ATOMIC STRUCTURE AND X RAY PRODUCTION · ATOMIC STRUCTURE AND X RAY PRODUCTION ... Atomic Structure...

© Barbara Lamb 2013

ATOMIC STRUCTURE AND

X RAY PRODUCTION

X-rays were discovered by Roentgen in 1895.

The nature was unknown and so they were called x-rays

They are high energy electromagnetic radiation part of the

electromagnetic spectrum

ELECTROMAGNETIC RADIATION

Electromagnetic radiation consists of waves

most are invisible (e.g. radiowaves, microwaves, ultraviolet light, x-

rays)

only a very small part is visible to humans

they are all part of the electromagnetic spectrum

X-rays consists of ‘wave packets’ of energy called photons

X-rays are short wavelength with high power

A photon = one quantum of energy

X-ray beam is made up from millions of individual x-ray photons


Energy = the ability to do work

Radiation = the transfer of energy in the form of waves or particles

SOURCES OF RADIATION

1.ARTIFICIAL BACKGROUND RADIATION

Fallout from nuclear explosions

Radioactive waste discharge from nuclear establishments

Average background radiation is approximately 2.7 mSv per year

2. MEDICAL AND DENTAL DIAGNOSTIC RADIATION X RAYS

3.RADIATION FROM OCCUPATIONAL EXPOSURE

4.NATURAL BACKGROUND RADIATION

Cosmic rays from the atmosphere

Gamma radiation from the rocks and soil in the earth’s crust

Radiation from ingested isotopes in certain food

Radon and its decay products. Radon is a gaseous decay product of

uranium naturally found in granite. As the gas it diffuses readily

from rocks through soil and can be trapped in poorly ventilated

houses. Cornwall and Scotland have house is built on granite

Average background radiation is approximately 2.7 mSv per year

MAN MADE X RAYS

They are produced when fast moving electrons collide with a metal

target and are suddenly brought to rest.The electrons kinetic

energy is transformed into

electromagnetic energy (x-ray photons)

X-rays for medical procedures are produced inside an x-ray tube


the primary function of an X-ray tube is to convert electrical energy

into X-rays

ATOMIC STRUCTURE

Atomic Structure - the building blocks of matter

Nucleus-protons and neutrons forming a dense centre of particles

Electrons- orbiting around nucleus in specific orbits

Atomic no is the no of protons in an atom

Atoms are minute particles held by nuclear or electric forces

The unit of energy in atomic structure is the electron volt

Neutral Atom-no of electrons=no of protons

Atoms at ground state are electrically neutral

positive protons = no of negative electrons

NUCLEUS

A typical nucleus is made up from particles known as protons and

neutrons, and carries a positive charge

Protons and neutrons are almost equal in mass (size)

1.66 x10-24 g


PROTONS AND NEUTRONS

Protons are positively-charged They will tend to repel each other because

each proton carries the same charge (i.e. like charges repel each other,

whereas unlike charges attract each other)

Neutrons have no charge therefore are electrically neutral They act as

a ‘glue’ to bind the nucleus together by counteracting the repulsive

forces between the protons

The charge of a nucleus is therefore POSITIVE

ELECTRONS

Electrons are negatively-charged

They are 1840 times smaller than protons and neutrons

The orbiting electrons are arranged in energy levels or shells

around the nucleus

These have increasing energy levels from the nucleus outwards

Electrons can move up and down the orbiting shells around the

nucleus

There are different energy levels labelled KLMNO outwards from

the nucleus

There is a maximum no of electrons allowed in each shell

K-2 L-8 M-18 N-32 O-50

The electrons must first fill the lowest available energy level/shell

available nearest the nucleus

When the level is full the next electron goes into the next highest

level(shell) available

Electron removed we now have a positive charged “positive ion”

This process is called “ionisation ”


Electron displaced to outer shell (higher energy) “excitation”

Atomic Number (Z) – the number of protons in the nucleus of an

atom

Neutron number(N)-the number of neutrons in the nucleus of an

atom

Atomic mass number(A)-sum of the numbers of protons and

number of neutrons in an atom

(A=Z+N )

Isotopes –atoms with the same atomic number (Z)but with

different atomic mass numbers (A) and hence different number of

neutrons(N)

Radioisotopes-isotopes with unstable nuclei which undergo

radioactive disintegration

Atoms with different Z numbers are known as Elements

Elements form the basis of the periodic table

No two elements have the same atomic number

The number of electrons in an atom determines its chemical

properties

In a neutral atom the number of orbiting electrons is equal to the

number of protons in the nucleus, ie the number of positive and

negative charges are balanced If an electron is removed the atom is no

longer electrically neutral but becomes positively charged, i.e. it

becomes a positive ion and is said to be ionised

Ionisation is the process whereby an electron is removed from an

atom to overcome the binding energy of attraction that keeps

electrons within their shells

The unit of energy in the atomic system is the electron

volt (eV)


X-RAY PRODUCTION

X RAYS ARE PRODUCED WHEN HIGH SPEED

ELECTRONS HIT A TARGET AND STOP!!!

ELECTRONS BOMBARD THE TARGET AND

LOSE ENERGY 99% HEAT AND 1% XRAYS

High voltage (potential difference between the cathode and anode )

accelerates electrons Kv


Current flows between the cathode and anode mA

Lead casing absorbs unwanted x rays

Oil casing removes the heat

Electrons are brought to rest at the target

Energy lost is HEAT AND X RAYS

Heat is dissipated

X rays are emitted through the lead window BEAM

RELATIONSHIP OF FOCAL SPOT ON IMAGE

BLURRING

PRINCIPLE OF LINE FOCUS

Lead casing absorbs the unwanted x rays only allowing the x rays

to pass out of the x ray window

The oil in the x ray tube removes heat

The copper block surrounding target removes the heat

X rays are emitted through the lead window

X RAY PRODUCTION


X-rays are produced at the anode as the result of collisions between

high speed bombarding electrons and the tungsten targets atoms

At the anode there are two different atomic processes

HEAT PRODUCING COLLISIONS

X RAY PRODUCING COLLISIONS

HEAT PRODUCING COLLISION

BOMBARDING ELECTRON IS DEFLECTED BY TUNGSTON ELECTRON

INCOMING

BOMBARDING ELECTRON

DEFLECTED

ELECTRON

HEAT

© 2012 Barbara H Lamb 54

Incoming electron collides with outer shell tungsten electron and displaces more peripherally

INCOMING

BOMBARDING ELECTRON

DEFLECTED

ELECTRON

HEAT


Heat producing -most common –millions of electrons and many outer

shell electrons

Each bombarding electron can collide many times producing

HEAT

Needs fast heat dissipation copper block


X-ray Spectra

The two ways that x-rays may be produced at the target tungsten anode

result in the


spectra:

1 Continuous spectrum

2 Characteristic spectrum

CONTINUOUS SPECTRUM

PROCESS 1

The incoming electron passes close to the nucleus it is pulled

towards the nucleus (positive and negative charges attract)

the electron slows down and change direction towards the nucleus

the electron loses a large amount of its energy, which is then

emitted as multiple x-ray photons

original electron is slowed down or stopped

Incoming electron passes close to the nucleus

Electron-dramatically slowed down-deflected by the nucleus-large loss of energy emitted as X Rays

INCOMING

BOMBARDING ELECTRON

X RAY

PHOTONS

EMISSION


PROCESS 1

X-rays produced in this way are referred to as Bremsstrahlung (or

Braking) Radiation

During this process many x-ray photons of different energies are

produced. These x-rays form the continuous spectrum

This type of interaction accounts for 85% of the x-rays produced by an

x-ray tube


Small deflections are most common, resulting in many useless low

energy photons (most will not leave the x-ray tube and those that do will

be filtered out)

ALUMINIUM FILTRATION are used at the x ray window

TO FILTER X RAYS

There are many fewer large deflections, therefore few high-energy

photons are produced

The maximum photon energy (Emax) is directly related to the kV

across the x-ray tube

This is formed from the range of x-rays produced by

Bremsstrahlung(process 1)

This is termed the continuous spectrum. The amount of deceleration

and degree of deflection of the incoming electron determines the

amount of energy lost. Therefore a wide range(spectrum) of x-ray

photons of differing energies can be produced


CHARACTERISTIC SPECTRUM

NK

L

M


X ray producing collisions B STAGE 1


N

66

BOMBARDING ELECTRON

INCOMING BOMBARDING ELECTRON

DEFLECTED ELECTRON


Incoming electron collides with inner shell electron-displaces it to an

outer excitation or displaces it from the atom ionisation large loss of

energy and subsequent X rays

X ray producing collisions B STAGE 2

OUTER SHELL ELECTRONS DROP INTO INNER SHELLS -X RAY EMISSION

69

OUTER SHELL ELECTRONSCASCADE TO FILL SPACE

ENERGY IS GIVEN OFF AS X RAYS

Remember that atoms have their electrons arranged in"shells" of

different energies

The K-shell (closest to the nucleus) is the lowest energy state of an

atom the incoming electron collides with a tungsten K-shell electron

and gives it enough energy to jump out of its shell ionisation

Following this ionisation, one of the atoms electrons of higher energy

(from an outer shell) falls into the K-shell


During this there is a specific loss of energy, which produces a x-ray

photon

characteristic of tungsten atoms

These x-rays form the

characteristic spectrum

CHARACTERISTIC SPECTRUM....recap

This is formed by the x ray photons characteristic of tungsten

atoms (process 2)

The photon lines are named K or L dependant on the shell from

which they have been emitted

Only K lines are of use as L lines are too low energy

The accelerated electron must have enough energy to displace the

K-shell tungsten electron out of its shell

Line spectra emitted from another element would plot onto the

figure at different photon energies

PROPERTIES AND CHARACTERISTICS OF X RAYS

X-rays are wave packets of energy of electromagnetic radiation that

originate at atomic level


Each wave packet is equivalent to a quantum of energy and is

called a Photon

An x-ray beam is made up of millions of photons of differing

energies

The x-ray beam can vary in its

a) INTENSITY (the quantity/number of photons in the beam) and

its

b) QUALITY (the energy carried by the photons which is a

measure of their penetrating power)

INTENSITY

The x-ray beam can vary in its intensity (the quantity/number of

photons in the beam)

QUALITY

The energy carried by the photons which is a measure of their

penetrating power)

FACTORS THAT CAN AFFECT THE INTENSITY

AND/OR THE QUALITY OF THE X-RAY BEAM ARE:

– Tube kilovoltage (kV)

– Tube current (mA)


– Distance from the target (d)

– Time/length if exposure (t)

– Filtration-usually aluminium filtration to remove harmful low energy

( soft) x-rays

– Target material

X RAYS

X-rays travel in straight lines in free space (e.g air)

In free space, x-rays obey the inverse square law

This means that doubling the distance from the x-ray source will

reduce the intensity to ¼ (* radiation protection)

DOUBLE THE DISTANCE FROM THE SOURCEQUARTER THE DOSE TO THE PATIENT


Shorter-wave x-rays have greater energy and can penetrate a

greater distance

Longer-wave x-rays (soft) have less energy and little penetrating

power

The energy x-rays carry can be attenuated by matter


X-rays are capable of producing ionisation (and therefore can

cause biological damage in living tissue)

X-rays are undetectable to the human senses

X-rays can affect film emulsion producing a visual image (the

radiograph) and can cause certain salts to fluoresce and emit light

– this is the principle behind the use of intensifying screens in

extra oral cassettes and digital sensors

INTERACTION OF XRAYS AND MATTER

Complete scatter-no loss of energy

Absorbed –total loss of energy

Scattered-some absorption and loss of energy

Transmitted unchanged

X Ray Photon

Scattered with no energy loss

X RAY PHOTON INTERACTION WITH TISSUES

Transmitted unchanged

Scattered withsome energy loss

Complete absorption

83

DEFINITION OF TERMS USED IN X RAY

INTERACTIONS


Scattering-change in direction of a photon with or without loss of

energy

Absorption-removal of energy from the beam

Attenuation -reduction in intensity of the beam caused by

absorption and scattering

Ionisation -removal of an electron from a neutral atom producing

a negative ion(the electron) and a positive ion(remaining atom)

Interactions important in X ray production

PHOTOELECTRIC EFFECT

COMPTON EFFECT

PHOTOELECTRIC EFFECT

N

89

INCOMING X RAY PHOTON interacts with inner shell electron N

90

INNER SHELL ELECTRON IS EJECTED

X RAY PHOTON ISTOTALLY ABSORBED


N

91

OUTER SHELL ELECTRONSCASCADE TO FILL SPACE

ENERGY IS GIVEN OFF AS LIGHT

Outer shell electron replaces inner shell------then ongoing

replacement of shell electrons

Electrons replacing inner shells release excess energy----LIGHT

AND HEAT

A free electron is captured and returns to NEUTRAL STATE

The photoelectron that was ejected behaves like the original x ray

photon

Many interactions—many ejected electrons as it goes through

tissues

The interaction is usually low energy photons almost equal to the

electron it displaces

These low energy electrons are responsible for the majority of ionising

interactions in tissues----possibly resulting in damage that can be caused

by X rays

LOW Kv EQUIPMENT results in HIGH ABSORPTION and HIGH

DOSE TO TISSUES

HIGH DOSE TO TISSUES =Good contrast Films

LOW Kv = HIGH CONTRAST


COMPTON EFFECT

Absorption and scattering process predominating with high

energy photons.

Incoming photon interacts with free or loosely bound outer shell

electron

Outer shell electron is ejected COMPTON RECOIL ELECTRON

With some of the energy of the photon (some absorption)

The ejected electron has more interactions

The remainder of the incoming photon energy is deflected (or

scattered) from its original path as a scattered photo

N

100

INCOMING X RAYPHOTON

X RAY PHOTON INTERACTS WITHOUTER SHELL ELECTRON

N

101

COMPTON RECOIL ELECTRON

OUTER SHELLELECTRON ISEJECTED AND THE INCOMING PHOTONSCATTERED

SCATTEREDPHOTON

The scattered photon may then

Undergo further Compton interactions in the tissues

Undergo photoelectric interaction with the tissues.

Escape from the tissues as scattered radiation.

The atomic stability is achieved by capturing another free electron.


Important points to note

Energy of the incoming photon is greater than the binding energy

of the outer shell electron

Incoming photon cannot distinguish between one free electron and

another– interaction is not dependent on an atomic number(Z)

therefore this interaction provides very little diagnostic

information as there is little discrimination between different

tissues on the final radiograph

This interaction predominates with high x-ray photon energies,

this explains by high-voltage x ray sets result in poor radiographic

contrast

The energy of the scattered photon(Es) is always less than the

energy of the incoming photon(E) depending on the energy given

to the recoil electron(e) Es =E-e

Scattered photons can be deflected in any direction but the angle

of scatter depends on their energy.

High energy scattered photons produce forward scatter

Low energy scattered photons produce back scatter

Forward scatter may reach the film and degrade the image but can

be removed by an anti-scatter grid

Overall result of the interaction is ionisation of the tissuesThis

interaction predominates with high x ray photon energies,which

explains why high voltage x ray sets result in radiographs with poor

contrast

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