Tube with x-rays

The tubes with x-rays are devices making it possible to produce X-rays, in general for three types of applications:

Radiography and Tomography (Medical imagery);
X-ray crystallography (Diffraction of x-rays, to also see the article Diffractometer );
analyzes chemical elementary by Spectrométrie of x-ray fluorescence.

There exist several types of tubes.

Principle of operation

Whatever the type of tube, the generation of x-rays is done according to the same principle.

High a electric tension (about 20 to 400 K V) is established between two electrode S. It occurs a then running of electron S of the Cathode towards the Anode (sometimes called “anticathode” or “target”).

The electrons are slowed down by the Atome S of the target, which causes a continuous Rayonnement of braking or Bremsstrahlung , of which part of the spectrum is in the field of x-rays.

These x-rays excite the atoms of the target, and those re-emit a X-radiation characteristic by the phenomenon of X-ray fluorescence.

The outgoing spectrum of the tube is thus the superposition of the radiation of braking and x-ray fluorescence of the target.

The tubes of x-rays have a Rendement extremely bad, the major part of the electric power (99%) is dissipated in the form of Chaleur. The tubes must thus be cooled, in general by a water circulation.

Tube of Crookes

Historically, the first tube with x-rays was invented by sir William Crookes. It was a question in the beginning of causing a Fluorescence luminous of mineral. The tube of Crookes is still called tube with discharge , gas-tube or tube with cold cathode .

It is about a bulb in Verre in which one makes the Vide; there remains a residual Pression of Air of approximately 100 Pa (approx. 1 Torr). It contains a metal Cathode, in Aluminum, of concave form to concentrate the flow of electron S, and a Anode, or “target”.

An induction coil provides an high voltage. It then occurs a Ionization of the residual air, in the form of a flash or “discharges”, which causes a flow of electrons of cathode towards the anode. This flow, called cathode ray, produces an electromagnetic radiation which is able to create light of Fluorescence on certain bodies like producing discharges of bodies electrified remotely. It also creates x-rays.

This tube makes it possible to create x-rays only intermittently. It is not used any more in the modern apparatuses.

Tube of Coolidge

The tube of Crookes was improved by William Coolidge in 1913. The tube of Coolidge , still called tube with hot cathode , is the tube most largely used. It is a thorough vacuum tube (approx. 10^-4 Pa, approx. 10^-6 Torr), covered with a leaded enclosure.

In the tube of Coolidge, the electrons are emitted by a Filament of Tungstène heated by an electric current (thermionic effect also used in the cathode tubes of Téléviseur). The filament constitutes the cathode of the tube. The high voltage is established between cathode and the anode, which accelerates the electrons emitted by the filament. These electrons come to strike the anode.

In the tubes called “to side opening”, the electrons are concentrated (focused) by a part called Wehnelt just placed after the filament.

From an electric point of view, one thus has:

a filament at the boundaries of which a low tension is established, in order to create a heating electric current (Joule effect);
in certain tubes, a particular pipe fitting having a slightly negative tension compared to the filament (i.e. compared to the two terminals of the filament), in order to push back the electrons resulting from the filament towards the center of the part; it is Wehnelt;
a target anode having a strongly positive tension compared to Wehnelt and with the filament.

One distinguishes two geometries from tube:

tubes with side opening: the filament is a rectilinear Solénoïde of axis and is placed vis-a-vis the target, which is bevelled; the trajectory of the electrons is a line
the tubes with frontal window: the filament is a solenoid with circular axis and surrounds the anode; the trajectory of the electrons is curved.

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