Changes

Thorium Oxide coated Iridium Hairpin Cathodes are used in [[LEED]] machines for the electron gun. They consist of a Hairpin shaped Iridium Wire, coated with Thorium Oxide.

Thorium Oxide coated Iridium Hairpin Cathodes are used in [[LEED]] machines for the electron gun. They consist of a Hairpin shaped Iridium Wire, coated with Thorium Oxide.

== Oxide Cathode ==

== Oxide Cathode ==

The downside to LaB6 cathodes is their proclivity to be poisend by carbon, thus requiring significantly higher vacuum then a tungsten hairpin to opperate. This is typically affected by the use of a differental pumping system, i.e. a Aperture and second pump, and is of the order >1 e-7 mBar. The pumping power to achieve such pressures is supplied by Ion pumps usually connected directly to the emission chamber.  

The downside to LaB6 cathodes is their proclivity to be poisend by carbon, thus requiring significantly higher vacuum then a tungsten hairpin to opperate. This is typically affected by the use of a differental pumping system, i.e. a Aperture and second pump, and is of the order >1 e-7 mBar. The pumping power to achieve such pressures is supplied by Ion pumps usually connected directly to the emission chamber.  

=== Sintered LaB6 ===

=== Sintered Polycrystalline LaB6 ===

These early types as stated previously are long square rods with a pyramid tip. The heating of these cathodes was typically done by either bombarding them with high current electron beams, typically radially, or by thermal absorbtion of a white hot tungsten filament in close proximity. The cathode would normally be assembled into a oven assembly which also acts as the whenelt cup, and mounted into the micoscope. Examples of an early use of this type of cathode is the [[Stereoscan S4-10|Cambridge Stereoscan S4-10]], which if ordered or upgraded to use LaB6 cathodes, had the ability to also be used with Tungsten Hairpin cathodes by simply replacing the LaB6 oven with a Tungsten Hairpin Wehnelt Assembly (as well as changing some settings).  

These early types as stated previously are long square rods with a pyramid tip. The heating of these cathodes was typically done by either bombarding them with high current electron beams, typically radially, or by thermal absorbtion of a white hot tungsten filament in close proximity. The cathode would normally be assembled into a oven assembly which also acts as the whenelt cup, and mounted into the micoscope. Examples of an early use of this type of cathode is the [[Stereoscan S4-10|Cambridge Stereoscan S4-10]], which if ordered or upgraded to use LaB6 cathodes, had the ability to also be used with Tungsten Hairpin cathodes by simply replacing the LaB6 oven with a Tungsten Hairpin Wehnelt Assembly (as well as changing some settings).  

Due to the complexity and requirement for a 4th conductor for the gun, sufficient room for the oven assembly, and being overall hard to set up and use, this design of cathode is now no longer used.  

Due to the complexity and requirement for a 4th conductor for the gun, sufficient room for the oven assembly, and being overall hard to set up and use, this design of cathode is now no longer used in SEM.

 

Disc type LaB6 cathodes are used in Ion Sources for particle accelerators, high current electron guns for accelerators and electron beam welding machines.  

=== Single Crystal LaB6 ===

=== Single Crystal LaB6 ===

Since this cathode type is much simpler to use then the sintered type, as well as being smaller, and offering a more even emission area current distribution, it has supplanted the older type mentioned above.  

Since this cathode type is much simpler to use then the sintered type, as well as being smaller, and offering a more even emission area current distribution, it has supplanted the older type mentioned above.  

= Field Emission Cathode =

= Field Emission Cathode =

This type of cathode is marked by its extremely small emission area, being down to a few nm in diameter, and thus overwhelming current density. It is however one of the hardest to produce and work with, as well as requiring the most stringent of vacuuo to opperate. This type of cathode can be subdevided into two main types, the cold field emission cathode, and the Schottky Hot Field Emission cathode. The vacuum required for the cold type is higher then that of the shottkey, these being in the range of <1e-10 mBar for the former, and <1e-8 mBar for the latter. The cathode takes the shape of a tungsten single crystal needle, typically mounted on a tungsten hairpin, with the tip having a radius of <100nm. These tips are, overly simplidied, made by etching the crystal in a solution of Sodium Hydroxide, half being sumberged and half being above water. The resultent needle is of remarkably small tip radius.  

This type of cathode is marked by its extremely small emission area, being down to a few nm in diameter, and thus overwhelming current density. It is however one of the hardest to produce and work with, as well as requiring the most stringent of vacuuo to opperate. This type of cathode can be subdevided into two main types, the cold field emission cathode, and the Schottky Hot Field Emission cathode. The vacuum required for the cold type is higher then that of the shottkey, these being in the range of <1e-10 mBar for the former, and <1e-8 mBar for the latter. The cathode takes the shape of a tungsten single crystal needle, typically mounted on a tungsten hairpin, with the tip having a radius of <100nm. These tips are, overly simplidied, made by etching the crystal in a solution of Sodium Hydroxide, half being sumberged and half being above water. The resultent needle is of remarkably small tip radius.  

= Photoelectric Cathode =

= Photoelectric Cathode =

A rare type of cathode used in ultra short pulse electron microscopes, is the photoelectric cathode. This usually consists of a metal surface of low work function, such as Cesium, which is illuminated by a suitable light source. The ara exposed thus, will emmit electrons via the photoelectric effect, which can then further be accelerated to form the electron beam of an electron microscope.  

A rare type of cathode used in ultra short pulse electron microscopes, is the photoelectric cathode. This usually consists of a metal surface of low work function, such as Cesium, which is illuminated by a suitable light source. The area exposed thus, will emit electrons via the photoelectric effect, which can then further be accelerated to form the electron beam of an electron microscope.

 

 

 

Photocathodes can be either of the Front or Rear illuminated type. Front illuminated being those deposited or composing a opaque surface which can be illuminated from the direction of electron emission. Backside illuminated cathodes are thin films deposited on a transparent medium such as glass, here the cathode is illuminated through the transparent medium and the electrons ejected from the opposite side.  

A common useage of this cathode type is within the [[photomultiplier]] tube of the secondary electron detector of a scanning electron microscope as well as night vision image intensification tubes. In the past, such cathodes also formed the basis of Videocon image capture tubes, used in television cameras and the first digital cameras.

Examples of a front side illuminated photocathode are some designs of vacuum tube photocell, where a metal plate in vacuum is coated with a layer of photocathode material, with a wire anode or similar design in front of it. Thus composing a light dependent diode, or photodiode.

Examples of a backside illuminated photocathode is the humble photomultiplier tube, most designs feature a semi transparent thin film of Bi-Alkali or similar material on the inside of the glass vacuum tube, with the dynodes situated behind it at some distance. Most photomultipliers made, are of this type, though there exist rare examples of photomultiplier tubes with a front side illuminated cathode on a metal plate similar to the Vacuum Tube photodiode. This design has the downside of lower acceptance angle then a backside illuminated photocathode.