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 byJJ 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 byRoentgen 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.

Crookes tube

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 comparisonwith 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-switchwhich 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