to know the basics of production of x-rays and interaction with matter

1. X-RAY
PRODUCTION AND
INTERACTIONH A R I B U D K E
3 R D Y E A R B S C . M I T
0 3

2. MAN BEHIND
IT ALL
Sir Wilhelm Conrad Roentgen
Wilhelm Roentgen, a German
professor of physics, was the first
person to discover electromagnetic
radiation in a wavelength range
commonly known as X-rays today.
To highlight the unknown nature of
his discovery, he called them X-rays
though they are still known as
Roentgen-rays as well. For his
remarkable achievement he was
honoured with the first Nobel Prize
in Physics in 1901.

3. A BRIEF LOOK BACK…
• Born on March 27, 1845 in the small town of Lennep (Rhine Province) in
Germany; Wilhelm Conrad Roentgen was the only child of a cloth merchant. He
was raised in the Netherlands because his family moved to Apeldoorn when he
was still three. For his early education he went to a boarding school in
Apeldoorn named, Institute of Martinus Herman van Doorn. He was not a
sparkling student rather he was keenly interested in nature during his young
years. In 1862, he joined Ambachts school; a technical school in Utrecht. There
he got involved in a contrivance against one of his teachers and was expelled
subsequently.
• In 1865, he studied mechanical engineering at the Federal Polytechnic Institute
in Zurich having failed to get admission in University of Utrecht lacking
required credentials. There he flourished greatly under the influence of the
teachers like Kundt and Clausius. He graduated from the University of Zurich
and received his Ph.D. in 1869. In the same year, he assisted Kundt and

4. ACCIDENTAL DISCOVERY
• Wilhelm Roentgen was already working on the effects of cathode rays during 1895,
before he actually discovered X-rays.
• His experiments involved the passing of electric current through gases at extremely
low pressure. On November 8, 1895 while he was experimenting, he observed that
certain rays were emitted during the passing of the current through discharge tube.
• His experiment that involved working in a totally dark room with a well covered
discharge tube resulted in the emission of rays which illuminated a barium
platinocyanide covered screen.
• The screen became fluorescent even though it was placed in the path of the rays, two
meters away from discharge tube.

5. NOT ALL WE KNOW

6. PROPERTIES OF X-RAYS
i. X-rays are electromagnetic radiations of shorter wavelength.
ii. They travel in straight line and with a velocity equal to light.
iii. They are not influenced by electric or magnetic fields.
iv. They penetrate through objects which are opaque to light.
v. Produce fluorescence in materials like cadmium tungstate, zinc cadmium sulphide,
etc.
vi. Affect the photographic film and form a latent image.
vii. Produce ionization and excitation in substances through which they pass.
viii. Produce chemical and biological changes after interaction.

8. HOW IT ALL BEGINS
• X-rays are produced when fast moving electrons are stopped by means of a target material.
• When the electron is suddenly stopped, its kinetic energy is converted into heat and x-rays.
• This conversion takes place within the target material, interaction of electron with the target
is the basis for x-ray production.

9. Current to the
filament
Thermionic
emission
• When a metal is heated, the atom absorbs
thermal energy and some of the electrons in
the metal acquire enough energy to allow
them to move a small distance from the
surface
Potential
difference applied
• A high potential difference is
applied between the cathode
and the anode to accelerate the
electrons
Interaction of
electron with
target
• Due to the potential
difference the electrons rush
towards the anode
Production of x-
rays

10. INTERACTION OF ELECTRONS WITH THE
TARGET (ANODE)
• Electron interacts with the target in four different ways
– Excitation of outer shell (heat)
– Ionization of outer shell (heat)
– Ionization of inner shell (k-characteristic spectrum)
– Interaction with the nuclear field (Bremsstrahlung/continuous spectrum)

11. NOT SO USEFUL INTERACTIONS
• Excitation of outer shell
– The incident electron interacts with an
electron in the outer shell of the
target.
– It transfers a small amount of energy
to the outer shell and displaces it to a
higher energy level
– The excited electron returns to the
original shell and the difference in
energy appears as heat in the target.
• Ionization of outer shell
– The incident electron interacts with an
outer shell electron of the target.
– It transfers sufficient energy and
removes an electron from the outer
shell.
– The displaced electron is called as
secondary electron which may further
produce ionization or excitation in
other atoms of the target and results
in heat production.

12. IONIZATION IN AN INNER SHELL
• This is the interaction between the
incident electron and the electron in
the inner shell.
• The incident electron transfers
sufficient energy and removes an
electron from the inner shell.
• The displaced electron further
produces ionization and excitation in
other atoms.
• The vacancy created is filled by an
electron moving inwards from the
outer shell.
• During this transition the difference
between the binding energies of the
two shells is given out as x-ray photon.
INTERACTION WITH NUCLEAR FIELD
• The incident electron passes close to
the nucleus of an atom in the target.
Since electron is a negative particle, it
is attracted towards the positive
nucleus.
• Therefore, the electron decelerates
and loses energy in the form of x-ray
photon.
• The energy of the x-ray photon
depends upon the degree to which the
electron is decelerated by the nuclear
attraction.
• The x-ray photon are known as
continuous x-ray or bremsstrahlung.
The bremsstrahlung is a German word
meaning ‘breaking radiation’.

13. X-RAY SPECTRA
• Two types of x-ray spectrum
X-ray spectrum
Continuous spectrum
Consists of x-ray photons
of all energies up to the
max.
Also called as white light
radiation
Characteristic spectrum
Consists of x-ray photons
of few discrete energies
Also called as line
spectrum and depends on
the atomic number of the
target

14. Examples
1. A K-shell electron is removed from a tungsten atom
and is replaced by an L-shell electron. What is the
energy of the characteristic x-ray that is emitted?
Solution: From the table below the ionization energy
of a K-shell electron of Tungsten is 69.5 keV and the
ionization energy of a L-shell electron of Tungsten is
12.1 keV therefore, the difference of these two
energies will be the energy given off.
69.5 keV – 12.1 keV = 57.4 keV
2. A K-shell electron is removed from a tungsten atom
and is replaced by an O-shell electron. What is the
energy of the characteristic x-ray that is emitted?
Solution: 69.4 keV
Electron transition from shell (KeV)
Shell L M N O P
K 57.4 66.7 68.9 69.4 69.5
L 9.3 11.5 12 12.1
M 2.2 2.7 2.8
N 0.52 0.6
O 0.08

15.  During this process, the electron while passing near the nucleus may suffer a sudden deceleration
by the action of coulomb forces of attraction. As a result the electron may loose energy, in the form
of Bremsstrahlung.
 The electron may have one or more such interactions and this may result in partial or complete loss
of energy.
 The amount of bremsstrahlung production is determined by the distance between the electron and
the nucleus.
 At very large distances, the coulombic forces are weak, only low energy x-rays are created. This has
higher possibility.
 When electrons are more close to the nucleus, more kinetic energy is lost, resulting in high energy
x-ray production. It has low probability of occurrence.
 Efficiency =
𝑜𝑢𝑡𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑒𝑚𝑖𝑡𝑡𝑒𝑑 𝑎𝑠 𝑥−𝑟𝑎𝑦𝑠
𝑖𝑛𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛
= 9× 10−10
× 𝑍 × 𝑉
Z – atomic number ; V – tube voltage
 X-ray production increases with increasing voltage and atomic number
 The efficiency of Tungsten target is found to be less than 1%. The rest of the input energy (99%)
appears as heat.

16.  Electron incident on the target may produce characteristic x-rays by ejecting an orbital
electron from the K shell, resulting in a vacancy in that shell leading to ionization of the atom.
 The outer orbital electron will fill the vacancy at the K shell
 Difference in binding energy of the two shells is radiated as x-ray photon, which is called the
characteristic radiation. It will only have discrete energies.
𝑬 = 𝒉𝒗
= EK – EL
= 69.5 keV – 12.5 keV
= 57.4 keV
 K characteristic x-rays are emitted only, if the incident electron have energies greater than
the binding energy of the K shell electron.
 As the energy of the incident electron increases above the threshold energy, the percentage
of characteristic x-ray also increases.

18. Three possible fates await each photon
1. It can penetrate the section of matter
without interacting.
2. It can interact with the matter and be
completely absorbed by depositing its
energy.
3. It can interact and be scattered or
deflected from its original direction and
deposit part of its energy.

19. ATTENUATION
 It refers to both absorption and scattering. When a photon passes through an
absorber of thickness x, both absorption and scattering takes place.
 Attenuation results in the transmission beam having lesser number of photons
LAC (linear attenuation coefficient) = it is defined as the reduction in the radiation
intensity per unit path length and is expressed in cm-1
HVL (half value layer) = it is the thickness of material that attenuates an x-ray beam by
50%
TVL (tenth value layer) = thickness of material that attenuates 90% of the x-ray beam.
 The LAC depends on the energy of the photons and the nature of the material. The
attenuation of photons is caused by four important processes
i. Coherent scattering
ii. Photoelectric effect
iii. Compton effect
iv. Pair production and Photo-disintegration

20. Coherent scattering
Thomson
A single electron is
involved in the
interaction
Rayleigh
Cooperative
interaction with all the
electrons of an atom
 These are the interactions in which radiation
undergoes a change in direction without a
change in wavelength.
 A low energy radiation encounters the
electrons of an atom and sets them into
vibration at the frequency of radiation.
 A vibrating electron being a charged particle,
emits radiation.
 Only type of interaction with radiation which
doesn’t result in ionization. Only effect is
change in direction of the incident radiation.

21. PHOTOELECTRIC EFFECT
• This is the interaction between the
incident photon and the electron in
the inner shell.
• The incident photon transfers
sufficient energy and removes an
electron from the inner shell.
• The vacancy created is filled by an
electron moving inwards from the
outer shell (L or M).
• During this transition the difference
between the binding energies of the
two shells is given out as x-ray photon.
• The atom after filling of the K shell
void becomes a positive ion

22. End products
1. Characteristic radiation
2. A negative electron (the photoelectron)
3. A positive ion (an atom deficient in one
electron)
Probability of occurrence
1. The incident photon must have sufficient
energy to overcome the binding energy of the
electron.
2. A photoelectric effect is likely to occur when
the photon energy and electron binding
energy are nearly same.
3. The tighter an electron is bound in its orbit
the more likely it is to be involved in a
photoelectric reaction.

23. APPLICATION IN DIAGNOSTIC
RADIOLOGY
It is used for mammography (imaging of breast tissue)
• Photoelectric effect is both good and bad
Good
It produces radiographic images of excellent quality because
It does not produce scatter radiation and
It enhances natural tissue contrast
Bad
Patients receive more radiation from photoelectric reaction

24. High energy incident
photon
Interaction with outermost
shell electron
Scattering of photon and
recoil electron
Formation of positive ion
Compton scattering
End products
1. Recoil electron
2. Scattered photon
3. Positive ion
Probability of occurrence
The probability of
occurrence of Compton
effect depends the total
number of electrons
available in the outermost
shell.

25. Pair production
• High energy photon
(≥1.02MeV)
• Interacts with the nuclear field
initially
• Photon disappears
• Appearance of an electron and
a positron (0.51MeV each)
Mid phase • The positron looses its energy
in the medium
• It combines with a free
electron and produces two
photons
later

26. Photodisintegration
• In photodisintegration, part of the
nucleus of an atom is ejected by a high
energy photon.
• The ejected photon may be a neutron, a
proton, an alpha particle or a cluster of
particles.
• The photon must have sufficient energy
to over come nuclear binding energies of
the order of 7-15 MeV.

27. Photo
disintegration
High energy
photon
7-15MeV
Nuclear interaction
Ejection of neutron,
proton, alpha or
cluster of particles

28. REFERENCE
• http://www.columbia.edu/itc/hs/dental/sophs/material/production_xrays.pdf
• https://explorable.com/wilhelm-conrad-roentgen
• Basic Radiological Physics by K Thayalan.
• Christensen’s Physics of Diagnostic Radiology
• https://www.radiologymasterclass.co.uk/tutorials/physics/x-ray_physics_production
• https://youtu.be/l32YoBUqJAg