Cathode ray
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Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two metal electrodes to which a voltage is applied, a cathode or negative electrode and an anode or positive electrode. They were discovered by German scientist Johann Hittorf in 1869 and in 1876 named by Eugen Goldstein kathodenstrahlen (cathode rays). Electrons were first discovered as the constituents of cathode rays. In 1897 British physicist J. J. Thompson showed the rays were composed of a previously unknown negatively charged particle, which was named electron.
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[edit] Description
Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a tube. To release electrons into the tube, they first must be detached from the atoms of the cathode. In early vacuum tubes (Crookes tubes) this was done solely by the high electrical potential between the anode and the cathode. In modern tubes this is assisted by making the cathode a thin wire filament and passing an electric current through it. The current heats the filament red hot. The increased random heat motion of the filament atoms assists in knocking electrons out of the atoms at the surface of the filament, into the evacuated space of the tube. This process is called thermionic emission and can reduce the anode to cathode voltage needed to obtain effective currents.
Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be travelling in straight lines from the cathode. After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube.
[edit] History
After the 1650 invention of the vacuum pump by Otto von Guericke, physicists began to experiment with passing high voltage electricity through rarefied air. In 1705, it was noted that electrostatic generator sparks travel a longer distance through low pressure air than through atmospheric pressure air.
[edit] Gas discharge tubes
In 1838, Michael Faraday passed a current through a rarefied air filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode). In 1857, German physicist and glassblower Heinrich Geissler sucked even more air out with an improved pump, to a pressure of around 10-3 atm and found that, instead of an arc, the glow filled the tube. The voltage applied between the two electrodes of the tubes, generated by an induction coil, was anywhere between a few kilovolts and 100 kV. These were called Geissler tubes, similar to today's neon lights.
The explanation of these effects was that the high voltage accelerated electrically charged atoms (ions) naturally present in the air of the tube. At low pressure, there was enough space between the gas atoms that the ions could accelerate to high enough speeds that when they struck another atom they knocked electrons off of it, creating more positive ions and free electrons in a chain reaction. The positive ions were all attracted to the cathode. When they struck it they knocked many electrons out of the metal. The free electrons were all attracted to the anode.
In the Geissler tubes, there was so much air that the electrons could only travel a tiny distance before colliding with an atom. The electrons in these tubes moved in a slow diffusion process, never gaining much speed, so these tubes didn't produce cathode rays. The glow in the gas was caused when the electrons or ions struck gas atoms, exciting their orbital electrons to higher energy levels. The electrons released this energy as light. This process is called fluorescence.
[edit] Cathode rays
By the 1870s, British physicist William Crookes and others were able to evacuate tubes to a lower pressure, below 10-6 atm. These were called Crookes tubes. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow.
What was happening was that as more air was pumped from the tubes, the electrons could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays.
When they reached the anode end of the tube, they were travelling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher energy levels, causing them to fluoresce. Later researchers painted the inside back wall with fluorescent chemicals such as zinc sulfide, to make the glow more visible.
Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist Johann Hittorf was first to realize that something must be travelling in straight lines from the cathode to cast the shadows. Eugen Goldstein named them cathode rays.
[edit] Discovery of the electron
At this time, atoms were the smallest particles known, and were believed to be indivisible. What carried electric currents was a mystery. During the last quarter of the 19th century many experiments were done to determine what cathode rays were. There were two theories. Crookes and Artur Shuster believed they were particles of "radiant matter", that is, electrically charged atoms. German scientists Eilhard Wiedemann, Heinrich Hertz and Goldstein believed they were "aether waves", some new form of electromagnetic radiation.
The debate was resolved in 1897 when J.J. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen. Therefore they were not atoms, but a new particle which he originally called "corpuscle" but was later named electron. He also showed they were identical with particles given off by photoelectric and radioactive materials.[1] It was quickly recognised that they are the particles that carry electric currents in metal wires, and carry the negative electric charge of the atom.
Thomson was given the 1906 Nobel prize for physics for this work. Philipp Lenard also contributed a great deal to cathode ray theory, winning the Nobel prize for physics in 1905 for his research on cathode rays and their properties.
[edit] Vacuum tubes
The gas ionization (or cold cathode) method of producing cathode rays used in Crookes tubes was unreliable, because it depended on the pressure of the residual air in the tube. Over time, the air was absorbed by the walls of the tube, and it quit working.
A more reliable and controllable method of producing cathode rays was investigated by Hittorf and Goldstein, and rediscovered by Thomas Edison in 1880. A cathode made of a filament heated red hot would release electrons into the tube by a process called thermionic emission. The first true electronic vacuum tubes, invented around 1906, used this hot cathode technique, and they superseded Crookes tubes. These tubes didn't need gas in them to work, so they were evacuated to a lower pressure, around 10-9 atm (10-4 P). The ionization method of creating cathode rays used in Crookes tubes is today only used in a few specialized gas discharge tubes such as krytrons.
The technology of manipulating electron beams pioneered in these early tubes was applied practically in the design of vacuum tubes, particularly in the invention of the cathode ray tube by Ferdinand Braun in 1897. and is today employed in sophisticated devices such as electron microscopes, electron beam lithography, and particle accelerators.
[edit] Particles of Cathode Rays
Like a wave:
- they travelled in straight lines
- Produced a shadow when obstructed by objects
- could pass through thin metal foils without disturbing them (Tested by New Zealander Ernest Rutherford using gold foil.)
These conflicting properties caused disruptions when trying to classify it as a wave or particle. Crookes insisted it was a particle, whilst Hertz maintained it was a wave. The debate was resolved when an electric field was used to deflect the rays by J. J. Thomson. This evidence that the beams were composed of particles was strong because scientists knew it was impossible to deflect electromagnetic waves with an electric field.
[edit] See also
- α (alpha) particles
- β (beta) particles
- Electron
- Electron beam processing
- Electron gun
- Electron irradiation
- Ionising radiation
- Particle accelerator
- Rays:
- Sterilisation (microbiology)
[edit] References
- ^ Thomson, J. J. (August 1901). "On bodies smaller than atoms". The Popular Science Monthly (Bonnier Corp.): 323-335. http://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323. Retrieved on 2009-06-21.
[edit] External links
