Crookes Tube

Crookes tube

Historically, the first X-ray tube was invented by sir William Crookes. It was used to make a visible fluorescence on minerals. The Crookes tube is also called discharge tube or cold cathode tube.

It is a glass bulb with around a thousandth of sea-level atmospheric pressure of air (approximately 100 pascals or 1 torr). It contains an aluminum cathode with a curved shape to concentrate the electron flow on the anode, or "target".

A high tension (known in the US as voltage) is made between the electrodes; this induces an ionization of the residual air, and thus an electron flow or "discharge" from the cathode to the anode. When these electrons hit the target, they are slowed down, producing the X-rays (Bremsstrahlung and X-ray fluorescence of the target).

This tube can not produce X-rays continuously. It is no longer used on modern devices.

See also Crookes tube and glow discharge tube.

[edit] Coolidge tube

Coolidge side-window tube (scheme)

• K: filament
• A: anode
• W_in and W_out: water inlet and outlet of the cooling device (C)

The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10^-4 Pa, or 10^-6 Torr).

In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.

There are two designs: end-window tubes and side-window tubes.

In the end-window tubes, the filament is around the anode, the electrons have a curved path.

What is special about side-window tubes is:

• An Electrostatic Lens to focus the beam onto a very small spot on the anode
• The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons:
  • Mechanically spun to increase the area heated by the beam.
  • Cooled by circulating coolant.
• The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current.
• The anode is usually made out of tungsten or molybdenum.
• The tube has a window designed for escape of the generated X-ray photons.

The power of a Coolidge tube usually ranges from 1 to 4 kW.