The document provides an overview of x-ray physics, including a history of x-rays, the electromagnetic spectrum, properties of x-rays, components of an x-ray tube, and interactions between x-rays and matter. It describes how x-rays are produced via thermionic emission in an x-ray tube, where high-speed electrons generated at the cathode strike the tungsten anode, producing bremsstrahlung and characteristic radiation. It also summarizes the photoelectric effect and Compton scattering that can occur when x-rays interact with matter.

1. Date 6/1/2021
X-ray Physics
Dr.Akshay M Joshi
DMRE

2. Introduction,History &
Basic Physics
Discovered in 1895 by German physicist Sir
wilhelm Conrad Roentgen.
Got First nobel prize in Physics in 1901

4. Nature of X-rays
✤ X-rays are classified as Electromagnetic radiation- wavelike like fluctuations
of electric & magnetic fields set up in space by oscillating electrons.
✤ Electromagnetic radiation consists of energy in small pockets called photons.
✤ Speed of X-rays = constant = Speed of light = 3 x 108 m/s
✤ The wavelength of X-rays are extremely short;in ordinary radiography,their
useful range extends from about 0.1Ao-0.5Ao
✤ The ultrashort wavelengths are associated with enormous frequencies from 3 x
1019 to 6 x 1018 Hertz.

5. Electromagnetic Spectrum
C=f x wavelength ; as wavelength decreases frequency increases and also the energy!

6. Properties of X-rays
✤ X-rays travel in straight lines.
✤ X-rays are electrically neutral.
✤ X-rays are polyenergetic and heterogeneous.(useful energy range in
conventional radiography is about 20-120kVp).
✤ X-rays travel at the speed of light-electromagnetic radiation.
✤ X-rays are highly penetrating,invisible rays.
✤ X-rays cannot be deflected by electric or magnetic field.

7. Properties of X-rays
✤ X-rays cannot be focused by lens.
✤ Photographic film is blackened by X-rays.
✤ Fluorescent materials glow when X-rays are directed at them.[making possible
their use in fluoroscopy].
✤ Produce chemical and biologic changes by ionisation and excitation.
✤ Liberate minute amounts of energies while passing through matter.
✤ X-rays interact with matter produce photoelectric and compton effect.

8. The X-ray tube
✤ Hot cathode - diode tube based on edison effect.

11. X-ray tube components
✤ Hot Filament : supplied by low voltage heating current ,serves as cathode(negative
electrode) releases electrons when heated.
✤ Focusing cup : focuses electrons on target.
✤ Target : tungsten target serves as anode or positive electrode.
✤ Copper stem : helps remove heat from target in stationary aode tubes.(copper is a better
conductor of heat).
✤ Glass envelope/tube : from which the air has been evacuated as completely as possible.
✤ Degassing : process of removing air from glass & metal parts by prolonged baking before
tube is sealed.

12. Why Vaccum???
Two reasons :
✤ To prevent collision of the high speed electrons with gas molecules,which
would cause significant slowing of electrons.
✤ To prevent Oxidation & burning out of filament.

13. X-ray Production

14. Two ways by which X-rays can be generated
✤ Whenever fast moving electrons undergo rapid deceleration.
✤ Electrons drop from an outer shell to a hole in an inner atomic shell.

15. What does X-ray tube do?
✤ Obtains free electrons———>Speeds them up ——->Finally stops them to
generate X-rays.

16. Steps in X-ray Production
✤ Obtaining free electrons by heating filament with low voltage circuit.
✤ Applying high potential difference across electrodes causing electrons to
race towards Anode.
✤ Focusing electrons on the focal spot of anode by focusing cup.
✤ Stopping the high speed electrons in target.

17. Separation of Electrons:
✤ Filament current supplying the filament causes it to become glowing hot or
Incandescent,with resulting separation of some of its outer orbital
electrons.
✤ Separated electrons escaping from the filament,form a cloud or space
charge nearby.
The electrons liberated in this manner are called THERMIONS & the
process of their liberation through the heating of a conductor by an
electric current is called THERMIONIC EMISSION.

18. Production of High speed electrons
✤ A high Potential difference is applied across the tube which gives the filament a
very high negative charge (cathode) & the target an equally high positive
charge(anode).
✤ The resulting strong electric field causes the space charge electrons to rush at
an extremely high speed through the tube from cathode to anode.
✤ This electron stream is expressed in milliAmperes,while total charge
transferred in the process is in mAs.
The speed of the electrons at 80kVp is about 80 percent of the speed of
light !

19. Focusing of the electrons
✤ The metal focusing cup around the cathode helps focus the electrons on the
focal spot of the anode by forming the narrow electron beam,narrower the
electron beam smaller the focal spot and sharper the xray images!

20. Stopping of the High Speed Electrons in Target
✤ When the fast electrons enter target of the x-ray tube,their kinetic energy
changes to Heat,light & X-rays.
✤ X-ray tube efficiency is such that at 80kVp only about 0.6 percent of this
energy is converted to x rays while the remaining 99.4 percent appears as
heat.

21. Electron Interactions with Target Atoms
1)Brems Radiation
✤ Upon approaching the strongly positive nuclear field of a target
atom,the negatively charged high speed electron is deviated
from its initial path because of the attraction between opposite
charges resulting in slow down or deceleration of
electrons,thereby losing some of its kinetic energy.
✤ The lost kinetic energy is radiated as an X-ray of equivalent
energy.a/k/a Brems radiation or braking radiation or
bremsstrahlung radiation.
✤ The deceleration of electrons depends also on atomic number
of target,thus targets of higher atomic number are more efficient
producers of Brems radiation.

22. 2)Characteristic Radiation

23. ✤ An electron with a sufficient minimum kinetic energy may interact with an
inner orbital electron of a target atom,ejecting it from its orbit creating a
hole .this makes atom unstable being ionized & in excited state.
✤ This space or hole is filled by electron transition from one of the outer shells
emitting the energy while transition which is emitted as characteristic x ray
because its energy is unique to target element & the involved shell.

24. Summary of Radiation
✤ Bremsstrahlung radiation :
✤ White,Polyenergetic radiation with a continuous range of energies & wavelengths due to
deceleration of electrons by strongly positive electric fields of nuclei in target atoms.
✤ Constitutes about 90 percent of emitted x rays when 80 - 100 kV is applied to the tube.
✤ Characteristic radiation :
✤ Consists of limited,discrete energies & wavelengths.
✤ Constitutes about 10 percent of emitted x rays when 80 - 100 kV is applied to the tube.
No characteristic radiation is produced at tube potentials less than 60 kV with a tungsten
target.

25. Efficiency of X-ray Production
✤ %efficiency = k x Z x kVp
Where, k = constant = 10-4
Z = atomic number of the target
kVp = peak kilovoltage.
Eg, At 80kVp with tungsten target (z=74)
Efficiency = 0.6% meaning of the total kinetic energy of the electron stream,only
about 0.6% appears as x rays and the remaining 99.4% as the heat in the anode when
applied potential is 80kVp.

26. Target Material - Tungsten
✤ Two things to remember while selecting target material :
✤ It must have a very high melting point to withstand the extremely high
temperature.
✤ It must have a high atomic number because
✤ The resulting characteristic radiation will be of high energy and more penetrating.
✤ There is increased production of Brems radiation.
Tungsten - Melting point 33700c ; Atomic Number = 74 satisfies both requirements.

27. ✤ Target Material in rotating anode radiographic tubes : Rhenium &
Tungsten coated on a Molybdenum-tungsten disc
✤ Mammographic tubes use Molybdenum (Z=42) targets to produce low
energy X-rays.

28. The Interactions Of Ionizing Radiation & Matter

29. ✤ On passing through a body of Matter,an X-ray beam undergoes
attenuation,that is progressive decrease in initial number of photons.
✤ Attenuation is due to two processes
✤ Absorption of some of the photons.
✤ Emission of scattered & secondary radiation.
✤ Loss of beam intensity due to inverse square law.
Scattered radiation - x ray photons that have undergone a change in
direction after interacting with atoms.
Secondary radiation - Characteristic radiation emitted by atoms after having
absorbed x ray photons.

30. ✤ Four kinds of interactions may occur between the photons and atoms in
their path,depending on the atomic number of the atoms and the energy of
the photons :
1. Photoelectric Interaction.
2. Coherent or Classical scattering.
3. Compton interaction with modified scattering.
4. Pair production.

31. Binding energy:
✤ The energy required to remove an electron from a
particular shell and beyond the range of the nuclear
positive electrostatic field is called the binding energy of
that shell.
✤ The shells are at a progressively higher energy levels the
farther they are located from the nucleus.
✤ Inner shell electrons are called as bound electrons
owing to high energy needed to remove electrons
✤ Almost no energy is required to liberate an electron from
outer shells,so they are called free or valence
electrons.
More the atomic number more the number of protons
more the binding energy.

32. Photoelectric Interaction (Photoelectric Effect)
✤ This type of interaction is most likely to occur when the energy hv of the
incident x-ray photon is slightly greater than the binding energy of the
electrons in one of the inner shells such as K or L.
✤ The incident photon gives up all of its energy to the atom (photon is truly
absorbed and disappears during the interaction).

34. ✤ The incident photon gives all of its energy to the atom——>Immediately the
atom responds by ejecting an electron,usually from K or L shell leaving the
hole in that shell——>Now the atom is Ionised positively & in an excited
state.
✤ So the energy of incident photon ultimately went to-
1. Free the electron fron its shell &
2. Set it in motion as a photoelectron,which ionizes atoms nearby &
releases secondary electrons.

35. ✤ In summary, the energy of the incoming photon in the photoelectric interaction involving K shell
has the following fate :
1. Photon enters the atom and completely disappears.
2. A K shell electron is ejected,leaving a hole.
3. Atom has excess energy-excited state.
4. A part of photons energy was used to liberate electron and rest to it kinetic energy; ejected
electron is a photoelectron,which releases secondaro electrons by ionising other atoms.
5. Hole in the k shell is filled by electron transition from a shell farther out,accompanied by
emission of a characteristic x ray photon.
6. Holes in successive shells are filled by electron transitions from shells still farther
accompanied by a corresponding characteristic photon;this sequence is a cascade
7. Sum of energies of all the characteristic photons equals binding energy of shell from
whichthe photoelectron originated,in this case the K shell.

36. Coherent or Unmodified Scattering

37. ✤ If a very low energy x-ray photon interacts with a firmly bound orbital
electrons,it may setthe electron into vibration producing an electromagnetic
wave identical in energy to that of incident photon,but differing in direction.
✤ Scattering without undergoing any change in wavelength,frequency or
energy.

38. Compton Interaction with Modified scattering

39. ✤ If an incident photon of sufficient energy encounters a loosely bound,outer
shell electron it may dislodge the electron & proceed in a diffraction
direction.
✤ The dislodged electron is called a compton or recoil electron.

40. ✤ As energy of incident photon increases probability of occurence of compton
interaction decreases but, at the same time the scattered photons have
more energy & tend to be scattered more and more in forward
direction,increasing the likelihood of their passing completely through the
body amd reaching the film constituting scattered radiation which in
radiography impairs image contrastby an overall fogging effect.

41. Pair Production

42. Anode
✤ Two main types of Anodes
✤ Stationary Anode - used in dentistry. Tungsten in a block of copper.
✤ Rotating Anode- Molybdenum disc coated with a tungsten - rhenium alloy to
serve as the focal track target.
✤ Rhenium diminishes roughening of the focal track with use,thereby
ensuring high x-ray output or emmissivity.
✤ For heavy duty tubes, a layer of graphite under the moly disc reduces the
weight of the large anodeand hastens the dissipation of heat.

43. “THE DARKROOMa quote
where the radiology begins & ends

44. ✤ The walls of darkroom should contain lead atleast 1.6mm thick.
✤ Radioactive materials should be located as remotely as possible
✤ Windows should be avoided.
✤ Should be readily accessible to plumbing and electrical services.
LOCATION

46. Building Essentials
✤ Passboxes are built into walls at appropriate location,they have two light tight and x-ray
proof doors that are so interlocked that both can’t be opened at same time.
✤ The cassettes after radiographic exposure are placed in passbox through the outside
door by the x-ray technician then removed through the inside door by the dark room
technician.
✤ Dark room walls should be covered with chemical resistant material made of special
paint,varnish or lacquer.
✤ Floors are covered with chemical resistant and stain proof material such as asphalt tile.
✤ Floors should be non skiddy made of porcelain or clay tile.

47. Entrances
✤ Single door - it must be light tight by weatherstripping,it is the simplest type.
✤ It should have an inside lock to prevent opening while films are being
processed.
✤ Small hall with two electrically interlocked doors
✤ One door cannot be opened until other is completely closed thereby
preventing light.
✤ There is separate door for emergency

49. ✤ Maze or labyrinth entrance
✤ One has to do a complete turn going through three doors it serves as a light trap.
✤ It requires no doors.
✤ Walls may be painted black
✤ Rarely used now-a-days.
✤ Two way revolving darkroom door
✤ Outer cylindrical chamber built into the wall,has two openings one into the dark room and
other into the light room
✤ The inner chamber has one door which is suspended at its top by a central bearing,can
easily be rotated by means of a hand rail until its opening coincides with either of the
openings in outer chamber.
✤ Entire unit is completely light proof and takes up very little space.

52. ✤ Ventilation- AC / Exhaust may provide adequate ventilation.
✤ Lighting - three types if illumination
✤ 1) safelight 2) general illumination 3) radioghic illumination.

53. ✤ Safelight -
✤ Source of light which will not fog films and provide adequate illumination
✤ Safelight lamps with adequate filters are used ,working distance not less
than 1 meter.
✤ General illumination :
✤ Source of overhead illumination for general purpose like
cleaning,changing solution etc.
✤ Radiographic Illumination :
✤ Fluorescent illuminator for viewing wet radiographs mounted over
washing compartment.

54. Chemistry of Radiography and film processing

55. ✤ An ordinary radiographic film emulsion contains silver halides in the form of
minute crystals that are invisible to naked eye consisting of mainly silver
bromide and small amount of silver iodide to enhance sensitivity.

57. Gurney-mott hypothesis of latent image
formation
✤ A photon entering a sensitized silver halide crystal may interact with a bromine
ion,liberating a loosely bound valence electron and leaving a neutral bromine atom.
✤ As the free electron drifts through the lattice,it may be trapped by a sensitivity speck
to which it imparts a negative charge.
✤ A migrant positive silver ion,attracted to the now negatively charged sensitivity
speck picks up the electron and becomes a neutral silver atom adhering to the
speck.this process is called as nucleation.
✤ The silver atoms deposited on sensitivity speck are too few in number to be
visible,so they constitute the latent image.

59. ✤ When the latent image is acted upon by reducing agents known as
devolopers,the process initiated by photon action is greatly speeded up;the
sensitivity speck serves as a devolopment centre for the entire crystal.
✤ The speck rapidly traps electrons from the reducing agents and attracts
more silver ions which become reduced to silver atoms and grow into
thread-like or in some film emulsions,tabular clusters of metallic silver.thus,
the dark areas consist of metallic silver in a very fine state of subdivision.

60. ✤ During the devolopment bromine ions diffuse out of the devoloped crystals
and into the solution.this in addition to the gradual exhaustion of the
reducing agents,eventually cause deterioration of the developer to the point
where it must be discarded.
✤ The unexopsed and underdeveloped silver halides are eliminated from the
emulsion by immersion in a fixing agent,ammonium thiosulfate,a process
knows as fixation. As a result,the areas from which the silver halides have
been removed become clear,while the black areas remain black since
metallic silver is not dissolved by fixing agent in the ordinary course of
processing.
✤ Prolonged immersion in fixing solution causes bleaching of the image.

61. ✤ The amount of blackening of particular area of a radiograph depends on the
amount of radiation it has received.this is the radiation that has passed
through the various thicknesses,types and densities of the tissue interposed
between the tube and the film and is called remnant or exit radiation.it
forms the aerial image.

62. Manual Precessing
✤ Steps in Manual processing
✤ Devolopment
✤ Rinsing
✤ Fixation
✤ Fixer neutralisation
✤ Washing
✤ Drying

63. ✤ Devolopment :
✤ Development in a solution containing an organic reducing agent, such as
hydroquinone and metol, to convert ionic silver to metallic silver.
✤ An alkali, sodium hydroxide, serves as an accelerator; and a restrainer
prevents chemical fogging, finally, an antioxidant, sodium sulfite, slows
oxidation of the devoloper by air.

64. ✤ Rinsing :
✤ Rinsing in a solution of acetic acid(vinegar) to neutralise the alkali carried
over from the devoloper, hereby stopping devolopment.

65. ✤ Fixation :
✤ In a solution of hypo to remove the unexposed, underdeveloped silver
halides and so preserve the image. A hardner alum, toughens the
emulsion, actually tanning it.

66. ✤ Fixer Neutralisation :
✤ Done in a special solution to remove any residual fixer,to help prevent
deterioration and discoloration of image.

67. ✤ Washing :
✤ Done by immersion of the films in running water for a prescribed time to
remove effectively any residual chemicals.

68. ✤ Drying :
✤ In a special cabinet preferably with a source of forced heat.
The minimum time possible for the manual processing is about 40 mins.

70. THANK YOU