrum – xrays of
different
energies
64
Energy (photons) are released when
the e passes close to the nucleus, then
changes direction
BREMS RADIATION
• Electron
• Passes by
nucleus
• Changes
direction
• Energy
released as a
PHOTON
Brem’s Radiation Animation
• http://www.coursewareobjects.com/objects
/mrophysics_v1/mod08/0816a.htm
Characteristic Radiation
• Projectile e- with high enough energy to
totally remove an inner-shell electron of
the tungsten target
• All tube interactions result in a loss of
kinetic energy from the projectile e• Characteristic x-rays are produced when
outer-shell e- fills an inner-shell void
CHARACTERISTIC (in tube)
• Electron hits inner
shell e in orbit –
knocked out &
creates a hole
• Other E’s want to
jump in
• Energy released
as PHOTONS
Notes from Slide 69
• Energy is created when the spaces in the
shells are filled. The electron energy is
converted in photon energy.
• With this interaction you get much more
photons created, much more damaging to
patients tissue. The only photons that get
tot the patient are those in the K shell.
• It is called
characteristic
because it is
characteristic of
the target element
in the energy of
the photon
produced
Characteristic Radiation Animation
• http://www.coursewareobjects.com/objects
/mrophysics_v1/mod08/0808a.htm
Tungsten Atom
Notes from Slide 73
• As this chart shows, the energy of x-rays that results
from electrons farther from the atom's nucleus is greatly
diminished. These x-rays have no value for diagnostic
imaging, although they can still have effects within the
body. Modern x-ray equipment is designed to minimize
such low-energy x-rays. This is why a transformer is
used in the x-ray machine to boost the voltage high
enough to produce higher-energy x-rays.
• Courtesy of Mosby’s Radiography Online. (Elsevier)