Cathode ray

As electron radiation, referred to as the radiation beam historically as cathode rays, a particle beam of electrons, that is, a propagation of electrons in a mostly gaseous material.

Generation

Technically generated radiation beam of electrons are called electron beam. The beam is generated technically usually with an electron gun, a beam system, as also occurs in the cathode ray tube ( Braun tube and CRT). The electrons are released from a hot cathode and accelerated by an electric field. Further acceleration can be done with particle accelerators ( linear accelerator, betatron, cyclotron, Mikrotron ).

With the discovery of cathode rays rays came through an opening in one of the cathode opposite (positive) anode, causing luminous phenomena. They called this obviously emanating from the cathode rays therefore as cathode rays. The costs incurred in cold cathode rays of a gas discharge designated to contrast as canal rays. Only later was it realized that the former consisted of electrons and the latter of (positive ) ions.

History

The investigations were stimulated by the search for the smallest particles of electricity, as they should exist according to Faraday's laws. Therefore were studied electrical processes in dilute gases and found luminous phenomena. Julius Plücker used gas discharge tubes in which the cathode were heated. He and his pupil Johann Hittorf found that

Shadow cross tube in operation

Applied magnetic field

Modified magnetic field

William Crookes, who invented the suitable for these investigations shadow cross tube, introduced in 1879 found that these rays also occurred in highly evacuated tubes in which were otherwise to detect any luminous phenomena of the gas discharge more. He also realized that she heated solid body and exerts pressure. This led to the conclusion that cathode rays apparently of particles ( corpuscles ) are made.

First cathode rays were systematically studied by Philipp Lenard in the 90s of the 19th century. He built for this purpose, the so-called Lenard window, consisting of a grid with a deposited metal film. He realized that the cathode rays could pass through a film of several thousand atomic layers. Lenard also realized that cathode rays exposed photographic plates and cause phosphorescence with suitable fabrics.

Properties

In contrast to the photons of electromagnetic radiation having a electron rest mass of 9,109 382 91 (40) · 10 to 31 kg, and an electric charge of 1.602 176 565 (35) · C 10 to 19 ( unit charge ). This creates an electron from moving charges, so it is an electric current and thus generates a magnetic field. Electron beams can therefore electrostatically and magnetically deflect with a deflection.

Due to the same charge of the electrons in the electron beam that has the tendency to diverge. Where one operates with magnetic or electrostatic focusing ( focusing) against (see electron optics ).

Scattering law

Lenard found the scattering law:

With:

There have been many attempts to determine the mass of the particles that made up the cathode rays. However, this only succeeded Joseph John Thomson ( 1856-1940 ). Thomson began a much-improved vacuum and could determine the ratio of charge to mass due to electrostatic deflection of the cathode rays.

Applications

The first major technical application found the electron beam as a directed beam of rays in the Braun tube, which was developed by Karl Ferdinand Braun ( 1850-1918 ). The cathode-ray is displayed on a fluorescent screen inside the tube when it is incident thereon. Applications of the cathode-ray oscillograph and the picture tube.

Accelerated pulsed deflected electron beams with relativistic speed used at synchrotrons, inter alia, as a source of electromagnetic radiation ( synchrotron radiation ) from the infrared up to soft gamma-rays (see also free-electron laser).

Electron beams interact strongly with matter, so, for example, heating a solid body when it is irradiated with electron beams. This is utilized, inter alia, to the melting of materials, for example, in electron beam melting or electron beam as a heater when the evaporator. About a corresponding beam guidance can also be structures in the micrometer range easily influenced, such as resistor trimming.

In metal processing electron high power ( 100 kW size ) for melting, tempering, annealing, drilling, engraving and welding are used. The processing is usually done in a vacuum ( at least 10-2 mbar). When electron beam welding at atmospheric pressure ( engl. non- vacuum electron beam welding, NVEBW ), an electron beam welding process, however, also happen under normal pressure. Here, the working distance between the jet exit and the workpiece 6-30 mm must be, the transition from high vacuum to atmospheric pressure happens over several pressure stages. In TIG welding, an electron beam is also used.

The electrons of an electron beam can be assigned according to their energy and wavelengths by Louis de Broglie, but they are not themselves electromagnetic wave. Your de Broglie wavelength is typical for energy while far below one nanometer. Therefore, electron beams have no limitations on the resolving power due to diffraction phenomena. Due to the strong interaction with matter, electrons are used for imaging and analysis of the internal structure and the surface of solids (see electron, photoelectron spectroscopy and electron probe microanalysis ). They are also suitable for the production of the finest structures in the nanometer range, for example, in electron beam lithography.