Modern x-ray tube

HISTORY OF
DEVELOPMENT OF
MODERN X-RAY TUBE
Presented by:
Sachidanand Giri
JR-2

CONTENT:
A.Introduction
B.History of development of x-ray tube
C.Types of x-ray tube
D.Components of modern x-ray tube:
1.cathode
2.anode
3.glass envelop
4.oil insulation
5.tube shield

INTRODUCTION:
 X-RAYS were discovered by Wilhelm Conrad Röntgen in
November, 1895, whilst he was experimenting with the passage
of electricity through a gas at very low pressure.
 The vital piece of his apparatus was a long glass vessel from
which as much air as possible had been removed and into each
end of which a short platinum electrode was sealed.

 When an electric discharge at high voltage was passed through the
almost evacuated tube, Röntgen noticed a glow on a piece of glass,
covered with zinc sulphide, which was lying a short distance from
the tube.
 The glow persisted even when the discharge tube was shrouded in
black paper, and Röntgen was quickly able to establish that the cause
was a undiscovered radiation.

 To it he gave the name X-rays, X being the established symbol
for the unknown quality.

 Although a modern X-ray tube bears no very obvious
resemblance to the discharge tube of Rontgen's apparatus, the
basic mechanism of X-ray production remains the same.
 X-rays are produced whenever high-speed electrons are
suddenly brought to rest, some of their kinetic energy, at least,
being converted into the electromagnetic radiation.

 In the original apparatus the source of the electrons was the
residual gas in the tube.
 Accelerated by the applied voltage they were brought to rest by
the glass end of the tube, whence the X-rays were emitted.
 Nowadays the electrons come, by thermionic emission, from an
electrically heated filament of tungsten.

History of development of x-ray
tube
 Sir William Morgan (1785), while investigating the discharge of
high tension current in perfect vacuum, obtained a vacuum so
high that there was no discharge.
 In one of his experiments, the glass cracked and Morgan
observed a display of colors, beginning with yellow-green and
followed by red, violet and blue.
 Unknown to him he was the first man to produce X-rays.

 In 1821, Michael Faraday conducted his first experiment on
electric discharges in partially evacuated glass vessels using a
vacuum pump built in 1650 by Otto von Geuricke.
 He described that the ‘voltaic arc’ was accompanied by
fluorescence of gas remaining within the vessel. He called the
fluorescence as ‘radiant matter’ and considered it as the fourth
state of matter.

Vacuum tube and pump used by
Michael Faraday

 Julius Plucker (1859) was the first to observe Green Glass
Fluorescence in partially evacuated discharge tubes.

 Wilhelm Hittorf (1870) improved vacuum pumps.
 He observed that the fluorescent discharge increased in size as
the tube was evacuated and identified the source of the
phenomenon as cathode and termed it as ‘cathode rays.’

 He found that these rays travelled in straight lines, produced
heat.
 Caused fluorescence on glass where they impinged, cast shadow
of the object placed in their way and were deflected by a
magnet.
 His work was subsequently verified by Eugen Goldstein (1879).

 In 1880s, Sir William Crookes described additional changes that
took place in the fluorescence.
 He considered ‘radiant matter’ to be the ‘ultra gaseous state.’
 He found that the freshly opened photographic plates were
strangely fogged and blackened.

 He referred to a ‘molecular’ and ‘emissive’ ray from his tube
which could only be seen when a fluorescent screen was placed
in the ray’s path beyond the tube.
 He had unconsciously and unknowingly generated X-rays.
 He subsequently redesigned the tube.

 Philip Lenard showed that cathode rays would pass through a special
aluminum window built into the wall of his discharge tube and
retained enough energy outside the tube also, to cause fluorescent
screen to glow.
 These rays caused air to glow in front of the window.
 This glow extended in all directions for about five centimeters in air
and became known as ‘Lenard’s Ray.’

 Lenard proposed the ‘Inverse Square Law.’
 In 1895, Jead Perrin stated that cathode rays were negatively
charged particles.
 In 1896, John Joseph Thomson discovered the ‘electron’.

Diagram of the cathode ray tube used by
JJ Thomson when he discovered the electron

 Roentgen, while experimenting and searching for the invisible
light rays turned on a low pressure Crooke’s tube ,completely
enclosed in heavy black paper and applied power to the
electrodes with a Ruhmkorff induction coil.
 Immediately he was surprised to see, a fluorescent screen,
covered with barium platinocyanide standing on a table, at some
distance away, started to glow brightly.

 When he interposed objects between the tube and the screen,
shadows were cast on the screen.
 These rays could not be reflected or refracted.
 They were unaffected by a magnetic or electric fields.
 He termed these rays “X-rays”, Since ‘X’ was considered the
American way to term the unknown, he ultimately, called them
‘Roentgen Rays.’

Hittorff-Crooke’s tubes, of the kind used by
Roentgen to discover X-rays

Ruhmkorff induction coil used to power early tubes

Types of x-ray tube:
1. Gas discharge tube/Crookes tube
2. Regulator Tubes
3. Vacuum Tube
4.Coolidge tube
5. Shockproof Dental X-ray Unit

Gas discharge Tubes/Crookes
tube
 Invented by Brittish physicist William Crookes, in early 1870s.
 Crookes tubes also called cold cathode tubes, meaning that they
do not have a heated filament in them that releases electrons .
 Early gas tubes depended upon the incomplete vacuum to
provide the source of electrons at the cathode.

Components of Crookes tube
 1. Tube envelop:
 Partially evacuated glass bulb of cerium.
 Low Air pressure 10−6 to 5×10−8 atmosphere.
 2.Anode: made of platinum with atomic number 74 and mp-1768*c
 3.Cathode:made of aluminium with atomic number 13 and mp-660*c
 4.Anticathode: made of copper plate
 DC current of high voltage (100kvp)

 When high voltage is applied to the tube, the electric
field accelerates the small number of electrically
charged ions and free electrons.
 The electrons collide with other gas molecules, knocking
electrons off them and creating more positive ions in a chain
reaction called a Townsend discharge.

 As the tube was used, the gas molecules combined with or were
trapped by vaporized residues from the anode and cathode
which gradually increases the vacuum.
 When the vacuum became too high, no X-rays were produced
and the tube was considered to be ‘cranky.’

 This ‘cranky’ tube could be heated by an alcohol lamp to drive
gas molecules from its walls which maintained continued
production of X-rays.

Regulator Tubes:
 To increase the longevity of X-ray tube, automatically self-
regulating and regenerative tubes were developed in 1896 by
Queen and Company.
 It utilized the principle that certain chemicals (caustic potash
and potassium permanganate) liberated gases upon heating and
absorbed them upon cooling.

 When the vacuum in the tube became high, resistance increased
and the current supplying the tube was diverted to the low
vacuum accessory bulb by means of adjustable wire.
 This resulted in heating of the caustic potash, which produced
gas and caused the vacuum in the main tube to be lowered
sufficiently to produce X-rays again.

Gas Regulated Tube (1902) in comparison
with the modern X-ray tube

Vacuum Tube
 In an effort to eliminate the gas and stabilize the operation of X-
ray tubes, J.E. Lilienfeld, an Austrian developed a tube in 1911
based on Field Current principles.
 The electrons were extracted from the cathode by using a high
potential across the tube.

 The operation of such a cold cathode tube was described as
“ticklish.”
 Due to the use of a curved cathode, charges became so crowded
on the curved part that they easily leaked (Lilienfeld Effect).
 So, to increase the drain of electrons from the cathode the
electrons were ejected from a pointed cathode.

Coolidge tube
 The real breakthrough in the tube design was the development
of the hot cathode tube by William David Coolidge, (1913).
 Coolidge used a coil of tungsten as the source of electrons (as a
filament cathode)in the new 2 tubes which out-performed
Lilienfeld’s cathode tube.

 It permitted:
• Greater flexibility in the quality and quantity of X-rays produced.
• Greater tube stability during the production of X-rays.
• Smaller tube size.
• Longer tube life.
• Direct operation from a transformer.

 An early problem with this new hot cathode tube was the
conduction of heat away from the tungsten target.
 A tungsten anode backed by copper was found to be the most
satisfactory method of dissipating heat rapidly; the heat was
conducted to the radiation fins at the end of the tube or by
circulating cold water through the anode stem.

 Until 1918, all X-ray tube cooling was provided by means of air
and water.
 Hirsh patented the idea of submerging the X-ray bulb in oil to
effect greater cooling of the anode and tube.
 In 1919, Harry Waite submerged the tube and transformer as a
single unit in the same oil bath.

Shockproof Dental X-ray Unit
 In 1918-1919, Coolidge and General Electric Co. introduced the
Victor CDX Shockproof Dental X-ray Unit , which eliminated
the exposed high tension wires.
 The principle of this design was to place the tube and high
voltage components in an oil filled grounded compartment
which acted as an electric insulator, coolant and radiation
shield.

Victor CDX – shockproof tube housing
(1919)

 The advantage of this tube was that the electrical and fire hazard
was eliminated.
 The anode and the tube length was reduced thereby permitting
more rapid removal of heat.

Components of Modern x-ray
tube
1.Cathode
2.Anode
3.Glass envelop
4.Oil insulation
5.Tube shield

Cathode:
 Has two main components: a)Filament b)Focusing cup
FILAMENT:
 The filament is the source of electrons within the x-ray tube.
 It is a coil of tungsten wire about 2mm in diameter and 1cm or less in length.
 Filament typically contain about 1% of thorium , which greatly increases the
release of electrons from the heated wire.
 It is mounted on two stiff wires that support it and carry the electric current.

 These two mounting wires lead through the glass envelope and
connect to both the high- and low-voltage electrical sources.
 The filament is heated by the flow of current from the low-
voltage source and emits electrons at a rate proportional to the
temperature of the filament.
 The filament has its own circuit powered by a relatively low
voltage (10-15 volts) and 4 –6 amperes.

 Most diagnostic X-ray tubes have two focal spots of different sizes
and these are obtained by having two filaments, each in its own
focusing cup, side by side in the filament assembly.
 By appropriate external switching either one or the other of the
filaments (not both) is selected for use.
 The small filament is designed to be used with relatively small parts
while the large filament is used when larger body parts are being x-
rayed.

Focusing Cup
 The filament lies in a focusing cup, a negatively charged
concave reflector made of molybdenum.
 The focusing cup electrostatically focuses the electrons emitted
by the incandescent filament into a narrow beam directed at a
small rectangular area on the anode called the focal spot.

 The electrons move in this direction because they are repelled by
the negatively charged cathode and attracted to the positively
charged anode.
 The x-ray tube is evacuated to prevent collision of the moving
electrons with gas molecules, which would significantly reduce
their speed.
 This also prevents oxidation and burnout of the filament.

 It is designed and shaped so that when the x-ray machine is powered
up, electrons will literally “boil” off the filament.
 It glows white hot and the electrons hover around the filament in a
“space charge” until the moment of exposure and then they
accelerate very rapidly towards the anode which is not very far
away.
 This process is known as “thermionic emission”/Edition effect.

Anode
 The anode consists of a tungsten target embedded in a copper stem.
 The purpose of the tungsten of the target in an x-ray tube is to
convert the kinetic energy of the electrons generated from the
filament into x-ray photons.
 This is an inefficient process with more than 99% of the electron
kinetic energy converted to heat.

 The target is made of tungsten, a material that has several
characteristics of an ideal target material.
 It has a high atomic number (74), high melting point(3380*c), high
thermal conductivity, and low vapor pressure at the working
temperatures of an x-ray tube.
 The tungsten target is typically embedded in a large block of copper
to dissipate heat.

 Copper, a good thermal conductor, dissipates heat from the
tungsten, thus reducing the risk of the target melting.
 In addition, insulating oil between the glass envelope and the
housing of the tube head carries heat away from the copper
stem.
 This type of anode is a stationary anode.

 The focal spot is the area on the target to which the focusing
cup directs the electrons from the filament.
 The sharpness of the radiographic image increases as the size of
the focal spot decreases.
 The heat generated per unit target area, however, becomes
greater as the focal spot decreases in size.

 To take advantage of a small focal spot while distributing
the electrons over a larger area of the target, the target is
placed at an angle to the central electron beam.
 The projection of the focal spot perpendicular to the central
electron beam (the effective focal spot) is smaller than the
actual size of the focal spot.
 This is known as the “Line Focus Principle’.

 Typically, the target is inclined about 20 degrees to the central ray
of the x-ray beam.
 This causes the effective focal spot to be almost 1x 1mm, as
opposed to the actual focal spot, which is about 1 x 3mm.
 The effect is a small apparent source of x rays and an increase in
sharpness of the image with a larger actual focal spot for heat
dissipation.

The angle of the target to the central ray of the x-ray beam has a
strong influence on the apparent size of the focal spot. The
projected effective focal spot is much smaller than the actual focal
spot size.

 Another method of dissipating the heat from a small focal spot is
to use a rotating anode.
 In this case the tungsten target is in the form of a beveled disk that
rotates when the tube is in operation.
 As a result, the electrons strike successive areas of the target,
widening the focal spot by an amount corresponding to the
circumference of the beveled disk and distributing the heat over
this expanded area.

 As a consequence, small focal spots can be used with tube currents of 100
to 500 miliamperes (mA), 10 to 50 times that possible with stationary
targets.
 The target and rotor (armature) of the motor lie within the x-ray tube, and
the stator coils (which drive the rotor at about 3000 revolutions per minute)
lie outside the tube.
 Such rotating anodes are not used in intraoral dental x-ray machines but
may be used in cephalometric units and in medical x-ray machines requiring
higher radiation output.

X-ray tube with a rotating anode, which allows heat
at the focal spot to spread out over a large surface
area

Glass Envelop
 The whole cathode and anode assembly are contained in a
evacuated glass envelop.
 It is a leaded glass vacuum tube that prevents X-rays from
escaping in all directions (radiation leakage).
 One central area of the leaded glass tube has a “window” that
permits the X-ray beam to exit the tube and directs the X-ray
beam towards the aluminum disk, lead collimator and PID.

Oil Insulation
 Expedite cooling and to insulate the tube, it is immersed in oil
to ensure that it is electrically insulated and so that the oil will
also help cool down the tube during operation.
 The cooling of the oil is sometimes assisted with a cooling fan.
 The oil serves two major purposes: One, it helps cool the x-ray
tube just like oil in your car helps cool the engine.

 Additionally, the oil helps insulate electrically for safety.
 And one additional thing it does is if you notice where the
primary beam exits the housing, the x-rays must pass through a
small thickness of oil before they emerge from the tube and this
contributes to minimal filtration or absorption of the x-ray beam
when you make an exposure.

 It is important that the oil should not become too hot since
insulators progressively lose their insulating properties as their
temperature rises.
 Metal bellows which extends as the heated oil expands.
 If the bellows expand beyond a certain amount (i.e., the oil has
exceeded a certain temperature) then they operate a micro-switch
which prevents operation of the tube until the oil has cooled
sufficiently.

Tube Shield
 The tube housing function to containing and supporting the X-ray
tube and oil, and protecting them from external damage.
 The metal tube shield has two other very important functions to
perform.
 Firstly, it provides a completely encircling metallic shield which,
because it is firmly connected electrically to earth potential, protects
the user from any possibility of electrical shock.

 The second purpose of the tube shield is to afford protection to the
radiographer and patient against unwanted X-radiation.
 It is arranged that any X-rays (both primary and secondary) which
are not within the wanted beam are attenuated by the shield.
 This is usually achieved by lining the steel shield with appropriate
thicknesses of lead, the actual thickness depending upon the likely
intensity of radiation reaching that portion of the shield.

 There is, an aperture opposite the target through which the
maximum size of useful beam can emerge.
 This beam is reduced to the size required, i.e., that needed to
just cover the film size in use, by a set of collimating
diaphragms or by a cone.

References:
1.Textbook of Dental and Maxillofacial Radiology
Freny R Karjodkar, Second Edition
2.Fundamental physics of radiology
Meredith/Massey , Third Edition
3.ORAL RADIOLOGY
Principles and Interpretation
White and Pharoah , Sixth Edition

Modern x-ray tube

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